Any point in getting a contact mic?

[dafduc]

Any of you guys use 'em?
If so, why?
What am I missing by not having one?

[Michael Jones]

A contact mike?
Is that like a boundary mike?

[dafduc]

Same thing - aka PZM.

Here's what Microphone University had to say:

Direct sound waves meeting a hard plane boundary will be reflected at the surface causing a 6dB acoustic pressure increase. A microphone diaphragm will therefore have a 6dB higher sensitivity when placed in the pressure zone, than an equal microphone placed in the free field. Diffuse sound will not be reflected at a plane boundary as diffuse sound has no direction (per definition). The PZM microphone will therefore give direct sound 3dB higher level than diffuse sound. If used outdoor PZM microphones have better conditions than conventional microphones with regards to wind noises, as the wind velocity in principle is 0 (zero) on the surface of the ground.

[Michael Jones]

I have a CAD STS100 boundary mike. I bought it a long time ago to use on piano. It has a REALLY HOT signal. I was dissappointed with it on piano. Didn't sound too good at all. Since then it's basically sat in the locker. I was thinking of using it as an overhead though, but I haven't tried it yet. It might work well as an overhead for a small classical ensemble.

I don't think you're missing out on much by not having one, but, you know, always nice to have choices.

[Harvey Gerst]

This is a weird topic.

Unless they've changed the name of the concept, a boundary mic ("Pressure Zone Mic") is not the same thing as a "contact" mic.

Contact mics picks up sound via direct contact with the source, such as an under the bridge piezo pickup on an acoustic guitar. Some older guitar contact mics had a built in suction cup to stick on the top of a guitar.

During WWII, pilots used contact mics placed directly on their throat to pickup only their voice and prevent wind and engine noise from drowning out the signal.

Most contact mics have poor frequency response and low output, making them pretty unusable for anything other than an unusual effect.

[chessrock]

Originally posted by Harvey Gerst
During WWII, pilots used contact mics placed directly on their throat to pickup only their voice and prevent wind and engine noise from drowning out the signal.

Wow. A DI for vocals. Cool.

[darrin_h2000]

I have a pair of modded PZMs and with them taped to the lid of a piano and the cover almost down and a blanket over the top does give me results.

Screwed to the ceiling they are good for overheads, and with the plexiglass connected at a 90 deg angle pionting to the 12th fret they are good for acoustic guitar.

Sit one on a pillow inside a bass drum its useful there too.

But not as good results as mikes designed for these uses, and if the room suck so will the results.

[Michael Jones]

I'm not sure that a PZM mike is the same thing as a boundary mike either.
I could be wrong, but I think they are 2 different animals entirely.

[Harvey Gerst]

While they are somewhat different, both use a plane surface to insure that the direct signal and the diffuse area signals (reflections) arrive at the capsule around the same time.

A PZM (Pressure Zone Microphone) is basically an omnidirectional mic pointed down close to a flat plate. A boundary mic is the same thing with the capsule pointed up.

Both use a large plane area to insure that the diffuse and direct signals are coherent. You can use other types of capsules in a boundary microphone to get different polar patterns.

You can make your own PZM by simply pointing something like an ECM8000 into a large piece of glass or any hard material, and spacing the element a short distance (1/16" to 1/8" away) from the flat plane.

You can cut a small hole in a large plate and stick the mic in from the back side to make it a boundary mic.

While the results and theory are similar, they are not "exactly" the same thing. Sorry for not making that clearerin my other post.

[Scooter B]

Bless you Harvey,

You certainly have extensive knowledge, an interest in teaching and a real gifting in being able to explain that knowledge to those of us without understanding.

Thanks for all your sharing of knowledge.

[Rossi]

I recently built my own PZM using those Panasonic measurement capsules (omni), so I did some research. There is a pretty good introduction into boundary mics on Crown's website, it's called "Crown Boundary Microphone Application Guide." According to them the capsule ought to be *very* near to the boundary to avoid treble rolloff and comb filter effects in the upper frequencies. The distance should be no more than about 1/8 of the highes wavelength. For 20 Khz that would be 0.85". Crown's own spacing is only 020". So you might wanna keep that in mind for your own PZM experiments.

BTW. my PZM sounds pretty good, but the noise level is no better that of my Radio Shack PZMs (when 12V powered). But then again, I'm still working to improve the beast.

[Michael Jones]

Originally posted by Scooter B
Bless you Harvey,
...being able to explain that knowledge to those of us without understanding.

HEY! I resemble that remark!