Help & Frequently Asked Questions

This is a phenomenon known as 'comb filtering'. It occurs when a sound is picked up by two mics at the same time. Because of the very slight differeces in the timing of the sounds arriving at the microphones, when the sounds from each are mixed together one is slightly 'out of phase' compared to the other.
When this happens, some frequencies effectivly cancel each other out; you hear a 'thin' sound because some of the original sound is simply not there!

A picture of the frequency plot of the two added signals has deep notches in it. The appearance resembles a comb - hence the name. See figs 1 and 2 below.

Fig 1 - original signal
	Fig 2 - effect of combining 2 of the above signals non-time aligned

In fact, this can occur whenever there is more than one mic open and picking up the same sound. Another situation when this can be a problem is when using multiple float or front of stage mics.

How can you avoid it? Well, the only sure way is to only have one mic open at any one time. You can make the effect almost negligable by making sure that one mic is at least 3dB lower than the other - this has been shown to reduce the possibility of combing occurring. This means that the operator will have to be alert and riding the faders as the action happens on stage. If the problem occurs with float mics, careful positioning of the mics and sympathetic blocking on the part of the director may help. If the float mics are far enough apart, then it is likely that any sound will have more than 3dB difference between any pair of mics. However, if they are spaced too far apart, you run the risk of leaving 'holes' in your coverage. Of course, if there is no action in these 'holes' then it does not matter.
Again, riding the float mic faders in line with the action is the best way of avoiding the problem.

There is another, very expensive, way of avoiding the problem, and that is to have a separate speaker system for each microphone. This is known as an 'A-B' system and was pioneered by Martin Levan and Andrew Bruce for shows such as 'Phanton of the Opera'. This means that the sounds from each mic are never 'mixed' and they cancel each other out. This might just be practical if there are two major characters who spend a lot of time singing together but other than for large West End shows it is a often too expensive and a time consuming option.