I will start this answer to your “question” with a conclusion; you need not concern yourself with the measurement phenomenon known as comb filtering. Since we produce multi-driver systems here at Axiom we have put much research into understanding the impact of comb filtering on the real world listening environment. There are huge benefits to using multiple drivers, especially in the area of being able to produce large dynamic range without distortion, so it is not something that should be casually thrown out of your design options. Certainly, on the surface, if you were to only look at the measurements taken by the microphone of comb filtering, without any further research, you would probably decide it is something that must be avoided. But this would be an over simplistic and very counter productive way to actually design a great sounding loudspeaker. It would be akin to simply taking a bunch of measurements and then go about drawing theoretical conclusions based on those measurements without feeling the need to do any real world testing of your theories. This will result in some great marketing propaganda and some great discussion material in which to back up your theories; but it would not result in a great sounding loudspeaker. The proper approach is to go through the somewhat painstaking process of understanding each measurement and its effect on the final listening experience, paying careful attention to the thresholds of audibility and the interrelationship with other measurements that may hold a greater significance to the end listening performance.
So let’s take a look at comb filtering in this light. First off comb filtering can be measured any time two drive units are playing the same frequency and the microphone is not located exactly equidistant between them. For example, if we take a standard stereo pair of speakers located say 8 feet apart, any movement of the microphone off centre, even as small as ½ inch, will show the affects of comb filtering. For more detail on this you can check out our newsletter article on comb filtering http://www.axiomaudio.com/archives/may2009.html#feature
. In order to avoid comb filtering occurring in our listening environment not only would we have to restrict ourselves to loudspeakers that only used one driver per frequency allocation, which would have huge detrimental affects on the ability to achieve great dynamic range, but we would have to restrict ourselves to only having one speaker in the room, in other words return to listening in mono through one speaker. Given the obvious enormous downside to doing this perhaps first we should review the real world results of comb filtering in our listening environment before rushing off and getting rid of all our speakers save one.
One of the more revealing tests we performed in our lab was to set up a double blind listening test where we had three identical speakers, in this case we used the M2, and compared only playing the single speaker in the middle against playing the two on the outside simultaneously. We conducted this test in two separate sessions; one with the three speakers placed side by side, which gave us about a 15” separation between the drivers playing simultaneously, and one with the two outer speakers placed one meter to each side of the centre speaker. The amplitude response was preset so the two speakers had the same output as the single speaker and only a mono source was used. Not to wander off into a completely different subject, but it is worth at least noting that these sorts of tests must be done double blind. Any attempt to do them sighted or with the participants knowing what is on test will result in the participants who understand the effects of comb filtering on the measurements becoming biased in their judgment. This holds true for virtually all tests in audio and we have proved this over and over.
The results of that test were a clear winner for the widely separated pair and a slight nod for the closely positioned pair; the single speaker lost in both tests. The moral of the story here is to be careful when interpreting your measurement results and falling into the trap of thinking a microphone is giving you the exact results that will be perceived by two ears and a brain. In the case of comb filtering such a simplistic approach to loudspeaker design would cause the designer to throw out all the benefits of multi-driver systems to gain nothing. Forget about having to choose between taking a small degradation in one area in order to achieve a much larger benefit in another, which would also be a valid design decision; in the case of avoiding comb filtering you have to give up a very large benefit to gain absolutely nothing. I think one of the most eloquent comments ever made regarding the dangers of taking microphone measurements for granted was by Dr. Floyd Toole in one of his papers written back when we were doing our research at the NRC; “A measurement microphone performs a simple transduction of the pressure summation at the diaphragm location, without regard for the direction or timing of the incident sounds. Two ears and a brain, however, are rather more elaborate in their processing”.