At 12:30 AM -0700 7/27/08, Eric Benjamin wrote:
>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 abou=
t
>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
>
Thanks Eric and Scott. You've got me curious. Audio Technica
describes the AT-3032's mic element as, "Fixed-charge back plate
permanently polarized condenser." Does anyone know what size the
3032' electret capsule is? Is it also in the 15-20mm range like the
EM-23? If so, Does anyone understand the probable reason for such
low noise performance from the 3032 and, if so, why is 14dB(A) often
regarded as exceptionally good FET performance for the EM-23?
Mic/Capsule Diaphragm Diameter Self-Noise
AT 3032 15-20mm ??? 8 dB(A) (E.B. tested)
http://tinyurl.com/5qlkyy
ME 67 15-20mm ??? 10 d(A) http://tinyurl.com/69f5gn
Telinga EM-23 19mm 14dB(A)
http://tinyurl.com/6f68dq (Primo Sheet)
Rode NT1-A 25mm 5.5 dB(A) http://tinyurl.com/66tr6x
Rob D.
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