by Gino Sigismondi
Gino Sigismondi from Shure explains some shortcomings of ceiling-mounted mics and list 3 basic rules of microphone placement that are violated.
OK, I need a favor. I’m going to make a statement that could very well put my job in jeopardy, I need you to promise not to tell any of my bosses here. I’m dead serious about this. Agreed? Here we go: Microphones are dumb.
I know what you’re thinking, “How can a representative of such a legendary manufacturer call their own products dumb?” Well, I don’t mean “dumb” in the “Wow, microphones are so dumb” sense. I mean, microphones have no inherent intelligence that allows them to reliably discern desired from undesired sound. Unlike the human auditory system, which allows a listener to concentrate on one conversation above the din in a crowded room, a microphone has no ability to “filter” unwanted sounds. And herein lies the problem with mounting microphones in the ceiling. It’s noisy near the ceiling (undesired sound), it’s far away from the talker (desired sound), and close to loudspeakers (more undesired sound). When placing microphones in a ceiling, no less than three basic rules of microphone placement are violated, including:
- Microphone-to-mouth distance should be less than half of critical distance (measured form the talkers mouth to the point where direct and reverberant sound are equal), when using a unidirectional microphone.
- Speech-to-background noise ratio at the microphone should be no less than 30 dB. Unfortunately, near the ceiling is typically the noisiest area of the room, due to air handling vents and general build-up of room noise. Compounding the problem is that pesky inverse square law, which results in 6 dB of loss every time the mic-to-mouth distance is doubled. If we accept that conversational speech at one foot equals 74 dB SPL, a ceiling-mounted mic picking up a seated talker will be lucky if it gets 60 dB SPL of desired sound. A quiet room might have background noise in the range of 50 dB SPL, leaving us 20 dB shy of the desired speech-to-background noise ratio. Remember, mics are dumb – they can’t tell the difference between speech and noise.
- PAG (Potential Acoustic Gain). If there will be local sound reinforcement, ceiling-mounted microphones are a major violation of at least two of the basic tenets of PAG: Keep the microphone close to the talker, and far away from the loudspeaker. In most cases, it will be impossible to achieve any significant reinforcement before the onset of feedback (although proper implementation of loudspeaker zones to mute loudspeakers closest to adjacent microphones will certainly help).
More detailed information on each of these topics is available from the Shure web site:
Critical Distance and Microphone Placement: Click here
Predicting Speech to Background Noise at the Microphone: Click here
Understanding Potential Acoustic Gain: Click here
This is a pretty strong case for not putting microphones in the ceiling, yet there are cases when ceiling-mounted mics are successful. These systems have been designed by world-class acoustical consultants, and are usually expensive and the rooms require extensive acoustical treatment. If all other microphone mounting options have been thoroughly explored and rejected, and a ceiling system is the only option that remains, contact an established acoustical consultant with experience designing ceiling systems.
And please, I have kids to put through college. Don’t tell anyone at Shure what I said about microphones…