Page 183 - Master Handbook of Acoustics

CHAPTER 10

Comb-Filter Effects

T he effect of a delayed reflection on the frequency response of a signal is often called a comb

filter. Comb filtering is a steady-state phenomenon. It has limited application to music and

speech, which are highly transient phenomena. With transient sounds, the audibility of a delayed
replica is more the result of successive sound events. A case might be made for combing effects
during brief periods of speech and music that approach steady state. However, the study of the
audible effects of delayed reflections is better handled with the generalized threshold approach of
sound reflection. Still, it is important to understand the nature of comb filtering, and know when it
will, and will not, pose an acoustical problem.

Comb Filters

A filter changes the frequency response or transfer function of a signal. For example, an electronic
filter might use active circuitry to attenuate low frequencies of a signal to reduce unwanted noise. A

mechanical filter could use a system of ports and cavities used to alter an acoustical signal, such as is
used in some microphones to adjust the pickup pattern.
In the days of multitrack tape recording, multiple-head tape recorders were used to provide
delayed replicas of sounds that were then mixed with the original sound to produce phasing and
flanging effects. These same effects can also be created electronically or algorithmically. Whatever
the means, these audible effects are the result of comb filters.

Superposition of Sound

Imagine a laboratory with a large tank of shallow water. Two stones are dropped in the tank

simultaneously. Each stone causes circular ripples to flow out from the drop points. Each set of
ripples expands through the other ripple pattern. We note that at any point in the water, the net effect
is the combination of both ripple patterns at that point. As we will see later, both constructive and
destructive interference will result. This is an example of superposition.
The principle of superposition states that every infinitesimal volume of a medium is capable of
transmitting many discrete disturbances in many different directions, all simultaneously and with no

detrimental effect on other disturbances. If you were able to observe and analyze the motion of a
single air particle at a given instant under the influence of several disturbances, you would find that
its motion is the vector sum of the various particle motions required by each of the disturbances
passing by. At that instant, the air particle moves with an amplitude and direction of vibration to
satisfy the requirements of each disturbance just as a water particle responds to each of several