The speed of sound is approximately 1,130′ per second in air, depending on the actual air temperature. Therefore, if you have a 1,130 Hz tone it will complete one full cycle in one foot.
Now suppose that you have a tone generator feeding a speaker, with two microphones as shown below (image A). Distance affects the phase. If the mics were both the same distance from the speaker, they would be in phase and would add together. The resulting tone would be twice the level (6dB) of either tone (image B). Similarly, if the second mic was 1′ further away from the speaker, the two sources would still be in phase and would again add together. If the second mic was only 6″ further away from the speaker than the first mic, the two sources would now be out of phase. This would cause the tones to cancel (image C). For the next section return to the previous setting, mic #2 is 1′ further away from the source than mic #1.
Frequency also affects the phase. At 565 Hz (1,130 hz/2) the tone will now complete a full cycle in 2′. As seen in the following example, the two tones now arrive out of phase and thus cancel (image D). At 1,695 Hz (1,130 Hz x 1.5) the two tones also arrive out of phase and cancel. However at 2,260 Hz (1,130 Hz x 2) the two tones arrive in phase and thus add. This effect, known as comb filtering, can be shown to repeat all the way up the frequency band.
The following graph shows the resultant gain verses frequency (image E). Note that when the two signals are equal, if they are exactly in phase they add 6dB, but if they are exactly out of phase, they totally cancel. In an actual situation, the effects would probably not be as pronounced, since the levels from the two mics would seldom be exactly equal. One good example of this situation is when two mics are (mistakenly) placed on each side of a lectern, with the idea that they will pick up the audio regardless of which way the speaker turns. This will result in poor sound quality. As the speaker turns his head, one mic can be closer than the other, thus introducing the comb filtering. Comb filtering will produce a hollow, diffuse, and thin sound.
Pick up the February issue of PS for Audio Phasing: Part II.
Al Whale is a Broadcast Technologist and Assistant Chief Engineer at CHBC-TV. He has also set up and operated sound systems and taught sound in many church settings. Reach him at firstname.lastname@example.org.