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Obtaining a good audio signal from one microphone is relatively simple. However, conference rooms often have multiple microphones located in close proximity to each other. With multiple microphones, there will be multiple copies of the same audio signal present, but the different distances between the talker and the various microphones will result in those signals being time shifted relative to each other. This causes a phenomenon called comb filtering, which is clearly visible in the frequency plot.
Comb filtering is also clearly audible and is often described as sounding like the talker is speaking through a tube. Here is an example of a single microphone followed by the same audio with the comb filtering caused by two microphones 36 inches apart.
In recording, the “3 to 1” rule is used for microphone placement. Whatever the distance is from the talker to their mic, all other mics must be at least 3 times that distance away from the talker’s mic.
While this is useful in a recording scenario, it is seldom appropriate for a conference room. With microphones spaced this far apart, someone will inevitably sit between the mics which will eliminate any benefit from the spacing.
The 3:1 rule was created to make comb filtering less audible, not eliminate it. Comb filtering is still clearly visible on the frequency plot and audible when listening, but the increased distance between microphones results in filter notches that are both more frequent and narrower than the filter notches caused by placing the microphones closer together. Filter notches that are narrower are less noticeable to the human ear, even if they are more numerous. Using the 3:1 rule will result in a better audio signal, but it is still not as good as a single microphone.
A better solution is to use an Automatic Microphone Mixer (AMM). The two types of AMM available in the Bose ControSpace® EX-1280C are the Gain Sharing AMM and the Gated AMM.
A Gain Sharing AMM allocates more gain to channels that have a larger input signal. A Gated AMM turns off (or turns down) any channel with an input signal below the threshold. Either type of AMM will generally be able to provide at least 20 dB of gain reduction on unused microphone channels.
When using an Automatic Microphone Mixer, microphones may be placed reasonably close to each other without needing to worry about comb filtering. The AMM ensures that the currently active microphone is substantially higher in level than other microphones in the room, and the resulting audio is almost indistinguishable from the result obtained by using a single microphone.
