What follows may be off topic for the majority of nature recordist
and for that I apologize.
Scott Foster wrote:
> I think it might be useful to point out here that "lavaliere" or
> miniature capsule microphones & "electret" are not synonymous, as has
> been implied in previous posts to this thread.
While it's true that some studio microphone are externally polarized and
some are electret,
so far as I know all lavalier microphones are electret types.
Eric wrote:
>> I don't believe that the performance of most of these microphones
>> is limited at all by the physics of microphones.
Scott Foster wrote:
> The noise vs diaphragm size question is not controversial nor a
> subject of debate within the the community of those who design,
> modify or repair condenser mics. Have you a scientific rebuttal to
> what is accepted knowledge on this?
First, let me ask you to note that I began the statement above with
"I don't believe...". That's another way of saying that it's my opinion.
And
when I said "these microphones", I was referring to the ones that are about
6 mm in diameter that are frequently used in small microphones like the
ones that were under discussion. But there is a reason for my belief.
A typical argument that is used to support the observation that
microphone noise scales with diaphragm size is that, if there were two
identical microphones and their outputs were summed together, then
the output voltage would go up by 6 dB and the noise would go up by
only 3 dB, thus giving an improvement of 3 dB in signal to noise ratio.
And this is what is observed in practice. Since this is a doubling of area
it would be reasonable to expect that micrphone SNR would increase
by 3 dB for each doubling of area and by 6 dB for each doubling of
diameter. Scaling from your post, in which you observed that the best
studio microphones have a self noise of about 6 dBA for a large diaphragm
(I'll assume that the diaphragm is 1", or 25 mm), we would expect that
6 mm microphones could conceivably have a self noise of 18.4 dBA.
So by that argument, we might hope, or wish that 6 mm lavalier
microphones would have a self noise that low. But they don't, at least
not yet. But we can hope.
In a smaller size class, there is the Knowles FG-3629, which has a
specified noise level of 25 dBA. The FG is 2.5 mm in diameter, but it
has a 2 mm diaphragm. Scaling from the 6 dBA 25 mm diameter gives
a target of 28 dBA, which the Knowles handily beats by 3 dB. Four
of these would fit inside the case of a typical 6 mm electret, and would
give a self noise of 19 dBA.
But here is another argument. There are special purpose low-noise
microphones made by Bruel & Kjaer (the type 4179) and by Gras
(the type 40HH) which perform far better than the scaling given above.
The 1/2 inch (12.5 mm) Gras 40 HH has a self noise of 6.5 dBA, and
the 1" B&K 4179 has a self noise of - 4 dBA, or 10 dB better than
that of our reference 1" studio microphone.
Obviously B&K and Gras went to extraordinary lengths to get that
noise performance, but it shows that it IS possible.
Eric Benjamin
"While a picture is worth a thousand words, a
sound is worth a thousand pictures." R. Murray Schafer via Bernie Krause
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