What is the use of Boundary/PZM Microphones?

Not long ago I was delivering a lecture introducing a group of 1st year students to a variety of microphones. I produced a Shure MX393C from my bag and placed it on the table, at which point a voice from the back of the room declared, ‘That’s like the ones they have down the police station!’  Not only was this a great ice-breaker for my new students, creating a good deal of merriment, but it also gave me an excellent intro into talking about boundary microphone techniques. Apart from imparting excellent clarity to the proceedings down the cop shop, these versatile and unobtrusive microphones have many other uses.

Shure MX393Cs at 90 degrees

Stereo pair of Shure MX393Cs on the floor of a theatre stage, set at 90 degrees

There are occasions when the huge, reverberant acoustic of a medieval cathedral can greatly enhance a performance. Choral music often sounds particularly impressive, floating in the vast empty space. However, there are also occasions when the blurring effect of multiple reflections can ruin the fine detail of a recording:-

CLICK HERE to listen to 2 very different versions of the same harp recording.

The first version, (though very grand), is considerably blurred by out of phase reflected sound arriving at the microphones. The mics used in version 1 are an M-S pair of AKG C414BTLs mounted on a stand 2-3m in front of the performer. On the floor at the base of the mic stand I also placed a pair of Shure MX393C (cardioid) boundary mics set at 90 degrees. Playing back this 2nd version of the same performance, recorded on the MX393s, it is hard to believe that this is the same concert! The reverberation of this enormous echoing space has all but vanished and the resulting recording is clean and sharp, and gives the impression of being much closer to the performer.

Sometimes known as Pressure Zone Microphones (PZMs), boundary layer microphones exploit a number of acoustic phenomena which occur at the point where sound hits a large, flat, reflective surface such as a floor, wall, or ceiling. Smaller surfaces such as tables can also be used, but may limit low end frequency response. If you are interested in understanding the theory behind these microphones and techniques for using them, this article from Crown Audio is a must read.

crown-guide-to-boundary-mics

Apart from concert recordings in difficult acoustics I have also used my Shure MX393s fixed to the inside of the lid of a grand piano. This works well, without the boominess often associated with close micing. Even works with the lid shut, giving excellent separation. Close to the front edge of a theatre stage facing the audience they are great for recording audience reaction. Facing up stage I have often used them, with excellent results, to reinforce tap dancing, drama, and singing.

If you are looking for a great bass drum mic the Shure Beta 91 is another boundary model  worth checking out. Either inside the drum or on the floor immediately in front it really delivers some punch. Particularly good for rock or metal.

On a number of occasions I have arrived at a venue to record a concert only to be told by the promoter that I am not allowed to have microphones in view of the audience! A representative from English Heritage once told me that microphones were not in keeping with the building! Mmm……….. The options for a sound engineer at this point are somewhat limited! Once you have abandoned the idea of punching the promoter it may be that, well placed, and unobtrusive boundary mics could save the day!

Technical Specifications for Shure MX393C

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4 responses to “What is the use of Boundary/PZM Microphones?

  1. I have one piece of MX393. What is best place inside the piano to record it with this (one) microphone?

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    • I have used one MX393 on a grand piano using gaffer tape to fix it to the under side of the piano lid. It was positioned almost in the centre of the lid pointing towards the keys, so that when the lid was closed (taking care not to crush the cable) the mic was about a foot back from the hammers. The exact position will depend on what sort of sound you want. For instance if you want more sound of the hammers striking the strings you may want to move the mic forward towards the keys. Also the balance of the left and right hands my be adjusted by moving the mic from right to left etc. I used the MX393 with the piano lid shut in order to gain separation from the band. It worked very well producing a nice clean, balanced, sound. However, I would imagine it might also work well with the lid of the piano open.
      Good Luck!

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  2. I listened to the Sound Cloud example. Remarkable. Very useful. Can the PZMs be used for recording a drum kit?

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    • Hi Ray,
      PZMs can work very well recording a drum kit. You may need to experiment with placement and distance. I have used a pair on the floor, one on either side of the bass drum,about 6ft apart, a couple of feet out from the kit. You could also try them as a crossed pair at a distance of a several feet in front of the kit. However, in that position you may end up with too much bass drum (depending on the drummer!) Certainly in a lively room PZMs will pick up a lot less of the unwanted reverberation. Anyhow, I’ll be interested to know how you get on.
      All the Best
      Martin.

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