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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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