Hi Klas,
--- In Klas Strandberg <>
wrote:
> For the naturesound recordist, I would say that the ideal preamp has
> two gain settings, say 30 and 60 db
Ideally, I think a recorder should have a variable gain knob, just
like on a mixer or outboard preamp, but none exist to my knowledge. To
make the level control on a recorder a variable gain control instead
of an attenuator creates various technical challenges (dc offset
shifts, control linearity, inability to go down to zero).
> Do you know of any simple trick to make a "silence chamber"??
Rob mentioned a method to make a silence chamber that sounded
impressive! However, for measuring mic noise, it should be possible to
construct a relatively small box a few feet long that can be sealed,
rather than a room that you can go inside. I have used such boxes with
a measurement microphone to measure various small devices, but not as
heavily damped and sealed that might be necessary to measure the self
noise level. I think there are various plans out there -- generally,
you use thick MDF particle board to construct a rigid box, and can add
damping sheets of dense rubber (a number of places sell them for
quieting computer cases or speaker cabinets). Then they are lined
inside with deep (like 3" pyramid) acoustic foam (this controls
reflections inside the box, but does not stop sound transmission
through the box). I'm not an expert at constructing such boxes, so I
don't know though that it would be suitable for measurement of a mic
with 14dBA self noise without additional measures (double walls,
fiberglass, rubber suspended inner chamber, etc) as Rob took for the
room. And, for any measurement, ideally you have a test environment
that is an order of magnitude less than the device under test. For the
Schoeps mic, fortunately you can remove the capsule and substitute a
test head that has a capacitor of the same value as the mic element.
This allows me to make measurements and not worry about acoustic
noise, for servicing the mic electronics. I'm not sure if this is an
absolutely valid measurement for mic self noise, or if any added noise
could be generated by the polarizing voltage across the actual mic
element. As far as comparing mics, you could start with the published
self noise figures for the microphone. That won't tell you if yours is
defective or if the manufacturer's figures are accurate of course. And
another interesting thing would be to determine if the self noise is
flat, or is dominant at perhaps the high frequencies, which would mean
that two mics with the same broadband noise level may not sound the
same to the ear.
- Adam
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