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Acoustic testplan and results from stone and timber circles

Please check the terminology section for some defined words. The authors of this web site have visited stone and timber circles some 15 days to make many acoustic observations.

Content

• The circles under investigation
• Dimensions of circles
• Archaeoastronomy
  
• Possible measurements
  
• Sound sources
• Equipment list
• Questionnaire
• Observations
  
• Dromagorteen stone circle (Nov. 2015)
  
• Observations in a garden (March 2016)
  
• Pre-test at Goseck and Pömmelte-Zackmünde (April, 2016)
• Pre-test at the Grange Circle B, Lough Gur (April, 2016)
• Test at Goseck and Pömmelte-Zackmünde (May, 2016)
• Summary of Heldenberg article (Sept, 2013)
• Performance test at the Grange Circle B (April 2017)
  
• Observations at Heldenberg (June 2017)
  
• Evaluation
• Influence of a terrain slope
  
• Auralisations
  
• Summary
• Press coverage
• Opportunities for the future
  
• References
• Abbreviations
  
• Acknowledgements
  

The circles under investigation

Three timber circles are investigated:

1. Goseck, Germany
   
2. Pömmelte-Zackmünde, Germany
3. Heldenberg, Austria

4. Dromagorteen stone circle, Kerry, Ireland
   
5. Grange Circle B, Lough Gur, Ireland

6. Mansfeld, Germany

7. a Home garden
   
8. a few open fields (semi-anechoic space) in Goseck, Berlin and Skerries
   

The dimensions of circles

Goseck, Germany

Reconstructed Goseck (from around 4800 BCE) has two tightly spaced (~4cm space between them) (timber) palisade rings with diameter of ~52m (1^st palisade) and ~60m (2^nd palisade). The posts have a diameter of 20cm and a height of 2.5 to 3m. The posts are quite straight and have no bark.

Weather conditions at Goseck are here.

Goseck timber circle [Bertemes, 2010]

Pömmelte-Zackmünde, Germany

Reconstructed Pömmelte-Zackmünde (from around 2300 BCE) has one tightly spaced (~4cm space between them) (timber) palisade rings, one medium spaced (~20cm space between palisade) and two widely spaced (~2m) (timber) palisade rings. The diameter of the tightly spaced palisade are resp. ~75m (1^st palisade) and ~115m, while the two widely spaced palisade are ~45 m and ~55 m in diameter. The posts have a diameter of 20cm and a height of 2.5 to 3m. The posts are a little bit crooked and have no bark.

Weather conditions at Pömmelte-Zackmünde are here.

Pömmelte-Zackmünde timber circle [Spatzier, 2012]

Grange Circle B, Ireland

The Grange Circle B at Lough Gur (from around 2900 BCE [Prendergast, 2016])  has an internal diameter of around 45.5m and its standing  stones are on average slightly higher than 1.5m (some are though higher). There is an earthen bank (~ 9m wide and ~1m high on the inside and ~2m on the outside) along the standing stones (on outside).

Weather conditions at Grange Circle B are here.

Grange Circle B [Ó Ríordáin, 1951  (adjusted to True North) and Google Earth]

Dromagorteen stone circle, Ireland

Dromagorteen stone circle (Early/Middle Bronze Age: 2000 to 1150 BCE), situated in the Bonane Heritage park, is of the type of Axial Stone Circle (ASC) with 13 stones. The circle has a diameter of around 10 m. This one might be too small to be compared with others.

Weather conditions at Dromagorteen are here.

Dromagorteen stone circle [after Bonane Heritage park, 2010]

Mansfeld, Germany

Mansfeld at the former Am Archäopark, which is a reconstruction of Schalkenburg, Quenstedt (from around 4800 BCE),  has a 1^st palisade with an short axis of 35m and a long axis of around 44m. Its 5^th palisade is around 90m by 120m. The reconstruction though has deteriorated so much in the last years (at least from 2005 on) that it is not presentative anymore. The posts had a diameter of ~27cm and a height of 2 to 3m. The posts were quite crooked and many had (peeling) bark. Mansfeld is build on a sloping terrain: the north east of Mansfeld is some 2m lower than the south west corner of the 1^st palisade (around 2.5deg). Quenstedt was build at the top of a hill [Kaufmann, 2011, page 109]. Quenstedt might have had tightly-spaced double palisade palisades (like Heldenberg) [Kaufmann, 2011, page 107].

Weather conditions at Mansfeld are here.

Mansfeld reconstruction from Quenstedt [Google Earth, 2009; Wikipedia]

Heldenberg, Austria

Heldenberg, which is a reconstruction of Schletz timber circle, Austria (from around 4850 to 2550 BCE, so similar to some of the Central Germany circles; like Goseck),  has two tightly-spaced palisade circles one with an axis of ~24m and the other one somewhat larger (~24.3m); and one widely (~1.5m) spaced palisade (~30m diameter). The tightly-spaced double palisades have gaps between posts of ~10cm. Seen from the centre, there are no gaps; posts of 2^nd palisade are shifted somewhat relative to the 1^st palisade. The posts have a diameter of ~25cm and a height of 2.8 to 3.2m. The posts are straight. Bark is still on them (no peeling bark, at least in 2013). Heldenberg is on a terrain slope of around 4.5 deg (Google Earth showed 5 deg).
Scheltz looks also to be on a slight ridge and thus on a sloping terrain (around 6 deg).

Re-reconstructed Heldenberg

The circles was re-reconstructed (in 2016), after the posts had rotted after the initial construction (13 years ago). The height of the posts on the inside of the palisades were on average ~1.9m while the posts on the outer side of the palisade were on average ~2.3m. This is some 0.9m lower than the initial reconstruction. The posts are still barked, but were loosing their bark. There were at some places considerable gaps between the posts.

Heldenberg reconstruction from Schletz [Google Earth, 2016]

Archaeoastronomy

Goseck

Likely winter solstice and summer solstice Sun rise/set, Beltane and Lughnasadh Sun rise/set.

Pömmelte-Zackmünde

Likely Imbolc and Samhain Sun rise.

Grange Circle B

Likely Beltane and Lughnasadh Sun rise.

Dromagorteen

Likely summer solstice Sun rise and major lunar standstill limit event rise.

Quenstedt

Likely summer solstice Sun rise/set and winter solstice Sun rise.

Schletz

A study on the archaoeastronomy of Austrian timber circles has been done as part of the ASTROSIM project [ASTROSIM]. The results are meager for the 32 Austrian KGAs investigate. No significant astronomical evident has been found for the positioning of entrances, it looks that entrances of the Austrian KGAs are positioned by looking at the lay of the land [Zotti, 2011].

Measurement methodologies

Room based acoustic measurements (see for instance [Ahnert]) might not be very effective in these large open space. Perhaps music-hall methodologies are more important. Perceptions of effects are just as important as measurements. There look to be three different measures:

• objective, purely based on measurements
• subjective
  
• psychoacoustic; including the effects of the human receptors and the brain
  
• cultural; what cultural recognises and defines as good or bad
  

• sensing and mapping space: archaeography
  sound source: none
  applicable freq. range: all
  [Graves, 2009; Blesser, 2007, 37-51]
• draw-away measurements
  sound source: noise, speech and music sounds
  applicable freq. range: all; for whispering gallery: high freq.
  determine the sound attenuation while varying the distance [Toole, 2013, chapter 4]. This will also result in the Strength G (at 10m from sound source) when compared to the monument and open field [Campanini, 2009] [Parker, 2013]
  Look also at the sound pressure level decay: inverse square law (open space), inverse of distance (whispering gallery wave; sound diffraction), or focal points.
  
• apparent source widening (ASW: sound stage illusions)
  sound source: noise/impulse sounds
  applicable freq. range: > 500 Hz
  this is due to reflections within ~80ms (Haas/Precedence-effect) (if RTT<80ms; distance listener and palisade < 14 m) [Toole, 2013, chapter 7]
• listener envelopment (LEV: surround illusions)
  sound source: noise/impulse sounds
  applicable freq. range: < 500 Hz
  this is due to reflections beyond ~ 80ms (if RTT>80ms; distance listener and palisade > 14 m) [Toole, 2013, chapter 7].
  
