Room Acoustics

Outdoor sound

In the open, sound spreads out in all directions- if it is unobstructed the sound intensity falls off as the distance from the source increases. (Remember that intensity is inversely proportional to the square of the distance from the source -- so that doubling the distance reduces intensity by a factor of four -- in tersm of sound intensity reduces by 6 dB. In fact we almost never have a situation where there are no absorbers of sound -- the ground, the grass, people etc are all absorbers of sound -- in the real world sound intensity may fall off 12 dB for each doubling of distance.

Enclosed areas

Most musical performances are given in enclosed places, and acousticians must design performance spaces so that the listening enviroment will be optimum.

When a sound wave strikes a surface and reflects, a certain amound of the intensity is lost (transferred into another form of energy (heat)) -- the amount lost depends on the material and its absorption quality.

The first sound we hear is the direct sound, followed by the first reflection , then more reflections, and reflections of reflections, until we have a diffuse mixture of sound waves filling the whole room, called reverberant sound.

If we had a room with non-absorbing walls (and we did not absorb any sound), the sound source would supply a certain amount of acoustic energy to the room, and this energy would fill the room and remain their for ever. In reality all rooms have a reverberation time:

Reverberation time is the time it takes for the original sound to be reduced to 1/1 millionth (10-6) of its original intensity.

Optimizing Reverb Time

Since reverb affects both speech and music -- it forces sounds to become mixed with the following sounds, and can create a muddy effect if reverberation time is too long. On the other hand, it is important to have enough reverberation to create a satisfactory sound intensity -- anyone who has been in an anechoic chamber (no reverb) very strange -- very muffled. The optimum reverberation is a compromise between the two extremes, and depends on what the room will be used for:

The average reverb time for a large autitorium is 1.5 - 2.0 sec.

Absorptions Coefficients:

All materials have an absorption coefficient -- this is the amount that they will absorb. From these figures, it is possible to determine the reverberation time of a room

• Marble = .01
• Rug = .39
• Perforated cane fiber = .70

Note also that coefficients change with the frequency

To determine and modify the reverberation time we also have to consider the seats, and the people. To accomodate the fact that you sometimes have audience, and sometimes not, most auditorium designers try to design seats that absorb the same amount as a person -- thus it doesn't matter whether the hall is full or empty.

Other Acoustic Considerations

1. another factor is uniformity of sound distribution (especially in corners or under balconies)
2. if walls are parallell, and good reflectors, a repeated echo (flutter) may occur -- to avoid this, either the walls can be angled, or the walls may be covered with absorbing material.
3. there is a factor called Intimacy that is important for our perception of sound. This is the time between the arrival of the direct sound and the first reflection -- this interval should not be more than 20 milliseconds (approximately 20 feet further in distance) This is the reason you sometimes see panels suspended from ceilings -- they create a false ceiling and minimize the time delay.

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