> Given microphone specs typically include either self-noise or SNR
> figures, though never that I have noticed "excess noise" figures,
> for practical purposes how do we determine excess noise?
John,
As I am a Sennheiser fan, I'll name them as quoting excess noise
figures. It is also used by B&K for their instrumentation mics which
incidentally, despite their price, are not necessarily good recording
mics. It is also a BBC Engineeering parameter. Thermal noise rises at
3dB per octave and so sounds as hiss and also really needs the 468
noise weighting. This however looks worse than the A-weighted figure
which is thus used for non-technical sales, but this peaks in the
lower KHz where the ear is less sensitive to low level noise.
If you look at a power spectrum of mic noise, you can sometimes see an
HF tip-up of 3dB per octave in the higher KHz. That is thermal noise
but not necessarily air thermal noise. While I'm being nerdy, the
absolute bottom limit of excess noise is 3dBs caused by air molecules
on both sides of the diaphragm. B&K claim a World record of 5DB.
Sennheiser claim around 23 dBs which includes a 468 weighting lift of
11 dBs making the absolute thermal excess noise about 12dBs. Their low
frequency absolute "self noise" is higher but in practice is less
noticeble.
> for practical purposes how do we determine excess noise? What do
> these figures look like when included on a spec sheet, how do we use
> them to determine the qualities of a mic compared to using
> self-noise figures?
I can't quote the exact figure for the thermal noise of air off hand,
but at 10 kHz it is near 0phons (20 microPascals SPL). Most people
cannot hear it but I claim to have heard it in my youth. :-) Excess
noise is simply the microphone "self noise" level in reference to this
fundamental point. The problem with "self noise figures" is that they
don't state a direct reference level and also use loud-sound
A-weighting. A quoted signal-to-noise ratio for a microphone is even
more nebulous.
Upsum for all of the above - beware of any noise figure and rely on
your ears. :-)
David
David Brinicombe
North Devon, UK
Cogito cogito ergo cogito sum - Ambrose Bierce
"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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