• discrete echoes (second image)
  sound source: impulse sounds
  applicable freq. range: all
  Experiencing discrete echoes in a time frame larger than (40 to) 80ms [Everest, 2001]. Related to reverberation time: T60 and EDT.
  
• increased Preference (using IACC [binaural])
  sound source: noise/impulse sounds
  applicable freq. range: all
  a change that might be perceivable for reflections larger than 10ms (if RTT>10ms, distance listener and palisade > 1.7 m) [Toole, 2013, chapter 7]
• timbre changes
  sound source: noise/impulse sounds
  applicable freq. range: all
  Other words for this: coloration. Can be due to resonance, filtering and/or comb effects.
  
• clarity of music (C₅₀, [Bradley, 2011])
  sound source: impulse sounds
  applicable freq. range: 500, 1000, 2000 and 4000 Hz
• clarity of speech (C₈₀, [Bradley, 2011])
  sound source: impulse sounds
  applicable freq. range: 500, 1000 and 2000 Hz
• chirp echo
  sound source: impulse sounds
  applicable freq. range: all
  acoustic convolution-scattering might happen when making impulse near the palisade. [Toole, 2013, section 6.5] [Lubman, 1998, including sound file and Lubman, 2008]
• directivity of the sound (using binaural measurements)
  sound source: noise sounds
  applicable freq. range: > 500 Hz
  [Toole, 2013, chapter 6]
• resonances can happen in timber circle
  sound source: special sounds
  applicable freq. range: < 500 Hz
  The diameter is 50m or longer so the first mode would be around 3 Hz. Not really something to observe/perceive or measure with available equipment, IMHO.
  [Toole, 2013, chapter 4.3]
• word clarity
  sound source: speech sounds
  applicable freq. range: all
  [Mallinger, 2011], [Toole, 2013, page 167]
  

Considerations for locations of the measurements:

• at places that could have acoustic differences, such as: wide or thin openings/entrances; in center; focal points of ellipses; close to palisade; on boundaries of 1.7m and 14m from the palisade; at archaeological important locations (burial pits); within palisades; between palisades; and outside palisades.
  
• if the monument is symmetric, only measurements on one of the symmetrical sides  is needed
• make sure that the sound source covers the whole area (as much as possible)
• take similar measurements at similar distance in open field

Sound sources

The following sound sources were considered (bold means that they were used during the experiments):

• Noise sounds
  
• pink noise
• brown noise
• Speech sounds
  
• babble sound
• whisper sound
  
• spoken words
• squealing pigs
  
• Special sounds (on computer)
  
• Exponential SineSweep (ESS)
  
• MLS
  
• Music sounds
  
• Bronze Age horn
• Neolithic flute
• Bull roarer
• Shamanic drum
• Bohdran
  
• Impulse sounds
  
• balloon
• hand clap
  
• paper banger
• pair of claves
• vocal Hip-Hop
  

Equipment list

• Power
  
• 12VDC car battery 60 or 80 Ah (RM)
  
• car battery charger 6 to 10 A (RM)
  
• 12VDC to 220VAC inverter 1200W (VR)
• 150 m power extension lead (RM)
• opened power closets at timber circles (RM)
  
• at least a 3-way extension lead with type C or F sockets (220VAC) (RM)
• batteries for any other equipment (VR)
  
• Sound sources
• sound system (2*50W) and 2 speakers (2*75/100W) (RM). Could only be used upto volume value 30 (the audio power is not measured/known).
  
• balloons (45cm from tip to mouth piece in blown state, which is approximately a diameter of 24cm) plus pin to pop on the balloon's side (RM)
• bull roarer (VR)
• shamanic drum (RM)
• bodhran (VR)
  
• CD with noise, speech and music sounds (each 15 mins) long (RM)
• USB flash drive with 20 sessions of 20 words to determine word clarity (RM)
  
• spare micro SD cards and spare CD with noise, speech and music sounds (each around 10min long); they are also on the shared dropbox directory: \Soundtest 09.04.2016\CDsounds (VR)
• small (flag based on A4) and large (based on a little larger than A3) paper bangers (several as they will break) (VR)
• Bluetooth stereo speaker (2*8W RMS) (VR) and 30W (FH)
  
• Recording
  
• two omni directional microphones (VR)
  
• binaural microphones (VR)
  
• wind muffler (VR)
  
• microphone boom (VR)
  
• headset (VR)
  
• Zoom H1 recorder plus spare battery (VR)
• SPL sound meter plus spare battery (VR)
• word clarity questionnaire (RM)
  
• Audio cabling and tools
  
• collection of connectors (3.5 mm, RCA, male, female, etc.) (VR)
  
• collection of RCA cables (VR)
  
• some tools (screwdriver, isolation tape, multi meter, zip tie straps, etc.) (VR)
• paper note books (VR, RM)
• camping table and folding chairs (to put sound system on) (RM)
• bin liners (RM)
• tape measure 100m plus tent peg (RM)
• Analytics and resources
• laptop plus charger (VR)
  
• RPlusD, Audacity and Aurora software (VR)
• Sound level, Hi-Q MP3 recorder, Geosurvey and SunSurveyor Apps (VR)
  
• relevant books, literature. (VR, RM)
• liability insurance (VR, RM)
  

Questionnaire

A questionnaire has been made for Goseck on SurveyMonkey. If you want to provide input, please do.

Dromagorteen stone circle (Nov. 2015)

Participant

Weather data

Equipment

The microphone (Zoom H1 recorder) was on top of the recumbent stone (Stone1) some 0.8 m high and the sources (balloon, paper banger) were at around 1.2 m high.

Measurements

• 3 msec
  traveled an extra 1 m, which could reflection against some stone near the portal stones or Stone1
  
• 12 msec
  traveled an extra 4 m, which could be a reflection from Stone4 or Stone9 to Stone1
  
• 33 msec
  traveled an extra 11 m, which could be a reflection from Stone1, MidStone and back to Stone1.
  

Impulse response from small balloon

Binaural recording

Sloping hill on the north-west side of the stone circle

Impulse response at Stone 1, by paper banger

Perceptions

• On the above binaural recording a kind of scanning reflection can be perceived, unfortunately a repeat impulse sound did not show this, but the angle/direction of the impulse sound could have been different.

Lessons learnt

• record at least at 24 bits and 48 kHz. And save the correct format to keep that 24bits and 48 kHz (in Audacity: File -> Export -> Save as type: 'Other uncompressed files' -> Options... -> Encoding: 'Signed 24-bit PCM' -> OK -> Save.
• Manipulate the IR the least possible. If one saves an IR, use enough trailing length; the clips at least three the length of the RT₆₀.
  
• test all equipment before going into the field. In the field it is sometimes very difficult to debug problems and to implement a solution.
• be sure one has plan B ideas/equipment/batteries/etc. in case plan A does not work.
  

Observations in Home garden

Participant

Victor Reijs

Binaural recording

Draw away measurements (loudness)

The question is: Would a 50W amplifier provide enough power for sound level measurements in open space aka timber circles?
An amplifier (Panasonic SG-HM-10L) was put at the top of a Home garden (where the house is). The walled garden is rectangular (~8m wide and ~15m deep) and its walls are covered with trees/bushes. There is though a glass conservatory for the first 4m. Brown noise was played and the audio power was around 10W.
The sound level in dB_SPL was measured while drawing away from the sound source, see below picture.

Sound level of brown noise

The dip around 5m could be due to the shadow-reflections from the conservatory.
There is some 4 dB between the power at 20m and 31m (to be compared to Grange Circle B) and there is some 2dB between the power at 25m and 30m (to be compared to Goseck).

