Is a PZM mic good for recording vocals?

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.

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...