Can I use regular omni as boundary/PZM mic?

[logey]

I've always loved playing with odd approaches to micing and have often thought about picking up a Crown or some-such PZM and playing around with it. Before I buy one though, I'd like to try one out...towards that end will I get the same or similar effect if I just tape a small diaphram omni to the wall? I built a couple of the Tape-Op omnis a while back and I could use one of those. I will experiment regardless as I'm now curious, but I would be interested in knowing the difference.
Thanks

[googacky]

read this thread from the Electrical Audio forum. It'll enlighten you.

[jca83]

that thread is very clever... and yes, putting omnis on the floor simulates the pzm/boundary experience VERY well. it's often a lot better, because you can use a lot nicer mic than what a regular boundary mic would be.

[mfdu]

totally fully. a big omni - i used to aim for about a foot or two in from the top corner of the room.

when i use a PZM it is usually as a poor cousin of the omni technique.

chris.

[Professor]

If I recall correctly, for it to be an actual "pressure zone microphone" the capsule should be facing towards the boundary (that is, looking down at the floor) and be placed within some critical distance which I believe is no more than something like 0.3 inches from the surface (which would make sense as 1/2 of a 20kHz wavelength). It then creates the very special circumstance and behavior of a PZM. Of course, if you try it out, you'll find that it can be pretty effective if the mic is facing the other direction (Audio technica makes some ceiling tile & in-table microphones for tele-conference use) or facing sideways. The distance to the surface is still important for minimizing phase distortions below 20kHz (20kHz has a wavelength of 0.678 inches) but the sideways capsules allow for different patterns in boundary mics. Crown makes a half-cardioid and half-hypercard. boundary mic, and MBHO makes the really cool 648-PZ which is a changable capsule boundary that accepts all their standard capsules (I have a pair, they're cool).

So if you have something like a pair of nice omnis, try pointing them at the floor from a distance less than 0.3 inches, and then try taping them to the floor sideways, and see what you like. I do recommend putting some kind of tiny barrier at the floor whether a thin piece of cloth or even just a layer or two of gaff tape to cushion the mic a little - it will prevent any dirt particles from vibrating against the mic body, and keep you from scratching up your nice mics.

-Jeremy

[joel hamilton]

Mic the reflection, not the source....

That is the fun thing about the technique. It "sounds closer to the mic" because the mic is recording the reflection from the boundary. You can rotate the mics to phase grate across the spectrum with various levels of success. Have a crashy drummer? rotate the mic until you get a little 5k notch. Too much boxy kick in the room mics? Rotate until you get a nice little drop at 220 (ish).

Phase relationships take the place of EQ. Even the relationship of the capsule to the wall, or floor, or whatever... other mics obviously....

[drumsound]

Mic the reflection, not the source....

That is the fun thing about the technique. It "sounds closer to the mic" because the mic is recording the reflection from the boundary. You can rotate the mics to phase grate across the spectrum with various levels of success. Have a crashy drummer? rotate the mic until you get a little 5k notch. Too much boxy kick in the room mics? Rotate until you get a nice little drop at 220 (ish).

Phase relationships take the place of EQ. Even the relationship of the capsule to the wall, or floor, or whatever... other mics obviously....

Isn't most EQ just screwing with phase electronicly? If so, moving the mic for does make a lot of sense in theory. We already know it works in practice.

[logey]

Thanks for all the feedback...sounds like in many ways I may be better off with a regular omni (or card?) than spending a bunch on a mic that can only be used as a boundary...

Mic the reflection, not the source....

You can rotate the mics to phase grate across the spectrum with various levels of success.

Joel...when you say "rotate the mics", are you referring to changing the angle of the mics to the boundary?

[joel hamilton]

Thanks for all the feedback...sounds like in many ways I may be better off with a regular omni (or card?) than spending a bunch on a mic that can only be used as a boundary...

Mic the reflection, not the source....

You can rotate the mics to phase grate across the spectrum with various levels of success.

Joel...when you say "rotate the mics", are you referring to changing the angle of the mics to the boundary?

Yes.

You wind up hearing the phase grating, like a notch filter sweeping across the sound as you hit certain perfect cancelling distances.... The phase between the boundary and capsule.... and reflection off the mic body... etc...

Try it, it will be obvious.

[kayagum]

Every once in a while, I read a thread like this one that completely opens up new recording possibilities.

It's threads like these that make TOMB so invaluable. Wow.

[Bill @ Irie Lab]

Not a PZM but a quick "field expediency" boundary microphone using a spare front-address omni.

Cut a hole in a large (3'X3' or 4'X4') sheet of thick packing foam the diameter of the mic body. Back the mic through the foam until the capsule is flush with the plane of the sheet. Secure the rear with more foam and rubber bands.

Presto! a large format hemispherical response boundary mic. Tape the gizmo to a couple of mic stands or lean against a chair facing kit, amp, or ensemble.

If you like the outcome make a more permenent rig using sheet rock and more durable stand and mic mounting hardware.

Good clean fun!

[Matti]

I had Crown 100?s instead of their PZM?s as allround plant mics in my documentary kit for Film/TV back then. I tryed to rig them like PZMs, 1 mm
from the surface and it was OK, and I had 4 of them in my mixers pocket!
Matti