From: "Volker Widmann" <>
>
> This a an interesting thought.....
> I recently read somewhere that even in recording studios there usually is a
> background noise of 20 dB.
> I wonder where that comes from....
>
> But seriously, can anybody verify this?
Sure, measure it with a very good sound meter. Note even sound meters
have limits as their mics have self noise too. You will not be able to
measure sounds quieter than the sound meter's mic.
> Maybe this is not a new discussion, but is there really any acoustic
> environment without any noise?
There is no zero in the noise scale, at least no point that's no sound,
it just keeps getting quieter and quieter. There is a arbitrary zero,
but that's supposed to be the minimum level that humans hear. (in
reality few humans do near that well)
> Are the mic self noise specs in relation to zero noise or to the the
> background noise of a laboratory?
Mic self noise is measured by exposing a mic to 94 dB sound. Then the
gain on the signal coming off is adjusted to max on the metering 0dB.
Then the sound is turned off and the self noise can be read off the same
meter. Signal to noise is the difference between 94 dB and that. The
other way expresses the level against the human scale. ie subtract
signal to noise from 94. When it's expressed in dBA, the original sound
source had the spectrum distribution that matches human hearing, if
expressed in dB the source sound was even spectrum, white noise.
Obviously this testing has to be done in a extremely quiet testing chamber.
Note, as such the specs are at a specific gain on the preamp. The level
goes up and down as the preamp is adjusted. If, say, you are recording a
quiet site and adjust the preamp to give a good record reading you will
have also amplified the self noise, making it louder than spec. Only if
you are recording a site with max noise at 94 dB and you set your meter
for that to hit zero will your meter be accurate as to self noise level.
For most nature recording your self noise is louder than the spec due to
preamp settings. Though it can be generally thought that your site's
dynamic range in your recording will be the difference between it's
measured sound level and the mic's self noise. A quiet site at 25dB
measured will only have a dynamic range against a MKH-20 of 15dB.
Against a Shure 183 it will have just a few dB. It's best not to
consider such numbers as absolutes, but more like good estimates. That,
of course, assumes your mic pre is not louder. So, self noise in the mic
sets one end of your dynamic range, and a absolute limit on how quiet a
site you can record with that mic.
Note with the self noise so close to the max sound at a quiet site it
becomes very important just what the self noise sounds like. It will be
audible, so the less noticed it is the better.
Walt
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