Digital Sound & Music: Concepts, Applications, & Science, Chapter 4, last updated 6/25/2013
Figure 4.32 Comb filtering frequency response of two sound sources one millisecond apart
We can try a similar experiment to try to hear the phenomenon of comb
filtering using just noise as our sound source. Recall that noise consists of
random combinations of sound frequencies, usually sound that is not wanted as
part of a signal. Two types of noise that a sound processing or analysis system
can generate artificially are white noise and pink noise (and there are others).
In white noise, there’s an approximately equal amount of each of the frequency
components across the range of frequencies within the signal. In pink noise,
there’s an approximately equal amount of the frequencies in each octave of
frequencies. (Octaves, as defined in Chapter 3, are spaced such that the beginning frequency of
one octave is ½ the beginning frequency of the next octave. Although each octave is twice as
wide as the previous one in the distance between its upper and lower frequencies octaves
sound like they are about the same width to human hearing.) The learning supplements to this
chapter include a demo of comb filtering using white and pink noise.
Comb filtering in the air is very audible, but it is also very inconsistent. In a comb-filtered
environment of sound, if you move your head just slightly to the right or left, you find that the
timing difference between the two sounds arriving at your ear changes. With a change in timing
comes a change in phase differences per frequency, resulting in comb filtering of some
frequencies but not others. Add to this the fact that the source sound is constantly changing, and,
all things considered, comb filtering in the air becomes something that is very difficult to control.
One way to tackle comb filtering in the air is to increase the delay between the two sound
sources. This may seem counter-intuitive since the difference in time is what caused this problem
in the first place. However, a larger delay results in comb filtering that starts at lower
frequencies, and as you move up the frequency scale, the cancellations and reinforcements get
close enough together that they happen within critical bands. The sum of cancellations and
reinforcements within a critical band essentially results in the same overall amplitude as would
have been there had there been no comb filtering. Since all frequencies within a critical band are
perceived as the same frequency, your brain glosses over the anomalies, and you end up not
noticing the destructive interference. (This is a oversimplification of the complex perceptual
influence of critical bands, but it gives you a basic understanding for our purposes.) In most
cases, once you get a timing difference that is larger than five milliseconds on a complex sound
that is constantly changing, the comb filtering in the air is not heard anymore. We explain this
point mathematically in Section 3.
The other strategy to fix comb filtering is to simply prevent identical sound waves from
interacting. In a perfect world, loudspeakers would have shutter cuts that would let you put the
sound into a confined portion of the room. This way the coverage pattern for each loudspeaker
Max Demo:
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