Talker-to-microphone distance

One of the most significant factors in determining the performance of an audio system is the distance from the talker to the nearest microphone. As the microphone is positioned farther away from the talker, the loudness of the speech reaching the microphone decreases, so the microphone's sensitivity to sound - its ability to "hear" - must be increased to compensate. This causes the microphone to pick up more of the desired sound (speech), but also to pick up more of everything else - background noise, reflections, sound from the loudspeakers, etc. The farther away the microphone is from the talker, the more hollow and noisy the sound will be.

So, when it comes to microphone placement, how far is "too far"? It so happens that there is a certain distance from the microphone beyond which a talker will sound hollow and difficult to understand, regardless of what type of microphone is used or which way it is pointed. This is called the Critical Distance, abbreviated D_c . The Critical Distance is different for every room, and is determined primarily by the volume of the room (in cubic feet) and the decay time. Those talkers who are at or beyond the Critical Distance from the nearest microphone will be difficult to understand, no matter what type of equipment is added to the audio system or how it is adjusted. Given that most classrooms have a Critical Distance of from two to five feet, the following guidelines should be used for determining acceptable distance from the talker to the nearest microphone:

• less than 2 feet is ideal
• between 2 and 3 feet is good
• between 3 and 5 feet is marginal
• more than 5 feet is unacceptable

In a typical classroom, microphones should be located less than two feet from talkers.

When in doubt, always place the microphones nearer to rather than farther from talkers. Moving microphones closer to talkers is the single most significant improvement that can be made to most classroom audio systems.