Lessons learnt

As the sound level of 'silence' was around 50dB_SPL, one would need at least some 20 dB above that for kind of accurate results. If the inverse square law holds, that would only give a maximum distance of  ~30m at 10W. If we increase the amplifier output to 50W (aka 6 dB more than 10W), the sound would reach a distance of some 60m. This is comparable to the diameter of the timber circles. So 50W amplifier looks to be just good enough.

Pre-test in early April, 2016

Participants

Reinhard Mussik, Finn Hockauf and Elke Hockauf

Test plan for Goseck and Pömmelte-Zackmünde

Measurements done, to get a feel of the timber circles, are around:

• Determine how far a 3W bluetooth speaker comes (please record/note: sound, location, condition and other things)
  
• Get a feel how the different (music and impulse) sounds work (please record/note: sound, location, condition and other things)
• Determine if there are obvious acoustical effects  (like: gain, reflections, echoes [second image] and chirp echoes) at certain locations (center, entrances, inside two palisades, near palisade, outside palisade , etc.) (please record/note: sound, location, condition and other things)
• Determine how close we can park and power closet.
• Determine if there are people living nearby who would want to know that we are working in evening/night at the timber circle
• Perhaps meet with collaborators.

Weather data

Goseck

Equipment

Impulse sound source: balloons ~45 cm (when blown up)
Music sound source: 3W loudspeaker
The microphone (Samsung Galaxy S5, not in flight-mode) and the sound sources were at around 1.2 m high.
No wind muffler was used.

Measurements and results

Perceptions

• The echoes (second image) were clearly audible. Also other visitors told us that they heard echoes.
• It was possible to speak with people in the inner circle at normal volume, regardless of the position of the speaker and the listener.
• The music from the 3W loudspeaker was well audible in the whole inner circle, regardless of the position of the loudspeaker and the listener. But it was not every time recorded by the smartphone mic.

Lessons learnt

• balloons with blown dimension of around 45cm (tip to mouth pice) work well
  
• have phones in flight mode to minimise interference noises
• handle the recording device with care, before the IR and after the IR, to minimise handle noises.
• use a wind muffler
• make sure the start of the IR does not peak to the max (so keep the level in mind).
• have enough sound source power
• better to take the measurement in the early morning because of the less: wind (and thus wind noise), tractors, airplanes and visitors.

Pömmelte-Zackmünde

Equipment

Measurements and results

Perceptions

• There were echoes (second image) in Pömmelte-Zackmünde, but not so powerful as in Goseck (see also below Spatzier's comment when the reconstruction was done).
• It was possible to speak with people in the inner circle in a loud voice. The quality depended on the position of the speaker and the listener.
• The music from the 3W loudspeaker was audible in the whole inner circle. But the sound quality depended of the position of the loudspeaker and the listener.
• When the loudspeaker and the listener were both at a distance of 2.5 m from the centre of the circle (possible focal points), the sound quality was the best. This could not be repeated during the next visit (May 2016).

Lessons learnt

• having phones in flight mode really works
  
• handling the recording device with care really reduces the noise
  
• have sound level meter, instead of recordings for determining draw-off values

Pre-test at the Grange Circle B, Lough Gur (April, 2016)

Participant

Victor Reijs

Weather data

Equipment

• Zoom H1 plus wind muffler and binaural microphones, microphone boom
  
• large paper banger
• 2*1W CD player
• CD with sounds (brown, pink noise, babble, Bronze Age Horn, whispering)
  
• claves
• balloons (15 and 10cm diameter)
  

Measurements

• draw-away measurements, inside and outside circle
• determine if a kind of chirp echo or whispering gallery effect happens.
• stress test the large paper banger

Results

• It looks that the draw-away power does not show focal points (at least not within 10 m range of the Rannach Crom Dubh).
• Cassidy [2016] shows reflection timings similar to my own (following distance and speed of sound).
• If the mic is 10m from the centre (where the paper banger is) in direction of Rannach Crom Dubh; the direct sound has less power (on average some 2dB) than the reflection from Rannach Crom Dubh (see below graph). This can also be clearly heard in the auralisation.
  
  Rannach Crom Dubh has a slightly titled angle [Cassidy, 2016, page 117]; so could this higher power be due to this slightly titled angle?
  The sound level of the reflection compared to the direct sound at different distances from the centre can be seen in below table and this graph (green squares): All these reflections are heard as second images.
  

  Location from centre [m]	Reflection time [msec]	Reflection-Direct sound level [dB]
  5	103	-7
  10	70	2
  15	38	10

  
  
• The reflection from Rannach Crom Dubh looks to be some 6 dB more powerful than the reflection from the standings stones near the passage. If no large standing stones are present, there is no real reflection.
  
• When recording 10m from the two stones shaping a V opposite the passage; beside the reflection of the paper banger from the stones shaping a V (an additional ~24m); the reflection of Rannach Crom Dubh (an additional ~ 45m); and also the reflection from the standing stones near the passage is recorded (an additional ~58m).
  
  
• no chirp echo effect was experienced.
• no whispering gallery effect was experienced (only the reflection of the opposite side is seen when using large paper banger at Rannach Crom Dubh).
• large paper banger lasted at least 60 bangs (using brown packing paper).

Stone circle's bank as noise barrier

The sound level on top of the bank is around 2dB lower than on the edge of the circle (~20m), while several meters further (but 'behind' the bank at field level, ~31m) it is some 19dB lower than at edge of circle. Compare this with the 4dB decrease in open air. Open air sound is some 3 times (15dB) louder then just behind the bank of Grange Circle B. The bank is an effective barrier, see also this section.

Perceptions

The following perceptions where made:

• Mickael Fenström experienced the following at Grange Circle B:

We found that the dominant reflection is from the Rannach Crom Dubh. Subjectively, the most exciting thing we heard was when we had one person chanting from the center of the circle towards Rannach Crom Dubh. When standing on the bank, behind the chanting person, an interesting ‘overdub’ effect could be heard. We speculated that this could be used as a ‘voice of the gods’ effect.

This could be related to the on average some 2 dB difference between direct sound and reflection against Rannach Crom Dubh. Although measurements need to be taken at the bank to determine if the reflection is there still more powerful than the direct sound (form the source in the centre).

• When standing beside Rannach Crom Dubh (or a tree or other large stone that reached up to head height), sounds are muffled due to the object and thus its shadow effects. It feels like a pressure difference in the ear (or is it the sound pressure difference between the two ears?).
• When walking from Rannach Crom Dubh to the centre, the timbre is changing (due to Haas or comb effect) and some few meters before reaching the centre the sounds is perceived to be flatter.
  

Lessons learnt

• Include silence on the recording, to be able to reference silence (or wind sounds...).
• Cars (~6dB above background), and to a lesser degrees bird singing (~4dB above background) can cause considerable loss in dynamic range of sound level measurements. Wind noises can start to be detrimental from wind force 3 (certainly its gusts).
• The wind muffler used was not effective above low 3 force.
  
• It is important to have at least one or two spare paper bangers!
• Check regularly that indeed the recorder is recording!
• Have something to put the loudspeaker at a higher level (1.5m).
• Make sure one measures in open field the impulse sound at 10m (for referencing to the strenGth parameter) and possible other references to compare.
  

Test in May, 2016

Participants

Victor Reijs, Reinhard Mussik, Finn Hockauf and Elke Hockauf

Weather data

Goseck

Draw-away measurements

In this picture the draw-away measurements at Goseck are shown (they start with the letter G and using Pink, Horn or Whisper sounds):

• There does not look to be a difference in Long (LA) of Short (SA) axis sound level measurements.
• If the end of the draw-away was a palisade, the sound power at that end was some 3 dB higher than if the end was an entrance. So almost 100% (resulting in this 3dB higher) is reflected back at a palisade.
  This 3 dB is though not visible in the below shadow measurements for Palisade (P) and Beltane (B); could this be because the Beltane opening is only ~60cm, while the entrance is ~250cm?
  

