David,
Thanks as always for your generous sharing of your knowledge and insight and
your insistence on clarity of terminology. You have helped me in this.
Unfortunately I know next to nothing about the technical details of all this,
have been groping my way to limited understanding, and am more likely to sow
confusion than clarity, and perhaps have already done so!
When I have thought about what microphone noise means, it has gone something
like this: If one were to stick a microphone in a vacuum, so that there was no
air movement affecting the capsule, what would the electrical output, in mV, be
from the microphone? In other words, what is the output that can be attributed
to the microphone's own circuitry?
Or, perhaps it is more accurate to think of it, as you describe, not as a
microphone in a vacuum, but as a microphone that is receiving random motion of
air molecules, but no pressure waves.
Whichever the case (how much of a difference does it make: vacuum vs. random
air molecules?), that output should be measurable, and can be expressed as mV
or as dBu, which is mV referred to 0.775 volts.
>From that noise output voltage, if one also knows the sensitivity of the
>microphone -- the voltage output from a known sound pressure level -- one can
>then express the microphone noise as an equivalent sound pressure level, 13dB
>SPL in the case of your MKH-416, 10dB SPL in the case of my ME-66
>(Sennheiser's product sheet calls this the "equivalent noise level." Rode also
>calls it "equivalent noise." AT just calls it "noise").
What one needs to know for the purpose of selecting a preamp that won't add
audible noise to the microphone noise is the mV output level of the microphone
(output in the absence of any signal) and a similar value for the preamp. Since
manufacturer's rarely give that number for the microphone, it must usually be
calculated from the values given for the "equivalent noise" or "self noise" or
just plain "noise," and the microphone sensitivity, both of which must refer to
94 dB SPL = 1 Pascal for any of this to work.
The "noise" value is given in dB SPL, which refers to 94 dB SPL = 1 Pascal. The
sensitivity refers Pascals to mV, and thus we arrive at the mV value we need.
Because preamp noise values are often given in dBu, we then convert preamp dBu
to mV so we are working with voltages that can be summed.
Happiness is when the sum of the mic output noise (in mV) and the preamp noise
(in mV - actually the square root of the sum of the squares of the values) is
less than 1 dB greater than the output noise of the microphone alone (or the
noise of the preamp alone in the unusual case of a preamp that is noisier than
the microphone).
Is there anything wrong with how I am thinking about this or how I have
described it?
Apologies to all of you who already understand this and have heard it a
thousand times and are sick of it!
John
John Crockett
Westminster, Vermont
--- In "Avocet" <> wrote:
>
> > Sorry, David, but EIN has nothing to do with microphones. It's the
> > noise of the mic preamp when terminated by the specified source
> > impedance (150 ohms). The Rane article follows.
>
> Dan,
>
> What we are discussing is comparing mics for noise. EIN can be
> expressed as the equivalent input whatever the energy type or source.
> When it applies to mics, it is referred to an equivalant sound input
> energy level. This shows the importance of being precise with
> definitions, something mic manufacturers are not good at. :-)
>
> "Excess noise", or "Equivalent noise level" is the EIN compared to the
> fundamental noise of the air. As an example for my MKH-416 mics it is
> 13dB with A-weighting. What this means is that the mic noise from
> capsule to output is 13dB higher than the thermal noise of the air
> itself as it would be heard through a perfectly noiseless mic,
> whatever the gain settings.
>
> The reason for using EIN sound level values is that it is difficult to
> define the sound energy entering the mic. The EIN sidesteps thia by
> using a known sound energy reference level so that both noise and
> signal levels effectively have the same signal path.
>
> Regarding gain, this can be the signal energy positive gain across an
> amplifier or negative gain across an attenuator. With a transducer
> such as a microphone, the gain can be defined across different types
> of energy such as sound energy to or from electrical energy, in other
> words with mics or loudspeakers. A tungsten lightbulb has a gain
> (efficieny) from electrical input to light output of around -30dBs.
>
> Decibels are a measure of energy no more or less, across any energy
> exchange. The difference between sunlight and moonlight (with
> frequency weighting from the atmosphere and our eyes) is 56dB.
>
> David
>
> David Brinicombe
> North Devon, UK
> Cogito cogito ergo cogito sum - Ambrose Bierce
>
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