Let’s say the sound system in the house of worship you’re working on goes into feedback whenever microphones pass under the loudspeaker array.
Worse yet, there are “soft spots” in some sections of the audience area.
Choir mics “squeal” before they are loud enough and the podium mic “rings” annoyingly for some presenters. You know that the system should be equalized to eliminate these problems.
So you install an equalizer and the feedback is reduced, but the soft spots persist and the system just doesn’t sound good.
But that’s why you, the consumate audio profesiponal, are there.
After some careful listening tests, a “problem area” within the room is chosen for the measurement mic placement.
This is a place in the seating where people complain that they can’t hear, or a place where the mic consistently goes into feedback, such as directly under the loudspeaker array. The measurement looks something like that shown in Figure 1.
Figure 1: Comb filter caused by a time offset between two loudspeakers. The audibility of comb filters has always been the subject of heated debate. While humans may not be very sensitive to narrow notches in the spectrum, the spacial lobing implied by the comb filter can excessively excite rooms and dramatically reduce gain-before-feedback.
The response clearly shows an acoustic “comb filter” that results from a time offset between two sound arrivals at the measurement position.
The measurer first makes certain that the secondary arrival isn’t simply the result of a bad mic placement (floor bounce, etc.) or loudspeaker placement (ceiling or wall bounce, etc.).
After ruling out these two possibilities, it becomes apparent that the multiple arrivals are due to the overlapping patterns of two loudspeakers being used to provide audience coverage.
Standing at the mic position and simply looking at the array, noting that you are clearly within the coverage pattern of two loudspeakers suspended over the stage, confirms the suspicion. Sound travels at a single constant speed.
Yet, in this case, there are two loudspeakers.
Therefore every location in the room that is receiving direct sound at equal level from both loudspeakers (except for the center line where the distance to each loudspeaker is exactly equal), will receive two signals arriving at different times.
This time offset causes the comb filtering.
Figure 2: Represents the lobing (a form of destructive interference) between two spaced loudspeakers at a single frequency.
An acoustic comb filter can produce undesirable coloration of the sound and loss of definition. It can even change where the sound seems to be coming from, ruining the “imaging” of the system.
The possible “options” are:
1. Set the analyzer resolution to smooth the comb filtering, and then adjust the equalizer for the desired response. This is not a solution. It just masks the problem.
2. Ignore the comb filtering and simply “notch” the frequencies that are prone to feedback. Even though this is a common approach, it is treating the symptom and not the problem.