John,
Sorry, a technical post but it is a complicated subject. :-)
> I also added calculations so that if one knows any two of the
> parameters one can calculate the third.
> 1) EIN (dBu)
> 2) Self Noise (dB SPL)
> 3) Sensitivity (mV/Pa)
Yes, these are comparable but only if the base definitions are the
same. I have seen different reference levels used by different
manufacturers which doesn't help with a global comparison of mics.
1) EIN is the sound level entering the mic which would be equivalent
to the total noises generated internally by the mic. Put another way,
it is the acoustic sound level which increases the mic output by 3dB
over the mic noise level. (The dB addition of equal random sounds.)
EIN provides, or should provide, a comparison between the internal
noise generated by different mics, irrespective of how they are
connected or the volume setting of a recorder.
EIN and self noise is meaningless without a weighting being declared.
Excess noise (including self noise) is measured or calculated against
the irreducable weighted thermal noise of the air, not the zero phons
level at 1KHz which I suspect is often used. It is impossible to quote
the fundamental minimum thermal noise of the air itself without a
weighting.
2) Self noise is the absolute mic output noise generated by the mic
capsule and the mic preamp stage minus the air thermal noise (which is
ususally small in comparison). The mic terminating impedance needs to
be specified as this affects the mic output level. Self noise may or
may not be quoted with the mic capsule disconnected. If the capsule is
connected, self noise is the total noise generated at the output by
the mic. With the capsule disconnected, this figure is just sales
talk.
Elecrical thermal noise is generated within the capsule circuit with
DC bias capacitor mics where the bias voltage is applied through a
very high value (and potentially very noisy) resistor (reduced at HF
by the capsule capacitance), so disconnecting the capsule makes any
self noise figure invalid. Electret mics also involve a permanently
charged electret and a high value resistor. Again, the weighting curve
used to make the measurement needs to be declared because various
noises have different frequency profiles.
The fundamental self noise of a perfect mic would be 3dB above air
thermal noise from the air behind the diaphragm. B & K claim to make
the quietist mic at 5dB excess noise, but this is a specialist
calibration mic not useful for general recording use.
3) Sensitivity is more easily measured with a calibrated sound source,
but as has already been pointed out, the termination impedance should
be specified.
Within a manufacturer, EIN values give a good equivalence which can
be used to compare mics, but between manufacturers and quality levels
such as comparing pro mics with affordable mics, we need to know more
about the measurement methods.
David
David Brinicombe
North Devon, UK
Cogito cogito ergo cogito sum - Ambrose Bierce
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