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Basic Knowledge About Room Acoustics - Part II

Direct vs. Ambient

Direct sound becomes weaker as it travels away from the sound source at a rate controlled by the inverse square law. When the distance from a sound source doubles, the sound level decreases by 6dB.

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 non-directional. Reverberation is an example of non-directional sound.

The ambient sound in a room becomes increasingly apparent as a microphone is placed further away from the direct sound source. The amount of direct sound relative to ambient sound can be controlled by the distance of the microphone from the sound source and to a lesser degree by the polar pattern of the microphone.

If the microphone is placed beyond a certain distance from the sound source the ambient sound will begin to dominate the recording and the desired balance may not be possible to achieve. This is known as the critical distance and becomes shorter as the ambient noise and reverberation increases forcing a closer placement of the microphone to the source.

Phase Relationships

The phase of a single frequency sound wave is always described relative to the starting point of the wave or 0??. The pressure change is zero at this point. The peak of the high pressure zone is at 90??, and the pressure change falls to zero again at 180??. The peak of the low pressure zone is at 270?? and the pressure change rises to zero at 360?? for the start of the next cycle.

Two identical sound waves starting at the same point in time are called in-phase and will sum together creating a single wave with double the amplitude but otherwise identical to the original sound wave. Two identical sound waves with one starting point occurring at the 180?? point of the other wave are said to be out of phase and the two waves will cancel each other out completely.

Most sound waves are not a single frequency but are made up of many frequencies. When identical multiple-frequency sound waves combine there are three possibilities for the resulting wave:

A doubling of the amplitude at all frequencies if the waves are in-phase

A complete cancellation at all frequencies if the waves are 180?? out-of-phase

Partial reinforcement or partial cancellation at various frequencies if the waves have an intermediate phase relationship.

The latter case is the most likely and the audible result is a seriously degraded frequency response called comb filtering. The pattern of peaks and dips resembles the teeth of a comb and the depth and location of these notches depends on the degree of phase shift.  

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