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May 1, 2009

Comb Filtering – Popular Misconceptions

Filed under: Audio Knowledge — Tags: , — Alan @ 9:01 am

Perhaps it seems odd to discuss the teeth of a comb in connection with loudspeaker sound reproduction or the propagation of real sound waves, but it is relevant.

Comb filtering is a catchy audio phrase that’s used in audio discussions on forums, in articles, and often in the context of critical comments about the specifics of a particular speaker design. The fact is that comb filtering is simply a measurement artifact and does not detract from the listening experience. The research shows that comb filtering is not detrimental to accurate loudspeaker sound reproduction; at worst, it’s irrelevant, at best it actually adds a pleasurable element of spaciousness to stereo and surround sound.

That said, you might ask if it’s a measurement artifact, and careful measurements are instrumental to the scientific approach to acoustics and loudspeaker design that Axiom espouses, then why don’t we hear comb filtering with music and speech?

Let’s break it down.

A Microphone Is Not Two Ears

It must be pointed out that a measurement microphone—even a very expensive lab-calibrated model like the one Axiom uses (a B & K)—is like a single ear with no brain. As human beings, we hear with two ears and a brain, the latter being an incredibly sophisticated audio processing unit that is constantly comparing signals received from our two ears and sorting out not only directional cues and amplitude (loudness) differences but also ignoring or disregarding information that might be confusing or detrimental to our sound localization, spatial perception and tonal identification abilities.

What Is Comb Filtering?

Simply stated, comb filtering is two signals arriving at the same location at different times. Because of the differences in the arrival times, the sound waves will have additions when they perfectly overlap and reinforce each other, and also have cancellations or nulls where they cancel each other out (the latter is called destructive interference). This occurs in virtually all speaker systems whose musical ranges overlap, where both drivers are reproducing the same sounds, as in stereo or surround sound, and because of multiple drivers with different physical locations used to cover the same frequency range.

To illustrate how a single measurement microphone “hears” or identifies comb filtering, we set up an interesting experiment in Axiom’s anechoic chamber. Two M2v2 bookshelf speakers were placed in the chamber 6 feet apart. The calibrated B & K microphone was placed 6.5 feet away and directly in the center in the sweet spot between the two speakers. A standard frequency sweep from 20 Hz to 20 kHz was played back over the two M2 speakers and we recorded the test sweep with the measurement microphone. The purple curve in Figure 1 shows the frequency response with the microphone exactly centered in the sweet spot between the two M2 speakers.

Figure 1

Figure 1

Then we moved the measuring microphone ½-inch to the side, off center from the sweet spot, and recorded another frequency-response curve. The green curve in Figure 1 shows the first comb cancellation effect at 15 kHz.

Then we moved the microphone 1 inch off center and ran another curve. In Figure 2, the green curve shows the next comb filter cancellation at 5.5 kHz. In Figure 3, the measurement microphone was moved 8 inches off center from the sweet spot. The dark greenish curve shows the pronounced comb-filtering cancellations beginning just below 1.5 kHz and extending all the way up to 18 kHz. The dips in response resemble the downward teeth of a comb, hence the name “comb filtering”.

Figure 1

Figure 2

Figure 1

Figure 3

The cancellations (dips) are what the single measurement microphone “hears” and measures using a full-frequency test sweep when the signals from the two M2 speakers don’t perfectly overlap. This seems like an acoustic effect that may be potentially nasty in nature and should be avoided. These are pronounced cancellations, yet when we play music or speech over a pair of M2 speakers, we don’t hear these comb filtering effects. Why is that?

How Does the Brain Deal With Comb Effects?

The precedence effect (previously known as the Haas Effect) dictates that our brain and ears pick out the location of a sound source that reaches our ears in the first few milliseconds of a sound’s arrival. The first sound to arrive at the ears enables you to determine the direction of the source. After hearing an initial signal, the brain will suppress any later-arriving signals, up to about 30 milliseconds. These later-arriving signals that show up with steady-state pink noise (within the 30-millisecond window) do not disrupt the brain’s precise localization mechanism. What occurs is that you do not “hear” the contributions of the later-arriving sounds from the adjacent drivers that are responsible for the measurement artifact of comb filtering. Or rather, your brain hears and processes them but disregards them lest they confuse our directional acuity; in fact all they do in the listener is create a sense of added spaciousness. Numerous scientific researchers, including definitive experiments conducted by Dr. Floyd Toole and Dr. Sean Olive, have verified this. Even in a room having lots of reflections, our brains correctly determine the direction of sounds. (By the way, sounds arriving at our ears after a delay of more than 30 milliseconds are perceived as a second sound or echo.)

Critics of comb filtering who believe it to be a big issue in speaker design have the option of listening in mono to avoid the comb filtering. But we all much prefer listening to music and vocalists in stereo—it’s far more spacious and realistic–and the reason is that our brains and two ears simply ignore those canceling signals that on paper show up with a test signal and a single microphone.

So put on a great recording, even in 5.1 channels or SACD/DVD-Audio multi-channel, pour a nice glass of wine or open a beer, and thrill to the realism and spaciousness of great musical reproduction. Stop obsessing over comb filtering; it doesn’t matter! – A.L.

(Enthusiasts who would like to read further about comb filtering and psycho-acoustics should explore Sound Reproduction: Loudspeakers and Rooms, by Floyd E. Toole, Focal Press. Available from Amazon.com)

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