Shadow effects

Several of shadow effects were experienced (mostly in timbre changes) at Goseck:

• A dynamic effect
  If one has pink noise in the middle of the circle and one walks between the two palisades; the walking provides a modulation of the sound level due to posts (shadow) and gaps (sound).
  
• A static effect
  Goseck has: a summer solstice area (S) where the posts are at a distance of some 50cm of each other, with gaps thus being ~30cm); a Beltane area (B) that is a gap of some 60cm; and between the two is an palisade area (P) where the posts are at their normal distance, say 24cm distance (gaps of some ~4cm). The sound levels measured  just inside the 1^st palisade and between the palisades can be seen in below table.
  

  	Summer (S)	Palisade (P)	Beltane (B)
  inside 1st palisade	63dBSPL	62.5dBSPL	64dBSPL
  between palisade	61.5dBSPL	58dBSPL	64dBSPL
  difference	1.5dB	4.5dB	0dB

  So if there is a 60cm wide opening (B), no reduction in sound level, behind the area with some ~30cm gaps (S) there is a reduction of 1.5dB, while behind the 1^st palisade with gaps ~5cm (P) there is a 4.5dB reduction. These differences are also heard in the recordings.
  It looks that the spectral power of the Summer area (orangy) is higher between 100 and 2,000Hz than for the Beltane area. From 2,000 to 7,000Hz the Beltane area has a higher spectral power (purply) than Summer area.
  

The reason could be that the Summer area is wider than the Beltane area and thus the Summer area has less attenuation for lower frequencies. The higher frequencies might be attenuated more by the few posts within the Summer area than the open area of Beltane.

If we compare the spectral power of Summer area with the Palisade area's, we get the below picture. One can see that the Summer area has for most frequencies a higher spectral power (orangy) than the Palisade area's (purply).

• noise barrier effect
  

  Several sessions were done with balloon pops recordings; from well outside the entrance of the circle (50 m from centre) to outside the back of the circle (-39m from centre). See below:

  
  
  

  There does not seems to be a large energy loss over 1^st and 2^nd palisades (remember that the variability of balloon pop is in the order of several dB, aka well audible), comparable to what is seen in open air. No significant step change is seen over these palisades, so the palisades at Goseck don't act as sound barrier, see also this section.
  

Whispering gallery effects

Although whispering galley effects were anecdotally mentioned [person at Goseck visitor centre, pers. comm 2016], such effects have not been experienced or recorded when using Babble, Whisper or Horn sounds. Whispering on one side of the 1^st palisade was not heard on its opposite side. The whispering gallery effect would be most pronounced beside the circumsphere and almost non audibility in the centre of the circle); but sound level measurements did not show such a difference.

Chirp echoes

Chirp echoes were heard near the 1^st and 2^nd palisades by popping balloons and hand clapping. One though has to exert considerable energy to generate it (compared to Chichen Itza where one has hard, crisper and regular stair surface).

IRs from centre

The IRs made show comparable aspects as during the April visit: some bouncing due to the 1^st and 2^nd palisade and bouncing of the far side of the palisade(s): flutterechoes. Also such behavior is seen outside the circle: 10m in front of the entrance; near the ditch; and on the bank.

IRs between palisades

Several IRs between the 1^st and 2^nd palisades were generated: in the April pre-test and repeated in the May test sessions. The following results have been seen in the measurements when popping the balloon in radial direction (towards 2^nd palisade):

• 0m arc length (impulse sound (balloon) and mic at same location)
  After the first direct sound peak, one dominant reflection is seen in the sound level graphs: around a time equivalent to 108m (is twice the diameter of the far 1^st palisade).
  
• 25m length (one 25m sector lengths between impulse sound (balloon) and mic)
  After the first direct sound peak, three dominant reflections are seen in the sound level graphs: the first one around a time equivalent to 7m (many (4??) bounces between the palisades [7+25=32m]), the second one around a time equivalent to 31m (aka twice the distance to the middle of the circle [31+25=56m]) and the third one a time equivalent to 78m (aka twice the diameter to the far 1^st palisade [78+25=103m]). The reflection from the middle of the circle is somewhat unexpected: in the April pre-test session there might have been people standing the middle while in the May test session there was a chair with speaker in the middle.
  
• 50m length (two 25m sector lengths)
  After the first direct sound peak, two dominant reflections are seen in the sound level graphs: around a time equivalent to 6m (aka twice the distance to the middle of the circle [6+50=56m]) and another time equivalent to 54m (aka twice the diameter to the far 1^st palisade [54+50=104m]). The reflection from the middle of the circle is somewhat unexpected: in the April pre-test session there might have been people standing the middle while in the May test session there was a chair with speaker in the middle.
  
• 75m length (three 25m sector lengths)
  After the first direct sound peak (in this case coming due to the chord distance: 56m), two dominant reflections are seen in the sound level graphs: the first one around a time equivalent to 17m (the arc length [17+56=73m]) and the second one around a time equivalent to 56m (aka four times the chord between the two points [56+56m=112]).

Below is a graphical view of the number of occurrences of peaks for different sector lengths between source and microphone (several measurements per sector lengths were done).

SketchUp was used to determine where the reflections might occur. Circles (red) were used (diameter; half of above mentioned distances), and ellipses (yellow) which shows any location that would provide reflections at the right time seen from the sector length (black line).

If the balloon is pop in a tangent direction (measured in May), the reflections from the middle or far 1^st palisade are significantly reduced or non existent.
Chirp echoes were perceived at several microphone locations.

Word clarity

No word clarity test has been performed, as one could understand each other in all areas within the 1^st palisade (and the background noise < 50dB_SPL is much lower than the speech).

Perceptions

• An interesting and possibly true statement is from Norma Literski-Henkel (2016) about Neolithic people:
  Vermutlich waren sie zunächst selbst überrascht, welche Effekte in der doppelten Palisadenkonstruktion entstehen.
• When clapping hands while coming closer to the 1^st palisade  (1 to 2m) the sounds became more muffled/duller. This could be the effect of decreased Preference, which happens when reflections come from closer than 1.4m.
  
• Clapping hands, near the 1^st and 2^nd palisade and as loud as possible, produced a chirp echo that was well perceived.
• It looks that the large paper banger (flap is somewhat larger than A3) is not able to survive (perhaps the strain/pressure is just too large for ordinary paper at this size). The small paper banger (flap based on A4) still works.
• It is important to keep the recording level as high as possible. As a nearby pop can generate a high sound level, the question is should one keep the same recording level for all further away recordings. Perhaps better that one adjusts the recording level in a controlled/deterministic way
• Two people were sitting and hand clapping in the centre of Goseck, with their backs to the listener. Half way between centre and 1^st palisade, a shamanic drummer was sitting. The listener was standing some 20m from the drummer and hand clappers (and looking at the backs of the hand clapping people on the right side). The direct sound of the hand clapping was not really heard (attenuated enough), but the hand clapping (its second image) came clearly from the left side.
  A thought to use this effect: Two hand clapping people would be back to back and one facing the listener. If they alternatively would make (clapping) sounds, it would appear to switch between left and right...

Press coverage

The Mitteldeutsche Zeitung, 'Das große Rauschen' (9/5/2016) published an article (with pictures) around the above acoustic research. Others were also published: Mitteldeutsche Zeitung, 'Erbauer von Goseck nutzten Klangeffekte' (11/5/2016); Magdeburger Volkstimme, 'Forscher finden steinzeitliches Tonstudio' (11/5/2016); ; Leipziger Volkszeitung, 'Goseck-Erbauer nutzten Klangeffekte (11/5/2016); Dresdner Neueste Nachrichte, 'Goseck-Erbauer nutzten Klangeffekte' (11/5/2016); Bild Halle, 'Schall-Geheimnis in Goseck entdeckt' (12/5^/2016); and Altmark Zeitung, 'Goseck-Erbauer nutzten Klangeffecte' (12/5/2016).
These news paper articles reached a readers' pool of around 1.25 million.

