The Decibel
 

"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.

They convert sound into a voltage, measure that voltage, and compare that
voltage to a predetermined voltage that corrseponds to a certain very faint
sound level.

Surely every power meter would give a different
reading depending on what its reference is for 0dB.

To be standard they use the same reference for 0 dB.

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

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.

The Decibel
 

In article , Arny Krueger
wrote:

"Jim Lesurf" wrote in message
...
[snip]

http://www.shure.com/pdf/specsheets/...dmics/sm57.pdf page 2.

says:

"Sensitivity (at 1,000 Hz)

"Open Circuit Voltage: -54.5 dBV/Pa* (1.9 mV)

"*(1 Pa = 94 dB SPL)

Seems pretty clear and usable, as far as it goes right?

Yes. Much more useful to quote dBV/Pa as this gives a better idea of the
reference level as a transducer sensitivity.

Note that 0 dB is 0.02 mPa or 2 1/100ths of a milliPascal. Milli is
worth 60 dB, 1/100 is worth 40 dB, and 2 is worth 6 dB. Add it all up
and you have a 94 dB difference. So, the *books* may lie, but they all
tell the same lie. In this case its a generally-understood and accepted
convention so its no lie at all!

;-)

I get the impression that this is an area where the books may give
differing explanations which end up much the same, The reason being they
are all starting from the same definition, but some of the book-authors may
not fully understand it. If so, no suprise. I've encountered the same sort
of 'diversity' many times in books on EM, and IT. :-)

Slainte,

Jim

--
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Audio Misc http://www.st-and.demon.co.uk/AudioMisc/index.html
Armstrong Audio http://www.st-and.demon.co.uk/Audio/armstrong.html
Barbirolli Soc. http://www.st-and.demon.co.uk/JBSoc/JBSoc.html

The Decibel
 

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.


--
Chris Morriss

The Decibel
 

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

The Decibel
 

In article , Chris Morriss
wrote:
[snip]

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.

Good thinking! :-) I do have some B&K mics and calibrators at work.
'Inherited' them along with an anechoic chamber some years ago. Didn't
think to look in the handbooks, but now you've mentioned it, it sounds like
a good idea.

Slainte,

Jim

--
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Audio Misc http://www.st-and.demon.co.uk/AudioMisc/index.html
Armstrong Audio http://www.st-and.demon.co.uk/Audio/armstrong.html
Barbirolli Soc. http://www.st-and.demon.co.uk/JBSoc/JBSoc.html

The Decibel
 

In message , Jim Lesurf
writes
In article , Chris Morriss
wrote:
[snip]

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.

Good thinking! :-) I do have some B&K mics and calibrators at work.
'Inherited' them along with an anechoic chamber some years ago. Didn't
think to look in the handbooks, but now you've mentioned it, it sounds like
a good idea.

Slainte,

Jim


If you've got a 4133 or 4134 going cheap then here's a buyer :-)
--
Chris Morriss