THORBURN ASSOCIATES

Acoustic and Technology Consultants

Designing Quality Environments

Ceiling Mounted Microphones -- A Solution or a Problem?

With hopes of eliminating tabletop or podium microphones, End Users, Designers and Architects frequently ask, "What about sticking them on the ceiling?" Usually, what seems like an easy solution is not the best solution.

Ceiling mounted microphones are not recommended for the following reasons:

First, loudspeakers are typically mounted in the ceiling. This places a ceiling mounted microphones closer to the loudspeakers than to the person talking. The nearness of the loudspeaker and microphone means that sound from the loudspeaker is likely to be louder than the speaker's voice. As shown in the diagram the primary sound picked up by the microphone is not the speaker's voice, but the sound from the loudspeaker. When this happens, the amplified signal is again picked up by the microphone, and that "high-pitched shriek" or feedback occurs. In other words, there is no "volume gain before feedback."

Second, the most effective distance of the microphone from the sound source must be considered. For vocal microphones in a band this distance is a few inches, for microphones in a conference room setting the optimum distance is usually 1 to 3 feet. Ceiling mounted installations will exceed this optimum distance, which allows the microphone to transmit the speaker's voice plus other unwanted sounds in the room (HVAC noise, fluorescent light buzz, etc.). The result is poor audio quality.

When can ceiling mounted microphones work?

• They can be used to record speech or music for an archive record, not to reinforce speech or music.

• They must be used in an extremely quiet room with a background noise criteria (NC) rating of no more than NC 20.

Michael Pettersen, Director of Consultant Liaison Program, Shure Brother's Inc. contributed to this article

© Thorburn Associates, Summer 1993.

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