Pömmelte-Zackmünde

Draw-away measurements

In this picture the draw-away measurements at Pömmelte-Zackmünde are shown (they start with the letter P and using Pink sound). No specific observation from these measurements. We first had equipment problems and then there was considerable wind (low force 4), so further measurements were not possible.

Comb effect

Brown noise draw-away measurements were done from the centre of Pömmelte-Zackmünd upto 7 steps away (1 step is approximately 1m). In the below spectrogram one can see; that when walking from 7 seven steps towards the centre (0 step) and then again 7 steps away, equally spaced bands of frequencies are attenuated and the bands move up in frequency when going further away from the centre. This is related to the comb effect: the interaction between the direct sound and the reflection against the ground plane.

Similar draw-away was done when using Shamanic drum (now up to 10 steps from centre). The sound changed considerable in timbre when closer than 7 steps from the centre; more high pitches were heard which can be seen to emerge in the spectrogram.

Chirp echoes and whispering gallery effects

IRs from centre

In general we see reflections due to a ballon pop: from the near palisade; one that has gone to the far side of the palisade (~0.22 sec = ~74m); and one that went between the near and far sides of the palisade (~0.42 sec = ~149m).
Depending on the distance from the near palisade, the reflections from the near palisade arrive faster after the direct sound; reflections from the far palisade keep the same timing from the direct sound (flutter echoes).
 
Most of the energy of the reflections are in the range between 150 and 400Hz, the later the less higher frequencies with every wave (which ends with another reflection against the far palisade reaching the observer). See below spectrogram.

A balloon pop has enough powerful frequencies between 150 and 2,000Hz, so the circle's acoustic properties have changed the original spectrum of the balloon pop during each wave.
See also section Reflections in circle for an evaluation of the results.

Word clarity

No word clarity test has been performed, as one could understand each other in all areas within the 1^st palisade (and the background noise < 50 dB_SPL is lower than the speech).

Observations at Heldenberg (Sept. 2013)

There have been done acoustical measurements Heldenberg on Sept. 2^nd, 2013 [Pomberger & Mühlhans, 2015].

Source source

Balloon pop (~25cm diameter) positioned in the centre. And pricked at the tip of the balloon (directed to the sky).
Source height at ~1.3m and receiver height at ~1.7m

Chirp echoes

One could detect a chirp echo when the observer is near the palisade and the balloon pop at the centre. It is expected that the chirp echo will be more pronounced with the balloon pop is near the palisade.

IRs from centre

The setup used:

In the centre the IR shows a flutter echo with a periodicy of ~70ms, which is equivalent to the diameter of the circle. The reasoning in the article is that the centre is a kind of focal point (Brennpunkt), but perhaps better to call it a focus point. The flutter echo is also pronounced in the centre as there is a perpendicular (white path) direction of the terrain slope and thus lowest loss for a flutter echo.
If the observation point is further from the centre, the perceived flutter echo is reduced according [Pomberger & Mühlhans, 2015, page 25]:

Auf halbem Radius des Kreises treffen die Echos jeweils zeitversetzt im Abstand von etwa 35ms ein, wodurch der Impuls in der Hörwahrnehming zu einem Schallereignis verschmilzt [precedence], das Flatter echo ist dort nur messtechnisch erfassbar.

Heldenberg IRs, balloon in centre and mic at 6m [IR from Mühlhans, 2015]

Heldenberg IRs, balloon in centre and mic at 6m: tangent and perpendicular to terrain slope [IRs from Mühlhans, 2015]

Heldenberg IRs, balloon in centre and mic at 12m: perpendicular and tangent to terrain slope [IRs from Mühlhans, 2015]

Tightly-spaced double palisades as noise barrier

The noise level difference between inside and outside the tightly-spaced double palisades is around 12 dB. This is considerable higher than at Goseck (a few dBs), which has no tightly-spaced double palisades. But lower than behind the bank of Grange Circle B (19 dB). See also this section.

Lesson learnt

The following ideas and lessons learnt are perceived:

• prick the balloon at the top, to give a more balanced N-wave response of the pop's impulse [Deihl, 1968].
• if a terrain slope; measure the IRs perpendicular and tangent to the terrain slope.
• would be interesting to have a balloon pop near the palisade, to hear if chirp echo occurs at Heldenberg.

Performance test at the Grange Circle B (April 2017)

Participants

Weather data

Equipment

• Zoom H1 plus wind muffler and binaural microphones, microphone boom
  
• 2*1W CD player
  
• CD with sounds (brown, pink noise, babble, Bronze Age Horn, whispering, squealing pigs)
  
• claves, bodhran, flute
• vocal Hip-Hop sounds (by Moley Ó Súilleabháin)
• chanting by Noirin Ni Riain and Moley Ó Súilleabháin

Experience performance of different sound sources

• The vocal Hip-Hop sounds  made by Moley Ó Súilleabháin had great clarify and definition when walking from middle towards the large stone Rannach Crom Dubh.
  

Observations at re-reconstructed Heldenberg (June 2017)

Participants

Victor Reijs, Reinhard Mussik, Finn Hockauf and Elke Hockauf

Dates

There have been done acoustical measurements at the re-reconstructed Heldenberg on June 2^nd to June 4^th, 2017.

Sources

Balloon pops
Exponential Sine Sweep (ESS: 50 - 4000 Hz in 10 sec) using 2*8W RMS loudspeaker (height ~1m above ground)
Zoom H1 plus wind muffler on microphone boom (height ~1.25m above ground)

Influence on response due to terrain slope of Heldenberg

The microphone was place on a boom in the centre of Heldenberg (see below picture).

The loudspeaker was moved in intervals of ~2.2m from the centre over three paths (see the open dots in above picture). The three paths being:

• the white path, which has a terrain rise of some 0.9m over the diameter of the circle (terrain slope ~2deg)
• the yellow path, which has a terrain rise of some 0.2m over the diameter of the circle (terrain slope ~0.5deg)
• the red path, which is more or less using the steepest terrain slope in the circle: a terrain rise of some 1.8m over the diameter of the circle (terrain slope ~4.5deg)

• The top row provides the two times four IRs on the White path: the Left side and on the Right side (referenced with ones back to the lowest entrance)
• The middle row provides the two times four IRs on the Yellow path: the Left side and on the Right side (referenced with ones back to the lowest entrance)
• The bottom row provides the two times four IRs on the red path: the UPper side (near the highest entrance) and on the DoWn side (near the lowest entrance)
  

Producing clave sounds at different heights

Comparing Heldenberg of 2013 and 2017

A balloon burst has been used to compare the T20 properties (remember that T20 is not really defined in a non diffusing environment) in 2013 (distance 6m by Mühlhans) and 2017 (distance ~4.4m  by Reijs):

Lessons implemented

The following ideas and lessons have been implemented:

• balloons were pricked at the top and the top directed towards the recorder. This to give a more balanced N-wave response of the pop's impulse [Deihl, 1968].
• the IRs perpendicular and tangent to the terrain slope have been extensively measured (see above).
• balloon pops have been done near the palisade, no chirp echos occurred at Heldenberg. Perhaps due to the smaller circle diameter and/or the peeling bark.

Evaluation

Flutter echoes in a circle

If we have a circle and the impulse sound is in the centre, the resulting peaks in the sound level change with the distance of the observer from the centre:

• the direct sound arrives (define this at t=0 and a normalised Sound level 70dB)
  
• then the 1^st reflection, due to direct sound, returning from the nearest palisade side
• then the 2^nd reflection returning from the furthest palisade side
• then the 3^rd reflection, due to 2^nd reflection, returning from the nearest palisade side
• then the 4^th reflection, due to 3^rd reflection, returning the furthest palisade side
• further reflections seem to be too low in power to come normally above the noise.

