Gregory Kunkel wrote:
> Marty
> I liked your inclusion of a superposition graph so
> much that I included it on my page.
> http://www.akwebb.com/gkweather/ambient/selfnoise.html
>
> Do you think these graphs have much use in comparing
> different microphones?
If you want to get into that you would have to do some sort of
standardization of the gain of your recorder (or whatever you have
between mic and getting a measurement). It would probably be pushing it.
In the official version the mic is exposed to a standard sound level
(usually 94dB) in a soundproof chamber. and the gain to the meter
adjusted to give a reading of OdB. Then the sound source is turned off
and the reading of the meter gives the self noise. Anyway, that's a
rough description. Often this is done just at one frequency, though as
you have seen it's variable with frequency. There are published
standards for how to test, but not always followed. And there are
various ways you can report the results. I prefer the raw reading in the
dBA scale. The S/N is usually, but not always, the difference between
the raw reading and the test signal reading. But often they don't report
the test level.
Comparing different microphones you can often find the noise spec. At
least on the upper levels of mics. Then you only have to check how that
manufacturer did it. Usually a individual mic manufacturer will test the
same for all models so comparison between his models is fairly
reasonable. Between different manufacturers it's more iffy.
Note getting into worshiping noise specs is going overboard. Take them
as approximate guidelines. There are a lot of other characteristics of
mics that may be more important.
Note also, the raw numbers don't tell you what the self noise sounds
like. Some mics have much more noticable self noise sounds than others,
crackles, buzzing or such like. A smooth hiss is usually the least
disturbing. The pillow test is very good for examining this. Since in
some of your recordings the self noise will be unavoidable, it's well to
know what it sounds like, how much you can let it intrude.
Self noise can also be considered some measure of the quietest
environment the mic can be used in. A good part of the time the base
noise of the environment exceeds that of the mic.
> As an old retired high school physics teacher, the
> discussion of transverse and longitudinal waves makes
> me want to break out the wave tanks and slinkys and do
> some demos.
>
> I'll resist the urge.
You will probably enjoy the modern version:
http://www.kettering.edu/~drussell/Demos.html
Some neat stuff there. There's probably some other stuff on the web too.
You had wave tanks? My HS only had string and wire, and it was before
slinkys. Though we did have springs. In fact my College physics did not
have wave tanks either.
Yes, resist. Get out there, point a mic at things and be happy with what
you get.
Walt
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