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are not at the same distance from the snare drum. On the other hand all these micropho-
nes will be mixed together on the mixing desk. On top of that, not all these microphones
will be identical so will not have the same frequency response, and the applied EQ on
each input channel on the mixing console will also be different. The result is that the one
original sound of the snare drum will be spread out with different delays and different EQ
and different levels over a number of channels on the mixing console. This phenomenon
is also known under the name of phase shift. The result is that the original sound is
smeared out: you loose definition, impact and imaging. Since everybody is still waiting
on the microphone manufacturers to develop an intelligent microphone which can dis-
tinguish between snare drum and other drum sounds, you can still buy an Apex dBG-8
to clean up your sound. Using the gate in a right way, we can try to minimize the effects
of phase shift. If the gate is set in a correct way, it will only let the sound of the snare
drum pass when it is hit, in the intermediate pauses; the gate will be closed and will not
let other drum sounds through. This goes of course for the other drum mics to. This only
works of course if the intended source is clear and distinguished from the others. That
is why there are normally no gates on the overhead microphones, these tend to pick up
the sound of the cymbals but also the overall sound of the complete drumkit.
Thus this now means that you can only use gates on drum mics? No, not at all. Let's
consider the backing vocals.
This leads us to another problem of multiple open microphones: Feedback! When a
microphone is picking up sound, the signal is amplified and fed to a loudspeaker, we
have a complex electro acoustic circuit. A tiny sound is picked up by the microphone, is
amplified, and fed to the loudspeaker. But some of the sound coming from the loudspea-
ker is picked up by that same microphone, amplified and fed to the same loudspeaker
and some of that sound is again picked up by that same microphone, amplified etc.....
When a certain gain is applied between the microphone and the loudspeaker, the circuit
will start to oscillate and produce what we call Feedback (Larsen effect). Nobody likes
feedback (as far as I know), not human beings, not amplifiers and definitely not spea-
kers and if more definitely not speaker tweeters. With only one microphone on stage
and open one can (via rather complex mathematics) calculate the possible gain before
feedback will occur. In other words: this will decide how loud you can go before you get
feedback. Based on this there is the rule of NOM (Number of Open Microphones). If we
have 2 microphones on stage, open and with the same gain, we will have to reduce the
overall gain by 3dB to avoid feedback (compared with one open microphone). If more
microphones are open you can use the following formula: NOM (in dB) = 10 log NOM .
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