In the below stylised and animated graph we see these peaks changing while the observer distance is increased in a single palisade ring. For comparison: A impulse sound at 27m from 1^st palisade with the observer at distance of 10m from impulse response: the measured impulse response at 10m at Goseck has been overlayed. This are clear examples of flutter echoes.
One sees more flutter echo peaks in the Goseck impulse response due to the 2^nd palisade reflections (8m or 23msec later).

Goseck IR, balloon in centre and mic at a several distance (diameter 52m)

The direct sound (per definition at 0msec) and the reflections (2^nd and 4^th) returning from the furthest palisade side are fixed in time (independent of the distance of the observer from the centre) [red lines]. The reflections (1^st and 3^rd) due to the nearest palisade side change in time (dependent of the distance of the observer from the centre) [purple lines].
The height of the lines depends on the total length of the sound path (and the number of reflections against furthest or nearest palisade).

When the reflections come into the 10 to 40msec range, timbre changes can happen. Below10msec preference can be lowered (see section on Discrete reflections).

This also maps well on theoretical flutter echoes in Heldenberg (mic at 6m):

Heldenberg IR, balloon in centre and mic at 6m (diameter 24m) [IR from Mühlhans, 2015]

So it looks that Goseck and Heldenberg (in tangent direction of terrain slope) behave similarly (even the loss due to a reflection against a palisade, is in both cases ~8dB).

A flutter echo can give an impression of a larger room then one expects, see this perception when flutter echo happens in a room. This is perhaps also the reason why in open air moment one gets the impression of a larger enclosed space when a flutter echoes happens.

Transparency of the palisades

The palisades of the circles have different area of gaps between the timber posts. Below are pictures of front view (and almost the full height of posts is included) of the 1^st palisades.

Site [picture taker]	Palisade picture	Straightness &bark	Terrain slope (deg)	Gap area [%]
Goseck [Reijs, 2016]		straight & no bark	~0	16
Pömmelte-Zackmünde [Reijs, 2016]		straight-ish & no bark	~0	15
Mansfeld [Meinike, 2005]		crooked &   many (peeling) bark	~2.5	11
Quenstedt (original of Mansfeld)	NA	NA	~0	NA
Heldenberg 2013 [J. Mühlhans, 2015]		straight & mainly bark	~4.5	~1 (as seen from centre)
Heldenberg 2017 [Reijs, 2017]		straight & mainly bark	~4.5	4
Schletz (original of Heldenberg)	NA	NA	~6	NA

The reflection energy of these palisades does not differ that much (max difference 0.4 dB for middle [(353 - 1414Hz] and low [88 - 353Hz] frequencies [Ahnert, page 37], which would not really be audible, as it is below the JND of 1dB). Diffraction might be different due to large variation of straightness of the posts.
Transparency of the palisades also will have an influence on the transmission of sound behind the palisade, the more dense the less energy.

Perceptions and stories

• Goseck:
  
  • From a visitor to Goseck [Reinhard, pers. comm. 2016]:
    When we were first in Goseck we could not recognise any sound effect. The cause was that we did not know that there could be a special sound. Only after the guided tour we could recognise these effects.
    The spatial impressions were still noticeable when there were some 100 people within the Goseck circle during the winter solstice festivities.
  • From an anonymous visitor during a windy day (anonymous, pers. comm. 2017):
    When I was in Goseck the last time it was very windy and the special acoustics, that many people told me about, was untraceable. 

• An observation when Pömmelte-Zackmünde was reconstructed [Spatzier, pers. comm. 2016]:

  "Rebuilding the Pömmelte site we started erecting the palisade. Even when just 1/2 to 3/4 was finished the sound in the centre was really clear and enhanced. After we erected the two inner post rings with lintels this enhancing effect is considerably reduced, although still perceptible.
  In comparison the acoustics of Goseck is better - comparable to the palisade reconstruction phase in Pömmelte, likely even a little better. This obviously results from Goseck consisting of two palisade without inner posts/post rings, that is an empty inner space, and because it is smaller than Pömmelte."
  

  

• Acoustical experiences [Meinike, pers. comm. 2016] at the reconstructed Quenstedt at Mansfeld, indicates that there were no acoustical effects perceived. She thinks this is possibly due to the sloping terrain, the gaps and/or the bark.
  
  
  
  1^st palisade at Mansfeld (photo by M. Meinike, May 3^rd, 2005)
  
  
  It is as of yet not possible to explain this lack of acoustical experience at Mansfeld, possible reason(s) could be:
  
• the terrain might be somewhat sloping (~2.5 deg: according tot Google Earth: 2m height difference within 1st palisade ring). It might be that this is the reason for a reduction of echoes, but still not sure.
  
• the Mansfeld posts are more crooked than Goseck (more diffraction towards different planes);
• many posts in Mansfeld still have (peeling) bark (more absorption, more scattering, more diffraction; although Heldenberg has posts with [non peeling] bark and a very good echo);
  
• the gaps between Mansfeld posts are somewhat smaller (the reflection though should not that different);
  
• the wind condition at Mansfeld could be different (not really as on May 3^rd, 2005 wind was low force 3). Wind can have an effect when it is more than 5m/sec (high force 3) and when distance between source and observer is much more than 30m [Albert, 2004, page 7];
  
• something else?
  

A video with sound made at the former site might help also, if available please inform us.

• Heldenberg has the posts in such a way that a gap is covered by another post (slightly more inside): tightly-spaced double palisades, when looking sideways one though can see the gaps  [Pomberger, 2015, page 23]. Heldenberg is on a terrain slope of around 4.5 deg.
  
  
  
  Posts placement at Heldenberg (J. Mühlhans)
  
  
• The Echo in the centre is really amazing!, according to Zotti [pers. comm. 2016].
  
• Goats roaming around the area of Heldenberg would pass a lot of time inside the timber circle (Aigner, 2017).
• The first time walking into Heldenberg, did not give me the same feel (sacred) or spatial impression as with Goseck or Pömmelte-Zackmünde. The reflections/echos were less obvious (Reijs, 2017).
• Goseck is isolated in the landscape and you have to go a long "processional way". All this gives a more sacred character to Goseck than Heldenberg. When we first entered the Goseck circle (we were alone), we just had a "scary" feeling and left it quickly. Maybe, the fact that Goseck was rebuilt on its original place (a place which was discovered or chosen as a sacred place for some reason millennia ago) was one important cause for this feeling? The considerable terrain slope at Heldenberg detracts the impression of being in a closed space. Furthermore the sloppy built palisade, the short - only indicative - ditch and wall, and the integration into a "theme park" (Neolithic village, falconry, museum ...) gave more the feeling of being in a playground than in a sacred enclosure. (Mussik&Hockauf, 2017)

Influence of a terrain slope

Would this have a relation with the acoustic aspects of tangent and perpendicular to the terrain slope? And would the same yield for German circles (which are from a different region/culture)? No real answer available(yet), but there might be a thesis in 2017/2018 that could provide the basic data for this evaluation (Michel, possibly 2017).
An initial comparison (based on tentative data gotten from Michel on position of the German circles) between the cumulative distributions of the terrain slope seen (dashed line) in Austrian and German circle can be see below (the continuous line would happen if both cumulative distributions would be the same):

There might indeed be a difference in terrain slope distribution (although it still needs to be investigated if the general distribution of terrain slope for the larger region in Austrian and German are different). It looks that more German circles than Austrian circle are build on flat terrain. The spread of the terrain slope is for both regions similar (maximum 7.5 deg).

Noise barriers

Noise barriers cause the receiver to experience a shadow effect, and sound reaches the receiver through diffraction.
There are two types of noise barriers that can be present at timber circles:

• timber palisade
• bank
  

For illustration purposes only

Here is a handy 2D interactive tool and a 1D from tool: source at 0m, barrier at 12m and receiver at 15m (more or less Heldenberg). One can derive at Heldenberg that the sound level before the barrier and after the barrier has a difference of around 15 dB (measured is 12 dB). For Grange Circle B, with its 9m wide bank (and stone circle at 1.5m) the tool gives around 10 dB, while measured is was around 19 dB.

