In message , Chris Morriss
writes
In message , Jim Lesurf
writes
In article , Arny Krueger
wrote:
"Nathan Higgins" -spam wrote in message


I understand what a decibel is and how it is calculated, what I don't
understand is how sound level meters reference the sound to give a
reading in dB.
[snip]

I have the figure 0.02 mPa in my head as a reference but should all
meters be calibrated to this pressure ?

Since I have just purchased a microphone, this prompts a similar question.

When I looked in the Maplin catalogue at the microphones there, many give a
'sensitivity' by simply quoting a value -XX dB. They don't give a reference
level. Some mics with dual output impedances quote -XX dB for one impedance
and -YY dB for the other.

I looked in my old copy of "Sound Recording Practice" edited by John
Borwick, and that describes several ways for defining a sensitivity. The
one that looks most probable is - IIUC via assuming 'typical speech at
20cm' (assumed 74dB SPL) - and then quoting microphone output w.r.t. 1V.
The Maplin mics all seem to have values in the range from around -65 dB to
-75 dB for 600 Ohm output.

So, my question is, is there now a fixed standard of the kind defined
above, or have I got the wrong end of the stick?

i.e. If I am understanding the sensitivity correctly, typical nearby speech
would give an output typically in the range 0.1 - 1 mV from these
microphones. (?)

Yes. AKA

2 x 10-5 N/m2 at 1000 Hz.

AKA

10-12 W/m2

AKA 2 x 10-5 Pa

AKA

0.02 mPa

Is there a gentleman's agreement to what 0db should be referenced to
with sound meters (i.e.. to measure audio in a concert).

Yes.

Does this mean that those amongst us who are *not* gentlemen will use a
different standard? :-)

Slainte,

Jim


With the B&K test mic (4133) that I've used for DECT and GSM handset
measurements, you calibrated the mic by using a reference pressure
generator that the mic plugs into. I think that 0dB Pa is with
reference to an SPL of 0 Pascal at the mic diaphragm.

I think if you look at the Bruel and Kjaer web site they have some info
about reference SPLs.


Whoops!

I meant 0 Pascal SPL at the mic is a reference level of +94db Pa.
--
Chris Morriss