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Primer: Acoustic Characteristics Of Live Sound Reinforcement
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Echo occurs when a reflected sound is delayed long enough (by a distant reflective surface) to be heard by the listener as a distinct repetition of the direct sound.

Reverberation consists of many reflections of a sound, maintaining the sound in a reflective space for a time even after the direct sound has stopped.

Standing waves in a room occur for certain frequencies related to the distance between parallel walls. The original sound and the reflected sound will begin to reinforce each other when the distance between two opposite walls is equal to a multiple of half the wavelength of the sound.

This happens primarily at low frequencies due to their longer wavelengths and relatively high energy.

2. Absorption - Some materials absorb sound rather than reflect it. Again, the efficiency of absorption is dependent on the wavelength. Thin absorbers like carpet and acoustic ceiling tiles can affect high frequencies only, while thick absorbers such as drapes, padded furniture and specially designed bass traps are required to attenuate low frequencies.

Reverberation in a room can be controlled by adding absorption: the more absorption the less reverberation. Clothed humans absorb mid and high frequencies well, so the presence or absence of an audience has a significant effect on the sound in an otherwise reverberant venue.

3. Diffraction - A sound wave will typically bend around obstacles in its path which are smaller than its wavelength. Because a low frequency sound wave is much longer than a high frequency wave, low frequencies will bend around objects that high frequencies cannot.

The effect is that high frequencies tend to have a higher directivity and are more easily blocked while low frequencies are essentially omnidirectional. In sound reinforcement, it is difficult to get good directional control at low frequencies for both microphones and loudspeakers.

4. Refraction - The bending of a sound wave as it passes through some change in the density of the environment. This effect is primarily noticeable outdoors at large distances from loudspeakers due to atmospheric effects such as wind or temperature gradients. The sound will appear to bend in a certain direction due to these effects.

Direct Vs Ambient Sound
A very important property of direct sound is that it becomes weaker as it travels away from the sound source. The amount of change is controlled by the inverse-square law which states that the level change is inversely proportional to the square of the distance change. When the distance from a sound source doubles, the sound level decreases by 6dB. This is a noticeable decrease.

For example, if the sound from a guitar amplifier is 100 dB SPL at 1 ft. from the cabinet it will be 94 dB at 2 ft., 88 dB at 4 ft., 82 dB at 8 ft., etc. Conversely, when the distance is cut in half the sound level increases by 6 dB: It will be 106 dB at 6 inches and 112 dB at 3 inches!

On the other hand, the ambient sound in a room is at nearly the same level throughout the room. This is because the ambient sound has been reflected many times within the room until it is essentially nondirectional. Reverberation is an example of non-directional sound.

For this reason the ambient sound of the room will become increasingly apparent as a microphone is placed further away from the direct sound source. In every room, there is a distance (measured from the sound source) where the direct sound and the reflected (or reverberant) sound become equal in intensity.

In acoustics, this is known as the Critical Distance. If a microphone is placed at the Critical Distance or farther, the sound quality picked up may be very poor. This sound is often described as “echoey”, reverberant, or “bottom of the barrel”. The reflected sound overlaps and blurs the direct sound.

Critical distance may be estimated by listening to a sound source at a very short distance, then moving away until the sound level no longer decreases but seems to be constant. That distance is critical distance.

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