The above information is for a noise barrier wall (a tightly spaced palisade), a bank's attenuation would be a few dB lower [BC Ministry of Transportation and Highways, 1997, page S-1, S-2].

Draw away measurements (loudness)

Equipment

An SPL meter was used at A-Weighting and Slow response.
The SPL meter was held at around 1m height and against the body of the measurer. The microphone directed to the speaker. The speakers were at around 75cm high. Everywhere the ground was covered with (mowed) grass, although at Pömmelte there was some wild plant growth (upto some 30cm).

Measurements

Naming convention of the measurements:

• First letter is the location: Home garden (H), Goseck (G), Pömmelte (P) and Field (F).
  
• Next two (if present) long what axis has been measured: Long Axis (LA) or Short Axis (SA) of the circle
• Next letter (if present) shows if walking towards Palisade (P) or Entrance (E)
  
• In most instances Pink noise was used, but sometimes Brown noise, Whisper or (Bronze Age) Horn.
  
• The distance in the below graph is measured from the centre of Goseck and Pömmelte or a reference point in the Home garden and Field.
• All graphs were normalised to the same dB_SPL at 10m (70.5dB_SPL).
  

The differences between the draw-away curves are not that large. The following evaluations can be made:

• The background noise was smaller than 50dB_SPL.
  
• All graphs are quite close to each other (except in case there was influence due to wind). There is a small difference between the (normalised) sound pressure levels measured with different sound sources, but all this might be within the standard deviation.
• Reproducibility of a measurement was around 1.5dB (1 sigma)
• The wind and/or speed direction fluctuations (irregular amplitude fluctuations: IAF [Brown, 2003, page 121]) caused fluctuations in the measured power (up to plus/minus 4 B [say 2 sigma]).
• The errors for F/Pink and PSAP/Pink (depicted as vertical error bars) are due to reproducibility and wind noise&direction fluctuations (each might have comparable influence).
• The observed average standard deviation between all measurement at a certain distance was 1.5 B (1 sigma).
• The average level (using a SPL meter of Radioshack) change looks to be around -6.7dB/dd (doubled distance), this is more than in a hemispherical environment, where it is around -4 to -5 dB/dd or free space -6dB/dd [Everest, 2001, page 88]. These level changes are for omni directional point source, a loudspeaker is directional and not point like. In open space: a speaker's near field (smaller than 1m) would change with -12dB/dd and the far field (larger than 1m) would be -6dB/dd [Crocker, 1998, page 8]. So this does not explain the sound level change of around -5.5 to -8dB/dd in open field compared to the hemispherical environment of -4 to -5dB/dd.
  
  
  

Lessens leanrt

• Use C-Weighting in the future (is closer to relative loudness perception) [Toole, 2013, page 346-348]

Reverberation time

In most cases, with the sound source in the centre and the mic at 10m from the centre: the reverberation time (T30; derived through the Open AIR web page [Murphy]) and ¿Volume? (defined by the height of the palisade/stones, by assuming the circle is closed with a [absorbent] plane) of the circles were derived:

• Pömmelte-Zackmünde (10m in circle): T30=0.70sec, ¿Volume?=9500m³
  
• Goseck (10m in circle): T30=0.57sec, ¿Volume?=4400m³
• Grange Circle B (10m to Dubh): T30=0.46sec, ¿Volume?=2400m³
• Heldenberg (2.2m to centre): T20=0.41sec, ¿Volume?=950m³
• Dromagorteen stone circle (10m to portal): T30=0.3sec, ¿Volume?=65m³

The optimum reverberation time [Chan; fesi, 2007, page 57; Everest, page 154; line graphs: Reijs, 2016]

The line graphs are based on [Stephens, 1950]:
K = T₆₀/( 0.0118*Volume^1/3 + 0.1070)
with:    Volume in m³
            T₆₀ = optimum 60dB reverberation time [sec] at 500 Hz
            K = (optimum) suitability: 4 for speech, 5 for orchestra, 6 for chorus

It is important to realise that the T30's (estimator of RT₆₀) relation with ¿Volume? might not be really applicable for this type of open air monuments, which does not really have a real Volume (see also [Pomberger & Mühlhans, 2015, page 25]):

• A relation between the (optimum) reverberation time and Volume is only defined in spaces where there is a diffused sound field, which is certainly not the case for these open space measurements.
  
• The ¿Volume? is based on the assumption that it is a closed space, while the 'ceiling' of the space is fully open.
  
• Reverberation randomises time [Blesser, 2007, page 246-247]. In the case of the timbre/stone circles there is almost no reverberation due to a random reflection process. There is some, but most energy  is due to discrete flutter echoes.
  

If the above optimum reverberation time-¿Volume? relation is though relevant in some way; studio (speech); cinemas; television studios; openplan offices; and pop and rock concerts are 'applicable' for the measured T30.

But there were forests in the past and these can have reverberation times between 1sec and 2sec [Shelly, 2013; Grzinich, 2015], so larger then perceived in timber circles (T30 of up to 0.7sec). Is the timber circle a symbolic version of the forest, to replicate the natural reflections and/or supernatural voices?
Also interesting that Spengler says that 'The character of the Faustian cathedrals is that of a forest' and that the 'Cathedral and organ form a symbolic unity like temple and statue'. Of course organs meld very with a cathedral due to the long reverberation times: 'the organ, which roars deep and high through our churches in tones that, [...] seem to know neither limit nor restraint' [Spengler, page 199-200; Schafer, 1977, page 23].

But caution is warranted in interpreting the optimum reverberation time! Clarity might be a much better measure for open air monuments.

Early Decay Time

It is interesting to note that not T30, but the EDT (Early Decay Time of the first 10 dB) is better related to the perceived reverberance [Bradley, 2011, page 714]. The EDT for the timbre/stone circles is broadly around 0.15sec, while the T30 is broadly around 0.6sec. An EDT shorter than the T30 would indicate an improved speech and music clarity [Brown, 2010]. The EDT of Grange Circle B is higher (0.5sec) due to the large reflection from Rannach Crom Dubh, here the C₅₀ is indeed low (-3dB). Clarity can of course also be measured with C₅₀ and C₈₀.

Thus the reverberations times (T30~0.6sec) are close to a present day living rooms [Evert, 2001, Figure 7-17], while the EDT (~0.15sec)  is close to a semi-anechoic space. Are timber circle perceive as larger enclosed space, because there is a disconnect between the perceived visible size of the open space circle [Bregman, 1994, page 307,312] and the perceived reverberation (the EDT or T30) of a dead room? Could this be similar to the Moon illusion effect [Ross, 2002]?
Are the multiple flutter echoes generating envelopment and thus causing this large spacial impression?  

C₅₀ and C₈₀

In most cases, with the sound source in the centre and the mic at 10m from the centre: the C₅₀ [weighted over 500, 1000, 2000 and 4000 Hz] and C₈₀ [averaged over 500, 1000 and 2000 Hz] are derived through the Open AIR web page. Here they are for the circles:

• Pömmelte-Zackmünde (10m in circle): C₅₀: 19.4dB, C₈₀: 19.1dB
• Goseck (10m in circle): C₅₀: 10.9dB, C₈₀: 9.3dB
• Grange Circle B (10m to Dubh): C₅₀: -3.1dB, C₈₀: 13.0dB
• Dromagorteen stone circle (10m to portal): C₅₀: 26.7dB, C₈₀: 29.4dB
  

Discrete reflections

The reflections can perhaps be evaluated using below graph that looks at a single reflection that has a certain delay after the first (in)direct sound (the provided lines are based on Figure 6.6, 6.14 and 6.15 of Toole, which relates to an anechoic space). The measurements at Goseck and Pömmelte-Zackmünde are of course not in an anechoic space, but in an semi-anechoic space, but it is expected that the behavior is not that far off. Furthermore the comparison is using castanet sounds, as Toole does not have other sound sources closer to Balloons (he investigated: clicks without any reverberation, Mozart music and speech).

