An acoustic comb filter is a symptom of a more significant problem. When two loudspeakers are placed in close proximity, the resultant distance offset will cause "lobing" in the speaker’s radiation pattern. Lobes can be described as "fingers" of sound pressure "maximums" in the three-dimensional space surrounding the array. The fingers are separated by nulls or axis of minimal sound pressure level. The fingers typically cause problems with microphones, since a mic is likely to feedback when it is placed within a lobe.

The nulls cause problems for the audience, since parts of the audio spectrum that are critical for speech intelligibility (understanding the words) are cancelled at some listener’s seats. When a series of these lobes and nulls exist, the visual representation of the frequency response at one listener position will resemble the teeth of a comb, with a sequence of peaks and valleys. This is a far cry from the "perfect" system response that would look more like a flat line. As such, a comb filter is the symptom of a spatial problem that has resulted from a loudspeaker selection and placement choice.

To illustrate, look at these (below, Figures 6) "simulations" of such a condition performed with the EASE sound system design software package. Two loudspeakers with low directivity control have been separated by two feet. The resultant does not represent accurate sound reproduction and can cause the afore-mentioned problems with acoustic gain and speech intelligibility.

Please note that it is certainly possible to build quality "arrayable" loudspeakers, and there are a number of good examples in the marketplace. However, all of them have several parameters in common:

1. Large physical size

2. Horn-loaded components

3. Aggressive pattern control to minimize interaction with adjacent loudspeakers

If these loudspeaker requirements present problems for a particular venue due to the required large physical size, then smaller loudspeakers can be used (usually in greater number) if they are placed sufficiently close to the listeners (i.e. exploded arrays or distributed systems). Figure 2 (on page 38) outlines the options, and there aren’t many.

Radio broadcast engineers have understood for years the importance of proper antenna array design to control lobing in RF radiation to steer their signal to certain areas within the listening range and away from others. For instance, if a station is licensed to radiate 50 killowatts of power, they can use an antenna array to steer the radiated signal up and down an interstate highway rather than out across a sparsely inhabited area. In fact, if they do it wrong, they can be in violation of federal law and therefore subject to prosecution. Loudspeaker array designers must work with the same physical laws and principles as antenna designers. The only difference is that they can’t be prosecuted for bad sound.

Specifically about Figure 6:

Balloon plots are useful because they show the three-dimensional radiation pattern from a loudspeaker or group of loudspeakers located at the center of the balloon. The plot describes what is happening at a single frequency. The plots can be generated for multiple frequencies to more fully describe the performance of an array. The balloon plot of a "perfect" loudspeaker would be the same, regardless of frequency.

Comb filtering in the magnitude response (a measurement at a single point in space) is evidence of lobing in the spatial radiation of the array. 1

Top of Figure 6 - The balloon plot displays the 3-dimensional sound radiation from the two-device array described in the text.

Bottom of Figure 6 - The traditional horizontal polar plot views the equator of the balloon as viewed from above for one frequency.

Pat Brown teaches the Syn-Aud-Con seminars and workshops. Synergetic Audio Concepts (Syn-Aud-Con) has been a leader in audio education since 1973. With nearly 15,000 "graduates" worldwide, Syn-Aud-Can is dedicated to teaching the basics of audio and acoustics. For more information, visit their website at http://www.synaudcon.com or call 1-800-796-2831.