A unidirectional microphone should be positioned no farther than 50 percent of the critical distance, e.g. if the critical distance is 10 feet, a unidirectional mic may be placed up to 5 feet from the sound source.
Highly reverberant rooms may require very close microphone placement. The amount of direct sound relative to ambient sound is controlled primarily by the distance of the microphone to the sound source and to a lesser degree by the directional pattern of the mic.
Phase Relationships & Interference Effects
The phase of a single frequency sound wave is always described relative to the starting point of the wave or 0 degrees. The pressure change is also zero at this point. The peak of the high pressure zone is at 90 degrees, the pressure change falls to zero again at 180 degrees, the peak of the low pressure zone is at 270 degrees, and the pressure change rises to zero at 360 degrees for the start of the next cycle.
Two identical sound waves starting at the same point in time are called “in-phase” and will sum together creating a single wave with double the amplitude but otherwise identical to the original waves. Two identical sound waves with one wave’s starting point occurring at the 180 degree point of the other wave are said to be “out of phase” and the two waves will cancel each other completely.
Sound pressure wave.
When two sound waves of the same single frequency but different starting points are combined the resulting wave is said to have “phase shift” or an apparent starting point somewhere between the original starting points.
This new wave will have the same frequency as the original waves but will have increased or decreased amplitude depending on the degree of phase difference. Phase shift, in this case, indicates that the 0 degree points of two identical waves are not the same.
Most sound waves are not a single frequency but are made up of many frequencies. When identical multiple frequency soundwaves combine there are three possibilities for the resulting wave: a doubling of amplitude at all frequencies if the waves are in phase, a complete cancellation at all frequencies if the waves are 180 degrees out of phase, or partial cancellation and partial reinforcement at various frequencies if the waves have intermediate phase relationship. The results may be heard as interference effects.
The first case is the basis for the increased sensitivity of boundary or surface-mount microphones. When a microphone element is placed very close to an acoustically reflective surface both the incident and reflected sound waves are in phase at the microphone.
This results in a 6 dB increase (doubling) in sensitivity, compared to the same microphone in free space. This occurs for reflected frequencies whose wavelength is greater than the distance from the microphone to the surface: if the distance is less than one-quarter inch this will be the case for frequencies up to at least 18 kHz.