Digital Sound & Music: Concepts, Applications, & Science, Chapter 4, last updated 6/25/2013
Given a delay t between two identical copies of a sound,
then the frequencies that will be combed out are
Equation 4.21 Comb filtering
For a 20 foot separation in distance, which creates a delay of 0.02 s, the combed
frequencies are 25 Hz, 50 Hz, 75 Hz, and so forth.
In Section 2, we made the point that comb filtering in the air can be
handled by increasing the delay between the two sound sources. A 40 foot
distance between two identical sound sources results in a 0.04 s delay, which then
combs out 12.5 Hz, 25 Hz, 37.5 Hz, 50 Hz, and so forth. The larger delay, the
lower the frequency at which combing begins, and the closer the combed
frequencies are to one another. You can see this in Figure 4.41. In the first graph,
a delay of 0.5682 ms combs out integer multiples of 880 Hz. In the second graph,
a delay of 2.2727 ms combs out integer multiples of 220 Hz.
If the delay is long enough, frequencies that are combed out are within the
same critical band as frequencies that are amplified. Recall that all frequencies in
a critical band are perceived as the same frequency. If one frequency is combed
out and another is amplified within the same critical band, the resulting perceived
amplitude of the frequency in that band is the same as would be heard without comb filtering.
Thus, a long enough delay mitigates effect of comb filtering. The exercise associated with this
section has you verify this point.