What is an example of an electret?

Posted: Tue May 16, 2006 9:03 pm

An Electet Condenser mic is when a backplate and diaphram are charged by an electret material which is in the diaphram or on the backplate.

Verse

A True Condenser where the diaphram and the backplate are charged with a voltage from a curcuit built into the mic

The deffinition for electret is a material that permanently stores an electrostatic charge.

What would an electret be? Does a capacitor qualify? A capacitor stores a charge but not permanently so I don't think it would be? So what is an electret? And which mics are considered Electret Condensers and which are considered true condensers? Thanks in advance.

Posted: Tue May 16, 2006 10:54 pm

Well BigAl, that is simply an excellent question! And to be honest with you, I'm not sure what the current, popular, theyelectret substance actually is? I can tell you that any microphone that is a condenser microphone, that accepts a "AA" battery, or phantom power of other than 48 volts, is a permanently polarized electret element.

Non-polarized real condenser/capacitor microphones require a full +48 volt supply. Those microphones will not work at a lesser voltage. Years ago, a lot of professionals felt that the electret condenser microphones were of a lesser quality. Of course that is not quite true anymore. There are many wonderful quality condenser microphones out there that are back electrets. For instances the Shure SM 81 is a back electret condenser microphone and is simply excellent.

I'm really looking forward to the real answer to your question as to what the substance is that makes it a permanently polarized diaphragm? I remember reading that the Japanese pioneered the use of back electret microphones in World War II. They began utilizing them for their zero pilots. The early ones had a huge problem with humidity from the pilots mouth's. As the pilot spoke into the microphone the humidity in his breath would short it out and he would fade right out making communications unintelligible. I remember reading what the substance was that they originally used and then later discovered a better substance that was less affected by humidity.

Microphones like the Neumann U87, which are true condenser microphones also have major problems with the capsules shorting out, particularly when recording vocals up close and personal without a pop filter! Go ahead, try breathing heavily on a condenser microphone and see how much the sound deteriorates!

Lots of heavy breathing
Ms. Remy Ann David
Ooooooooo!

Posted: Wed May 17, 2006 8:26 am

Don't forget the KSM27, 32, and 44!

Posted: Wed May 17, 2006 10:36 am

AKG C2000B (99% sure).

I think there is a drawback to back electrets, that their polarisation becomes degraded in the same way that rechargeable batteries loose capacity with time. This may just be theoretical. I don't know the timescale with mics- 20 years?

Posted: Wed May 17, 2006 11:12 am

I am assuming that Prepolarized condenser is synonimus to electret? Is that correct? Because the AKG C2000B says that the transducer type is a Prepolarized condenser.

Posted: Wed May 17, 2006 10:48 pm

Arthur is exactly right.

BigAL, your assumptions are also correct.

jahtao, I have 2 pair of SM 81s. One pair from 1979 and the other from 1991 and they still all sound great! Of course anything will deteriorate over time, especially depending on what kind of usage and environment that it has been in. I have cleaned many a capsule over the years and because of the way that non-permanently polarized capsules work like Neumann U87s, KM84/5/6 works, they are essentially electrostatic air cleaners! A good reason why one should use foam pop filters religiously. I believe you should always put a condom on your microphone for protection and hygienic cleanliness. If you are bold enough to try and clean capsules, only use distilled water and fine natural hair miniature paintbrushes and go very lightly as the " sputtered gold" is only a couple of microns thick and easily removed if you're cleaning is too aggressive.

I've changed my procedure
I now use shampoo for my capsule
Ms. Remy Ann David

Posted: Wed May 17, 2006 11:15 pm

RemyRAD wrote:

...foam pop filters religiously. I believe you should always put a condom on your microphone for protection and hygienic cleanliness...

Now the foam pop filters your talking about are those the same that they use say on live news casts. The foam bubble on top of the capsule of the mic? Is there an advantage to these vs using the round pop filter infront of the mice? Or will they work essentially the same?

Posted: Wed May 17, 2006 11:15 pm

Unless I'm mistaken, the DPA 4006 microphones are technically electrets, as well

Posted: Wed May 17, 2006 11:47 pm

I am not expert, so please take this with a little scepticism.

As far as I know there are three types of condensor mics currently in widespread use:

- externally polarized. This is the type we generally use in a studio. Examples are abundant, most of the Chinese mics, almost all of the Neumanns and so on. The capsule receives an external polarizing voltage. It might come from a battery (as in the older versions of Neumann u87). It might be derived from 48V phantom simply using a resistor, a perfectly good and low-priced solution. Or it could have an elaborate step-up voltage generator to raise it to higher levels, as in say KM 184 that uses about 60V xxx text removed here xxx / gunnar.

- radio-frequency modulation mics. The only examples I know of today are the Sennheiser MKH series (I have some and like them). The mic capsule is used in an oscillator circuit around 10 Mhz and is used to modulate the frequency or phase (not sure which). This is then demodulated into sound. There are a number of advantages to this technique, as well as probably some disadvantages. One advantage of the technique is that it allows you to use normal bipolar transistors in the circuit instead of the FET needed in other condensor mics. Bipolar transistors are less noisy than FET-s, witnessed by the very low noise level of the MKH-series. Another advantage is that the mics are much less sensitive to humidity, not surprisingly they have become a favourite choice among film people recording out doors.

- electret mics. These are also called prepolarized. The word electret has sort of become tainted in the public mind with the notion that it means low quality and is often avoided. How wrong, some of the worlds best mics are electrets. The idea is that an electrostatic field is permanently burned into some material. I believe the principle is old, but it really took hold in the 1960-s with new plastic materials. Early mics could have the front membrane polarized, but the much better technique of having the back-plate polarized is almost exclusively used in mics today. Examples of electret mics are most of the DPA mics (including the 4006), most measurement mics, the AKG 1000/2000/3000 and so on. All mobile phone microphones are electrets as far as I know.

There is nothing inherently better with any of the techniques as far as sound goes, all have advantages and disadvantages. It is all about how the mic including its electronics is made. Electrets are often used in low-priced applications, say mobile phones, as they do not require the circuits to step-up the voltage. This makes the total solution simpler and less costly. But electrets are also selected by DPA as they find their characteristics to make for really good mics (and the 4003/4006 are in my mind the best small omni mic the world has on offer).

The electret material is "bombarded" with electrons during production. These will slowly evaporate from the material. Depending on exactly how pure the material is and how it is treated this is a process that may take several hundred years. I believe DPA expects their mic to survie one hundred years and not go outside the original measurement curve (ie a few tenths of dBs change in that time).

Gunnar

Posted: Thu May 18, 2006 12:20 am

Gunnar is quite correct. The DPA (formally B&K) microphones are electrets. I don't own any but I used some in the past and they are totally awesome microphones! Almost too clean for me? Gunnar is obviously quite knowledgeable. I think his understanding goes beyond mine? I was not even aware that the KM184 or the Schoeps, utilized a 60 volt polarization? Pretty cool eh?

Ouch! I guess I shouldn't lick my microphones?
Ms. Remy Ann David

Posted: Thu May 18, 2006 11:03 am

Note: I went back and edited my post. I am positively sure that the KM184 converts DC to about 60V, but even when looking for it I cannot find that the Schoeps mics does, so I removed that part. Sorry for any problems this might have created.

Gunnar

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