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Is a PZM mic good for recording vocals?

Is a PZM mic good for recording vocals?

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A recent question received by Audio Masterclass went something like this…

“I was talking to someone the other day about recording vocals – as most audiophiles do – and was told that good quality recordings could be made with a PZM fixed to the wall just above head height. The singer feels a bit odd singing to a wall but you can hang the words and notes on the wall. I said that while I understand the ability to get good sound in theory from a PZM, a singer who feels odd and has to read words isn't going to sing well but if they got used to singing to a wall and knew the song well, do you think a PZM would be suitable in a booth? In an open room I guess it would pick up background sounds really well.”

Firstly, what is a pressure zone microphone? Invented, or at least first popularized, by Crown, it puts the diaphragm of the mic very close to a flat metal plate that is intended to be mounted on a larger flat surface, such as a wall, rather than a conventional stand. This type of microphone is now more generally known as a boundary effect microphone.

So what is the boundary effect? It concerns a sound wave mixing with its own reflection. This will cause interference resulting in comb filtering.

Comb filtering, as you can see, causes a number of nulls in the frequency response, usually more than shown in the diagram. Clearly these nulls are far from desirable.

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The nulls are caused by reflection from the surface, but suppose that the surface is very close to the diaphragm of the microphone. In fact, within around 7 mm or so, something wonderful happens – the reflection strengthens the original signal, with no nulls.

In practice the characteristic sound quality of the PZM is very clear, and clear over a greater distance than conventional microphones.

But going back to vocals, it is likely that the sound of a PZM would be rather too clear, resulting in a subjective thinness in the voice, in contrast to today's fashion for a very full, rich sound.

For other instruments though, the PZM offers a different sound quality and makes an interesting stereo pair when spaced apart. When you are bored with conventional mics on stands, try a PZM or two and experiment with their unique sonic character.

And by all means try them on vocals. Nothing ventured…

David Mellor

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David Mellor