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Several microphones. Three to One Rule

Interference is the phenomenon of amplification of oscillations at some points in space and weakening of oscillations at other points as a result of the superposition of two or more waves arriving at these points. Wave interference is possible if the phase difference of the waves is constant in time, i.e., the waves are coherent. Wave interference is valid for waves of any nature and frequency.

When recording from several microphones, it is necessary to take care of their phasing: the signals from the microphones should not fall into the recording path in the opposite phase, since this can lead to their mutual compensation up to the complete disappearance. Without noticeable distortion of the timbre caused by interference of direct and reflected signals, the microphone can be brought closer to any reflecting surface at a distance of at least 1-1.5 m.

All efforts to improve the quality of the recording may be in vain if you do not follow the obvious rules. One common mistake is the manual use of a microphone. It can be considered justified only when recording the voice of a performer in motion. Manual use of a microphone requires a lot of experience. Watching on TV singers, almost juggling with a microphone, know that in most cases you hear not a “live” performance, but a phonogram. Most performers who receive a microphone in their hands do not know how to use it correctly. The most common mistakes are the use of the microphone at too close a distance, unnecessarily sharp and unnecessary movements to them, the movement of the fingers covering the microphone. In extreme cases, you can come to terms with this once a “live” performance, but not during a recording session.

Therefore, when we need to use several microphones, or use microphones mounted on a table, the resulting interference can be minimized using the “three to one” rule. In the case of using several microphones, the rule says: the distance between the microphones should be at least three times the distance from each microphone to the corresponding sound source …

Three to One Rule

The sound that the remote microphone picks up will be at least 12 dB quieter than the sound that the near one will pick up.
This ensures that the audible effects of the ridge will decrease by at least the same amount.

Multiple Microphones (Three-to-One Rule)

If installed on a table with a reflective surface, the microphone should be one and a half times farther from it than from the speaker (sound source). This ensures minimal audibility of the effects of interference.

Strictly speaking, the rule is based on the behavior of omnidirectional microphones. It can be followed less strictly in the case of “correctly targeted” unidirectional microphones, but it is still the basic rule for the worst cases.

Microphone Phase Effects

One of the effects that can often be heard during dubbing occurs when the microphones are located equally close to the same sound source, such as a participant in an event served by a conference system:

A microphone amplifies sound waves from a signal source (speaker, presiding, deputy)
The signal from the microphone passes through the sound path (preamplifier, processing devices, power amplifier)
Amplified signal is sent to the speakers
The microphone picks up the signal from the speaker

Thus, the so-called feedback loop arises:

The incoming sound will be amplified to the level of the original sound picked up by the microphone, as a result of which the speaker system (loudspeaker) starts to “whistle” and resonate.

But this is not all that defines the “gain index” in a particular case.
There is another factor – selective filtering.

The effect of acoustic interference is the result of using a large number of microphones and can occur even when using two microphones. Selective filtering is the result of signal mixing. It is characterized by the use of the highest and lowest sections of the audible frequency range. Since the microphones are not at the same distance from the sound source, the sound picked up by a more distant microphone will play with a longer delay than the sound from a microphone located closer …
The result is a phase shift and, as a result, selective filtering.

Most often this is due to the phase difference of the sounds coming into the microphones

If microphones receive the same sound from different places, phase summation or subtraction may occur. Subtraction occurs when two microphones receive the same sound, but with opposite phases. This is undesirable. A microphone with a different radiation pattern may reduce the reception of unnecessary sound, or physical isolation will help

In the case of a conference system, isolation of participants is not possible. In this case, the choice of microphone type depends on the characteristics of reception suppression outside the main axis.

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