> I have many other formulas for converting dB to mV/Pa and converting
> between dBV and dBu for example, but these are the two I have been
> using a lot to educate myself about how these parameters interact.
Just to be picky, mv/Pa cannot be "converted" to dB, but you can refer
it to phons at 1KHz as 2Pa SPL corresponds to 100phons (1Pa is
94phons) unweighted. We can't hear 0phons so that is only a
theoretical reference. NB the phon is what we hear but is only defined
at 1KHz.
Noise figures are the most difficult to pin down as there isn't a
simple quotable fundamental noise level to refer to without quoting
weighting. The fundamental thermal air noise is proportionate to
frequency at the rate of 3dB per octave, so any noise level has to be
against a weighted reference. Thermal noise at exactly 1kHz is 0Pa SPL
or -infinity phons as the bandwidth is zero. See the problem?
Manufacturers like to use A-weighting as it gives better figures, but
the best weighting is ITU-R 468 which actually corresponds to what we
hear at low levels.
EIN figures are referred to (generally) A-weighted thermal noise, but
I suspect that some are referred to 0phons or 20microPascals at 1KHz
which is not the same thing and the wrong frequency.
I was taught that "dBm" was the reading on a meter, specifically a VU
averaging meter, referred to 0.775 volts RMS sine wave. Most sounds we
record are not sine wave, and most meters now are peak reading, but we
can forget the difference in practice, but it does mean a few dB
difference in relative noise levels. Audacity shows both in the
waveform display. If you want to be really confused about various
decibels, read Wiki. It needs rewriting.
As an aside, compression raises the average level compared with the
peak level and is used to make adverts sound louder.
What really matters is what a mic sounds like to our ears, and specs
are a guide, but not a reliable rule to go by. The beleivability of
noise figures is dependant on the price bracket.
David
David Brinicombe
North Devon, UK
Cogito cogito ergo cogito sum - Ambrose Bierce
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