Who invented the electret (condenser) microphone?

Dr West, I presume. How do I know it? Kareem told me. That Kareem. The Basketball Kareem. Kareem Abdul-Jabbar. In his blog. No kidding. Quoth Kareem: “In 1962, Dr. [James] West and his partner Gerhard Sessler invented the electret microphone used in almost 90% of all microphones built today — over 1 billion a year.”

Electret (also called “condenser”) microphones are the type generally used in lavalier (or lapel) microphones. (Hello every TV anchor and guest in recent history. How do we hear you? It’s electret!) Electret mics are used in all mini-sized microphones.

I love how I came to learn of Dr. West, electret mic’s co-inventor. Though I have passing awareness (heh. pun unintended) of Pro Basketball, and have heard the name of Kareem Abdul-Jabbar, and I read a local (L.A.)  blog by Tony Pierce, who became the blog editor at the L.A. Times, and though I saw an announcement that he got Kareem to start blogging, I had no idea. I yawned, oh, it’s a sports blog. Whatever. Didn’t even click through. I had no idea that Mr. Abdul-Jabbar loves history, and has written history books. His blog bio states that he has authored “four bestselling history books intended to popularize the contributions of African-Americans to American culture and history.”

Among his posts, are African-American History month-themed posts describing African-American inventors of techy things, what with an IBM tech fellow story and the Dr. West co-inventor of the electret mic, which surely is topical to this very website. Oh, and there’s a follow-up post with Q&A, it leads off with an answer to an astute question posed by a 10-year-old.

Okay, more about those condenser microphones. In addition to lapel mics, all the miniature microphones tend to be electret microphones. So the built-in mics in recorders, whether the old cassette player or the mics in the little digital voice recorders, or the compact flash recorders such as Zoom handy H2 or Zoom H4 or the Marantz PMD 620 or other new recorders that I need to blog about, are all stocked with electret microphones. I safely say so, based on their compact size alone (in other words, I’ll say it without looking up all the product specs). Your ability today to easily record conversations owes a debt of gratitude to the work of Dr. James West. Over 1 Billion served.

So people, give him a shout out. Without him, the endeavors that I write about on this blog would not be possible. And a shout out to Kareem. if it weren’t for him, I wouldn’t have known it.

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Posted by Susan A. Kitchens on February 25, 2008 in • AudioAudio: Hardware
6 CommentsPermalink

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Comments

This is indeed interesting, but there are subtle technical items which I think deserve additional explanation.

A microphone, as we all know, converts the motion of air molecules to an electrical signal. It, like the loudspeaker (that does the reverse: converts an electrical siganal to moving air molecules) are generically referred to as “transducers” from the latin “lead across”, leading the information across a boundary from one representation to another.

Now, microphones are classified as to their basic technology. The classic ones are: carbon, dynamic, ribbon, and condenser. Each has their own plusses and minues.

The condenser microphone uses two plates separated by a minute difference. There is a charge differential across these plates. When the flexible one moves, a voltage is produced. This weak signal is made more robust and then becomes the microphone output.

There are two ways to impose a charge on the two plates of the capacitor (“condenser” is an archaic term for “capacitor” but it has stuck to the microphone application in many circles):
  - provide an external polarizing voltage
  - capture the charge permanently in the capsule
This second choice, the captured charge, is what defines an electret microphone. The electret is a subset of the universe of capacitor or condeser microphones.

It used to be that the charge would leak off in a few years, but that has been solved. It also used to be that electret microphones were inferior to externally polarized condenser microphones. That, too, is no longer the case.

The DPA 4006-TL microphone pair that I bought to be my “good pair” of microphones earlier this year are electret condenser microphones (at $1800 or so each, plus accessories). ( http://www.dpamicrophones.com ) These are full-sized microphones, not miniature. They are about the same size as the AKG C-451 microphones that I’ve owned several of since the early 1970s (prior to the commercial deployment of electret microphones). These require 9-52 V of phantom power from the mixer or recorder and generate a polarizing voltage of about 60 volts using a DC-to-DC converter inside the microphone. Many of the Neumann microphones ( http://www.neumann.com ) directly apply the 48 V phantom powering to the capsule through filtering.

In the mainstream microphone market, only Sennheiser in some of their classic condenser mics like the MKH-416 ( http://www.sennheiser.com ) use a different topology where the capsule is part of a tuned RF (radio frequency) signal generator and the sound waves change the frequency of the RF generator and that is fed to a standard FM-radio-type demodulator all within the microphone—again these are about the same size as the other two mentioned above, except the sample that is still in production has a longer “shotgun” or “interference” tube on the front to make it more directional.

Of all of the microphone types mentioned above, the highest-quality microphones in common use today are all condenser microphones (electret or externally polarized).

Some high quality applications still use ribbon microphones. This works by a non-magnetic ribbon being placed in a strong magnetic field. As this ribbon moves with the field, a small electric current is generated.

Dynamic microphones are found mostly in the more inexpensive (and rugged) lines for musicians and broadcasters. Dynamic mics are like speakers in reverse. There is a coil of wire placed in a magnetic field. As the moving air molecules move the diaphragm which moves the coil, a small electric current is generated. This is then amplified in the mixer or recorder.

The big difference between ribbon and dynamic microphones since they are essentially similar in technology (conductor cutting a magnetic field, movement generating current) is in the mass of what is moved. The ribbon is a very light weight assembly while the dynamic is a heavier assembly which does not follow the moving air molecules quite as well.

The caution with ribbon microphones is never blow into them as you’ll stretch the ribbon beyond repair.

Carbon microphones used to be what was in your telephone handset (although in the early days, better units were used for radio broadcasting). Carbon microphones actually change resistance in proportion to the sound waves, but are relatively noisy and high distortion.

Richard L. Hess  on 02/25  at  11:38 AM

Richard,

Thanks for pointing out that electret microphones comprise more than the “small” ones I alluded to. I took the intro to audio class at PCC and learned the basic differences between the microphone types, but I don’t know them in my heart of hearts, mind of minds, gut of guts (yet). Well, and I wanted to keep the post short and make my noon appointment and write a post, too. I appreciate your detailed description!! Many thanks.

Susan A. Kitchens  on 02/25  at  01:53 PM

But I found this..

“Lars J. Stenberg of Roskilde, Denmark, has developed an electret condensor microphone preamplifier that is insensitive to leakage currents at the input.

According to the U.S. Patent & Trademark Office: “A preamplifier having extremely high input impedance amplifies the electrical signal output from an electret condenser microphone without suffering from the effects of a direct current (DC) leakage current at the input. The preamplifier circuit includes a pair of cross-coupled positive-negative junction diodes setting the input impedance, a Positive-type metal-oxide semiconductor field-effect transistor device, and a load resistor configured similarly to a ... “

Is it the same device?

Electric Geek  on 05/06  at  05:36 AM

The electret is part of the actual capsule that changes acoustical (mechanical) vibrations into an electrical signal. The preamplifier you cite turns the electret capsule’s fragile signal into one that can be sent over a 100m cable or more.

Richard L. Hess  on 12/01  at  09:22 PM

I work with Dr. James West’s niece.  She told me that Dr. West works at John Hopkins in Maryland and I can contact him through the Engineering Department. I would like to e-mail him, do you have his e-mail address?

Linda Kozden  on 05/21  at  07:01 PM

Linda, the only thing I know is what I read on the site I linked to. I don’t have any contact information.

Susan A. Kitchens  on 05/21  at  08:59 PM

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