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How a boundary effect microphone gives crystal clear sound

How a boundary effect microphone gives crystal clear sound


One of the comments on my recent article on recording drums with a single microphone pointed out that a Shure SM57 lying on the floor would act as a boundary effect microphone.

This is true, well true-ish. Since it was lying on carpet, then the reflection that is an important feature of how boundary effect microphones work would be largely absent. Even so, it is a reasonably close approximation.

I remember when boundary effect microphones first become popular, in the early 1980s. But first, what is the problem that needs to be cured?

The problem for traditional stand-mounted microphones is caused by reflections from hard flat surfaces.

If the surface is distant, more than five meters or so, then it is likely that the refection will do no harm. Indeed, it will add to the ambience of the room.

But the closer the mic is to the reflecting surface, then the stronger that reflection is – as seen by the mic – and the closer it is in time to the original signal.

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So imagine a singer singing into an omnidirectional microphone. The singer is directly facing a hard, reflecting surface, such as a plaster-coated wall, at a distance of 1 meter.

The singer's voice goes directly into the microphone, but also past the microphone. The sound rebounds and enters the microphone after a time delay of just under 6 milliseconds and mixes with the direct signal..

This is exactly the right amount of time for frequency components around 85 Hz to come back close to 180 degrees out of phase. In other words, around this frequency band, the reflected signal is the same as the direct signal, but upside down.

And guess what – the reflected signal cancels the direct signal around the frequency of 85 Hz. Actually, it won't completely cancel even at exactly 85 Hz because the reflected signal will be weaker. But still, the effect will be significant.

At 170 Hz, the reflection will come back in phase and reinforce the direct signal. At 255 Hz it will be out of phase again, and at 425 Hz, and 595 Hz, and at intervals of 170 Hz all the way up the frequency band.

This is known as 'comb filtering' because of the regular series of notches all the way up the frequency band. And it doesn't sound good.

You may of course ask why anyone would be so stupid as to be recording in this way? Well they wouldn't, but the fact is that reflections from hard, flat surfaces are all around and they often have a negative effect on a recording, even if only slight.

So what can be done about this?

Strange as it may seem, the answer is to move the mic closer to a hard reflecting surface.

Try it with a stand-mounted mic and you will hear the effects of comb filtering more and more clearly the closer you get.

But something special happens if you place the diaphragm of the mic within around 7 mm of the surface…

This distance is so small that there simply isn't time for even the highest audio frequencies to come back out of phase. So the entire audio band is reflected in phase, and it reinforces the direct signal.

So not only does boundary mounting remove the effect of the hard, flat, reflecting surface, it gives us a doubling of signal strength!

You can try this with an ordinary microphone, but for best results you need a microphone that is specially designed for the purpose – a boundary effect microphone.

A boundary effect microphone is generally made as a hard, flat plate with mic capsule either suspended over the plate or mounted within it.

The mic should be attached to a hard, flat surface to work at its best. The floor will often do just fine.

The surface also needs to be quite large. Low frequencies are only reflected by large surfaces. So if you mount a boundary effect microphone on a panel of around 1 meter square, then frequencies below around 340 Hz will not be reflected strongly. This results in a 6 dB dip in the frequency response below 340 Hz.

The net result is a sound that is amazingly clear, compared to a stand-mounted mic. It isn't suitable for all purposes but it is a very useful tool to have available.

Now let me tell you a story…

I was the restaurant of Sarm West studios interviewing recording engineer Paul Gomersall who was working on a George Michael session.

The room was quite reverberant and I wanted to get a clear recording, so I used a miniature clip-on mic to get the mic as close to the sound source as possible. This was connected to my recording Walkman, which is a handy gadget for recording interviews.

But then the programmer on the session turned up as well, so I suddenly found a whole conversation going on that I wanted to capture clearly.

Thinking on my feet, I decided that the best thing to do would be to gaffer tape the mic onto the nearby wall, so that it acted as a boundary effect microphone.

It worked brilliantly well and every word of the conversation was crystal clear.

The only slight problem was that when we had finished and I took the mic off the wall, a patch of paint came off too! I managed to sneak out quietly.

I even got to pat George Michael's dog.

Boundary effect microphones are not for everyday use in every situation. But when crystal clarity is your aim, then they are certainly worth consideration.

David Mellor

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David Mellor