On several occasions in the past, we’ve talked about the problems caused by poor audio design and more specifically the placement of microphones in meeting rooms. That’s because its one of the most common mistakes we see made in meeting room designs, and also probably one of the most common sources for dissatisfaction amongst users of the rooms.
Our team recently came across another resource from Biamp (titled “Mic placement”) , which talks about the basic principles that should be applied to get microphone placement right in ANY room. In the article, they describe some of the key concerns to consider when deciding on locations for microphones in a room.
The key points that the article covers are:
Place the microphone as close as possible to the intended sound source: Close miking always sounds better — because as distance from any source increases the apparent volume drops and it approaches the ambient noise level of the space. As we get further from the source, we also have to increase the microphone’s analog input gain to maintain the same relative level. This makes the microphone more sensitive to extraneous noises that we don’t want to capture — noises such as projector fans, squeaky chairs, rustling papers, or traffic driving past the building.
Obtain higher signal-to-noise ratios: To be understood clearly, a speech signal must be at least 6dB above, or twice as loud as, the background noise level. This means that the speech must have a minimum signal-to-noise ratio of 6dB (recall that dB is a ratio expression), so 6dB denotes a 2:1 ratio. Obtaining higher signal-to-noise ratios will result in higher quality signals.
Increase your direct-to-reverberant ratio: This is a measure of reverberation which occurs due to sound reflections in larger rooms or due to poor acoustic treatment. It is a measure of the level of the direct sound, compared to the level of the reverberant sound picked up by the microphone. There are only two ways to raise the direct-to-reverberant ratio: either acoustically treat the room to absorb reflections and reduce reverberation (which is often impractical) or move the microphone closer to the talker.
Follow the 3-to1 rule for mic placement: This rule states that if you measure the distance from the sound source (e.g., the talker) to the nearest microphone, then the next closest microphone must be at least three times that distance away. The 3-to-1 Rule is meant to prevent the same sound source from being picked up at the same level by multiple microphones at slightly different times. In larger spaces it becomes necessary to use automixers to solve problems when the 3-to-1 rule ceases to work.
(Content and image credit: www.biamp.com)