Hi Adam!
I have tried small boxes a few times, but always met some kind of
trouble. Usually, at night, when there is no wind at all and I switch
off all electricity - then I can measure some 13 - 14 dbA.
At all other times, the needle goes +/- 2-3 db, even if I have hidden
the mic into something.
I know that Schoeps sells a round anechoic chamber of some kind, but
I am not sure if it really isolates outside sounds or just dampens
reflections. And I dare not ask for the price...
I will ask at a nearby industry if they can make me a small box out
of concrete. Or perhaps put sand bags over the mic? (I'll bet there
are ants making noise in the sand...)
Klas.
At 22:24 2007-01-13, you wrote:
>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
>
>
>
>
>"Microphones are not ears,
>Loudspeakers are not birds,
>A listening room is not nature."
>Klas Strandberg
>Yahoo! Groups Links
>
>
>
Telinga Microphones, Botarbo,
S-748 96 Tobo, Sweden.
Phone & fax int + 295 310 01
email:
website: www.telinga.com
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