The sound system in your house of worship 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 have heard that the system should be equalized to eliminate these problems. You buy an equalizer and the feedback is reduced, but the soft spots persist and the system just doesn’t sound good.

So you decide to obtain the services of an audio professional – someone who has invested the time and money required to become proficient at performing high-resolution acoustic measurements. 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.

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.

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. See Figure 2.

Figure 2: Represents the lobing (a form of destructive interference) between two spaced loudspeakers at a single frequency.

The audio professional’s 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. Excessive frequency notching can ruin the sound of the system. Why filter out sound that needs to be there?

3. Conclude that humans aren’t all that sensitive to narrow notches in the spectrum, so the comb filters are just something that we can live with. This is rationalizing the problem and is simply not true. It’s usually the explanation provided by someone who is responsible for the problem in the first place!

4. Get out your old one-third octave real-time analyzer. You can’t see the comb filters on it. For many years, audio professionals did not have high-resolution analyzers that could identify arrival time problems. The system response looked fine on a one-third octave analyzer, but it still sounded bad. Today’ analyzers are vastly more powerful and can reveal much more about the nature of a sound problem.

5. Inform the owner that the current loudspeaker placement has created some problems that cannot be "corrected" electronically. The only real solution is to relocate the existing loudspeakers or redesign the array.

Unfortunately, the sad reality is that only the last option is likely to fix the system. (See Figures 3-5)

Figure 3: Making the loudspeakers occupy the same physical position (impossible).

Figure 4: Placing a greater physical distance between the loudspeakers.

Figure 5: Use of aggressive pattern control to reduce the overlap.