Audio experts on microphone levels and pressure zone mics

-January 10, 2017

Many engineers and audio people don’t realize just how much signal can come out of a microphone. A hot mic with large signal can output many volts. That a dynamic microphone does this with a 120 ohm impedance is all the more remarkable (Figure 1). My pal, audio guru Steve Williams, recalled an entry in the Ampex Audio Digest bulletin board system (BBS):
“The circa 1968 demo for those who didn't understand how much level could come out of a microphone (like those who were still trying to do rock-n-roll with Altec 1567 mixers with no input attenuators): Take your Simpson 260. Set it on the 2.5 VAC range, and connect it to the output pins of a Shure vocal mic. Hand the mic to someone with a good set of lungs, and have them do a good rock-n-roll scream. I never saw anyone pin the meter at 2.5 volts, but 1.5 volts was easily achieved.” anon

Figure 1 This plot shows microphone levels of a speaking voice can easily reach 0.1 volts. Shouts, screams, or loud music can be 20 times greater. (Source: Clifton Laboratories)

Now I see why audio amplifier guys like ±15 volt power for those microphone pre-amps. When my buddies at the National Semiconductor audio group (now Texas Instruments) wanted a 44-volt process for the LM4562 amplifier, the process group fought them, saying all they had characterized was 40-volt process. Undaunted, the audio design and apps team winnowed down the number of devices, and then ran characterization tests for that limited set of devices at 44 volts. The apps people also insisted the part be offered in a metal can package, as well as DIP and SOIC.

The part was a great success and many engineers used it for industrial or scientific or medical (ISM) applications. With a hermetic metal-can package it might find applications in high-reliability military and telecom uses.

While talking about microphones with Williams and our friend Vini Carter, we got on the subject of pressure-zone microphones (PZM). I had heard about these when I read Frank Zappa’s autobiography. Zappa used these to record symphony orchestras and noted the musicians did not take the mics seriously since they were just little plates he glued to the wall or floor.

The theory is pretty compelling. If audio standing waves create “hot spots” of various frequency at various places in the room, just put a mic on the wall or floor, where there can be no standing wave. I sent Vini Carter a picture of a PZM I bought at the Silicon Valley eFlea years ago (Figure 2).


Figure 2 This Crown PZM-6S pressure-zone microphone even has screw-holes so you can mount it on the floor or wall.

To my delight, Carter was well aware of the technology, writing back:
“I was part of the class at Syn-Aud-Con that helped in the development of the PZM back in 1978. The technique was first shown to us by Ed Long and Ron Wickersham. We were all aware of the problems of reflected sound combining with direct sound to cause combing interference to the frequency response. Not only did PZMs eliminate that, it also gave any microphone 3dB more output. We were given tiny Knowles microphones and instructed to take them home and experiment with different configurations. Knowles made tiny omni, cardioid, and even bi-directional microphones.”

“I only had two issues with this technique. Firstly, the low-frequency response is limited by the size of the boundary you use. The frequency response for a symphony orchestra requires a boundary 30 feet high and 30 feet wide. Secondly, they couldn't be used as a co-incident pair. Any advantages gained from eliminating the first reflection would be lost by using a spaced pair of microphones. That having been said, I have used the PZM technique with various mics, often laying them on a piece of cloth or foam like the Electro-Voice "mic mouse" on the floor, or on a conference table (Figure 3).”

“Back in the day, when we did the Golden Circle Shows, we used as many as 32 channels plus a sub-mix for the strings. Each instrument in the orchestra had a microphone, plus wireless mics for the entertainers. It was not my idea of a good time. Steve manned the patch bay, I operated the board, and Bob and Cathy Gunter routed the mics on stage to various snakes leading to the booth. I have tried to avoid that form of crazy life ever since.”

“Most of the orchestral recording I do is co-incident M-S (mid-side), and the M mic is an omni, not a cardioid. The S mic is always a ribbon, even though I own an AKG large-diaphragm differential condenser. I place the mics very close to the front of the orchestra because of the omni M, and quite high in the air. Unlike a wind ensemble, an orchestra does not have enough space to "come together" at the conductor's ears; the group is just too wide and too deep into the back of the stage. Extra mics are for special, more complicated programs i.e. children's choruses, narrators, non-operatic vocals, and very soft instruments like Native American flute or classical guitar.”


Figure 3 This Electro Voice “mic mouse” foam pad can cushion a PZM microphone, perhaps on a conference table. (Source: gearslutz.com)

Well there you have it, not one but two audio gurus with some cool tips and history. While on the subject of levels, I should mention how astonished I was when I hooked a scope up to my Stratocaster. Same deal, signals way over a volt. I was trying to design a JFET amplifier (Figure 4), but I realized I needed more of an attenuator than an amplifier. Unlike a dynamic microphone’s 120-ohm impedance, a Strat pickup measured more like 80,000Ω, so a JFET buffer is a good idea. You can get discrete FETs from my pals at Linear Systems, while Analog Devices and Burr Brown make good JFET amplifiers, as does Linear Technology.


Figure 4 These JFET amplifier prototypes would work well with the high-impedance output of a Stratocaster guitar. You don’t need much.

Oh, and that Zappa book had a quote from Gustave Flaubert: “Be regular and orderly in your life, so that you may be violent and original in your work.” I am still working on that, so are most of my hoarder friends.

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