If there is a question mark before a label in the legend of the above picture, the line has been derived by the web-author through analogy of other acoustic. If the first reflection is above the dotted line, that reflection would be perceived as a second image by the listener (e.g. the red squares). If the first reflection is between the dashed and dotted line, then the location of the direct sound will be spread (ASW; e.g. yellow squares with red X). Between continuous line and dashed line Precedence (Haas effect) occurs for the direct sound location (e.g. yellow squares with blue *). And if the first reflection is below the continuous line, the reflection is not detected. Low Preference happens when the first reflections is earlier than ~0.01sec, while for first reflections between ~0.01 and ~0.04sec timbre changes (comb effect) can occur.

If more than one reflection happens after the direct sound, it looks that the latest reflection determines if one experiences a second image.
For instance in Shamanic drum recordings, the experiences anywhere in the circle is of a second image, because the reflection against the far side of 1^st palisade at Pömmelte-Zackmünde is always ~0.22 sec after the direct sound (blue diamonds). If one removes the reflection due to this far side 1^st palisade reflection, the remaining reflection (e.g. some of the red diamonds in above picture) determines more or less if there is a second (third?) image or not.
For music (red dotted line) any reflection would not really provide a second image. For speech (green dotted line), these remaining reflections would  perhaps provide some second imagine might when one is near the sound source, but in that case the second image sound level might be quite small compared to direct sound.
This can be seen also in the below picture, where one can see from 0.08sec the extrapolated echo (dotted) lines for different sound types (somewhat following the theory of section 'Large spaces' of Everest, 2001, page 359). The diamond symbols are the reflection sound levels as measured from Shamanic drum sounds in Pömmelte-Zackmünde, while the delay is related to the receiver distance from the 1^st palisade.

So percussion instruments (diamond symbols well above black dotted line) have an advantage in Goseck and  Pömmelte-Zackmünde. This can be linked perhaps to the following quote of HaYa RomanowSky on Goseck:

Wonderfull place in the world, my best place for drumming and shamanic circle for freedom and peace with hayayoga.

Simulating a timber circle on sloping terrain

To determine the effect of sloping terrain, a model of a timber circle has been build in SketchUp Make (radius ~52m, sound source 1m meter from centre and observer 10m from sound source). Here is a picture of such a timber circle on slopping terrain:

Auralisation

Some auralisations have been made of a drum sound (recorded in anechoic room; available on Open AIR library [Murphy]), which has been convolved (using Aurora Plug-in for Audacity 2.0.0) with the IRs derived from our measurements:
<below auralisations are somewhat harsh [used too short IRs and sometimes there was even overload of mic]; new auralisations will be included soon>

• Pömmelte-Zackmünde
• 0m from 1^st palisade (no chirp echo)
• 10m from centre
  
• Goseck
• 0m from 1^st palisade (gives an idea of chirp echo)
• 10m from centre
• 39m from centre on bank
• 40m from centre, 10m from entrance on outside
  
• Grange Circle B
• 10m from centre in direction of Rannach Crom Dubh (the 1^st reflection is loader than the direct sound)
• 10m from centre in direction of passage
• 10m from centre in direction of stones shaping a V
• Dromagorteen stone circle
  
• 0m from recumbent
• 10m from portal in direction of recumbent
• Heldenberg
• 6m from the centre

Summary

Some aspects to be reconsidered in the future for future acoustic measurements at timber/stone circles:

• shadowing is experienced, when walking between the palisades and openings.
  Pink noise source in middle, observer between palisades.
  Only investigated in Goseck.
• In Grange Circle B we saw a difference inside and outside the circle of some 19dB. A similar effect might have happen if Goseck had a bank of some 3m (according to archaeology). Heldenberg's tightly-spaced double palisades also has an attenuation of some 12dB. So attenuation would be quite high indeed as described in noise barrier section.
  
• chirp echo at 1^st and at 2^nd palisades at Goseck.
  Impulse source and observer at same place near the palisade (although it is also heard when observer is further away).
  Not at Pömmelte-Zackmünde and Heldenberg: due to more irregular positioning of the posts. And not at Grange Circle B: too irregular standing stone positioning. Not investigated at Dromagorteen.
  
• Preference effect (reflections smaller than 2m from palisade), only experienced when making sound at observer location.
  Hand clapping was used.
  Only investigated in Goseck.
• discrete reflections due to 1^st palisade, both at Goseck and Pömmelte-Zackmünde. Similarly at Dromagorteen and Grange Circle B (due to standing stones). This provides apparent source widening (ASW) and envelopment (LEV).
  Impulse sounds (balloons) were used to record this.
• discrete reflections due to the two palisades at Goseck. This is unique to Goseck and makes Goseck acoustically richer than Pömmelte-Zackmünde. This provides additional apparent source widening (ASW) and envelopment (LEV).
  Impulse sounds (balloons) were used to record this.
• experience of being in a confined space (due to reflections) instead of open field; even though that the draw-off measurements are comparable.
  Mainly pink noise was used to record this.
  In all investigated sites, but not really present at Dromagorteen (too small?).
• no whispering gallery effects in all investigated sites (used humans whispering who could not hear each other when standing opposite sides of 1^st palisades).
• Vocal Hip-Hop sounds had a high clarity in the circle and specifically at the large stone Rannach Crom Dubh, this was not really experienced in Heldenberg.
  
• It is possible that the posts in former times were: different heights; more crooked; more gaps; and/or remaining/peeling bark; thus chancing the reflection and increasing diffraction and scattering to different directions
• The flutter echos can be observed when using percussion instruments, but not when using orchestra or choral sounds. This maps on the perception seen by [Pomberger & Mühlhans, 2015, page 26].
• A flutter echo can give an impression of a larger room then one expects, see this perception when flutter echo happens in a room. This is perhaps is also the reason why in open air moment one gets the impression of a larger enclosed space when a flutter echoes happens. Neolithic humans have no experience of large closed sites, but they still could have experienced the timber circle as a difference place due to its acoustics (and not only because it gives a visual boundary).
• The experience one gets in a timber circle is mainly due to the reflections (and echos) within a certain time frame (nearer than 40ms and longer than that). They give your the idea of envelopment and source width. One could explain the experience difference due to the resulting from flutter echos (between the two palisade sides) and other echos/reflections.

Lessons learned

• synchronise the clocks and watches on all devices
• note properly where the (binaural) microphone(s) is pointing to
• put phones in flight mode or off
  
• check if recorder is recording
• use wind muffler or cotton wool
• record impulse responses when wind is lowest and when the bird don't sing yet (before sunrise). Interesting to learn that birds sing in the morning (dawn chorus) due to consistency of the acoustic properties [Brown, 2003]:
  

  The average quality of dawn transmissions was not significantly greater than those of midday, in either habitat. Rather, the clear advantage gained by transmitting signals at dawn was a reduction in the variability in transmission quality.
  

• make sure there is 2 to 5 seconds of silence after the IR (the longer the RT₆₀, the more is needed).
  
• walk around and listen for: (chirp/flutter) echoes; reflections, timbre-, pressure-, loudness- and/or feeling-changes. Record the spot, time, what experienced, etc.

Speculative workflow how aural aspects emerge in buildings

1. Social needs determines size of audience
2. Building technology limits size & geometry
3. Size, materials, & shape determine acoustics
4. Once constructed, listeners adapt to space
5. Experience enhances subjective quality
6. Future spaces replicate earlier spaces
7. Incremental evolution by trial and error [Blesser, 2011, slide 23]
   

Opportunities for the future

References

Terminology

Terminology that is in bold is used on this web page (and underscored is terminology good to know/explain in general). It is important to state that no good or bad should be linked to the below terminology. Good and bad are quite cultural depending [Blesser, 2007], and as we don't know the behavior of the culture involved, no value statement is linked to any of the below terms.

Abbreviations

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