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Sound effects in a room

• Influence of the ear
The ear does have some averaging function over a third of an octave (an octave is the frequency range between f and 2*f)
Also keep in mind that due to the equal-loudness contour, the ear is not always the proper instrument to determine the real sound-pressure level.

• Room dimensions
These basically determine the normal modes in the room. These normal modes should be measured with the speaker and microphone in opposite three wall corners.
• Number of the normal modes, especially in low frequency range
Coloration can happen in a small room when (Gilford rule):
• axial modes are at almost the same frequency or
• when there is a major gap (more than 20 Hz) between the axial modes.
Both issues change the sound impression of a room.
Coloration makes speech or music different; part of the sound spectrum is like in open air and the rest is as in an enclosed space.

• Gilford rule on modal coloration (Everst [2001], page 351)
• Modal coloration most of the time could happen between 80 and 300 Hz
  
• In neolithic times humans were smaller
  
• human pitch range  ~ 6% higher: 115 - 140 Hz
• Monte Carlo analysis with above pitch range and chamber size: 1.5 - 6 [m], some 5500 runs
• Gilford-rules on modal coloration:
• major gap (>20 Hz) between axial modes: ~20%
• two axial modes in 5 Hz range: ~15%
• ~90% has at least one axial mode is in the human pitch range!?
  
• Thus modal coloration occurs by chance in at least 30% of cases
  
• Absorption coefficient
The absorption coefficient of the walls does not change the normal mode's position, but makes its energy lower/higher. The absorption coefficient is dependent on frequency.
• Modal bandwidth [Hz]
The bandwidth of the normal modes is influenced by the absorption of the walls/air. The more absorption the larger the modal bandwidth is.
• Reverberation time [sec] or Decay rate for low frequencies [dB/sec]
The reverberation time (RT) is defined when a uniform and diffused sound distribution exists in a space: so at 'high' frequencies where a lot of normal modes exist (normally measured as a property of an octave).  Not really defined for low frequencies (when the normal modes are still quite scarce, thus below F₂), where the Decay rate would be more appropriate (normally measured at the frequency of the normal mode; for instance in a 3D waterfall frequency response graph).
But in most literature the reverberation time is used in both instances. In case of looking at normal modes:
RT₆₀ = 2.2/'Modal bandwidth'

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Calculator
Modal bandwidth: [Hz]

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reverberation time (RT₆₀): [sec]

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Depending on the reverberation time, the sounds reverberates longer or shorter in the space.
If reverberation time is different from the ideal situation, a room will sound different than expected:
Ideal values of RT₆₀ (from Everst [2001], page 423) Longer reverberation times can make speech difficult to understand (e.g. the end of a word tends to be masked by the decaying volume of the start of the word).

Non uniform reverberation time

If non uniform reverberation time/decay rate within the frequency range is present, certain frequencies can exist longer than others: like a tendency of ringing tones or booms.

Psychological effect
Anxiety, etc. can happen with low frequency sound (infrasound)


Noticeable echo's

These are normally not present in small room like, megalithic monuments, but can happen in caves, etc.
In that case one can experience discrete echo's.

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