On Mon, 25 Apr 2005 14:48:38 -0700, Wes Stewart
wrote:

It may come as a surprise to our correspondent who likes to disparage
"gurus" that "standard-gain" antennas are widely used as reference
standards. To head off the question of how the standard gain is
determined, that is done by testing three "identical" antennas in
pairs; each one against the other two, with one the source and the
other the receiver. A bit of algebra and you have the gain of each
one individually.

http://www.mi-technologies.com/literature/a00-044.pdf


Hi All,

The method described by the paper offered above is a commonplace of
Metrology called "Reciprocity." I have calibrated precision
microphones against this method, and the error math offered is
consistent with my experience (much less the actual values offered as
examples).

As an aside, this method is also as old as the pyramids - literally.
The Egyptians planned their blocks of granite to have nearly flat
faces to within 10s of microinches using three blocks, by abrading one
against the other and then rotating their positions.

Accuracy is far more a matter of protocol or technique than it is
about a ruler (or other scale).

73's
Richard Clark, KB7QHC

On Tue, 26 Apr 2005 00:04:02 -0700, Richard Clark
wrote:

On Mon, 25 Apr 2005 14:48:38 -0700, Wes Stewart
wrote:

It may come as a surprise to our correspondent who likes to disparage
"gurus" that "standard-gain" antennas are widely used as reference
standards. To head off the question of how the standard gain is
determined, that is done by testing three "identical" antennas in
pairs; each one against the other two, with one the source and the
other the receiver. A bit of algebra and you have the gain of each
one individually.

http://www.mi-technologies.com/literature/a00-044.pdf


Hi All,

The method described by the paper offered above is a commonplace of
Metrology called "Reciprocity." I have calibrated precision
microphones against this method, and the error math offered is
consistent with my experience (much less the actual values offered as
examples).

It is also a method used for determining the phase noise of low noise
oscillators.


As an aside, this method is also as old as the pyramids - literally.
The Egyptians planned their blocks of granite to have nearly flat
faces to within 10s of microinches using three blocks, by abrading one
against the other and then rotating their positions.

Accuracy is far more a matter of protocol or technique than it is
about a ruler (or other scale).

73's
Richard Clark, KB7QHC

Richard Clark wrote:

Hi All,

The method described by the paper offered above is a commonplace of
Metrology called "Reciprocity." I have calibrated precision
microphones against this method, and the error math offered is
consistent with my experience (much less the actual values offered as
examples).


Any references on microphone calibration? Maybe a short tutorial? That
is something I have a need to do.

tom
K0TAR

On Tue, 26 Apr 2005 20:28:01 -0500, Tom Ring
wrote:
Any references on microphone calibration? Maybe a short tutorial? That
is something I have a need to do.

Hi Tom,

Standard microphones (I am being quite specific in terminology here)?

I googled with the terms
B&K microphone reciprocity
and the first hit looks as good as any:
http://www.bksv.com/pdf/Bv0051.pdf
As a treat, it offers a discussion of matching with transmission line
metaphors.

I should point out that reciprocity means exactly that! The
microphone should be capable as acting as a loudspeaker (certainly not
too loud) when driven. Standard microphones are capable of accuracies
in the 1/100ths of a dB (and this is an extremely conservative
statement). If you are playing with retail microphones, and follow
the math, you should be able to cobble up something to the nearest
1/4th dB.

If your application conforms to this discussion, you may visit the
Brüel & Kjær website to find deeper references. They are the
pre-eminent makers of precision sound equipment. As I pointed out in
another posting relating to the poverty of academia on many technical
subjects, the commercial field often leads the way in actual
instruction.

73's
Richard Clark, KB7QHC

Richard Clark wrote:

On Tue, 26 Apr 2005 20:28:01 -0500, Tom Ring
wrote:

Any references on microphone calibration? Maybe a short tutorial? That
is something I have a need to do.


Hi Tom,

Standard microphones (I am being quite specific in terminology here)?

I googled with the terms
B&K microphone reciprocity
snip
73's
Richard Clark, KB7QHC

Thanks.

tom
K0TAR

On Tue, 26 Apr 2005 20:28:01 -0500, Tom Ring
wrote:

Any references on microphone calibration? Maybe a short tutorial? That
is something I have a need to do.

Hi Tom,

As a second thought, you may not be in the market for the reciprocity
technique (it does require that you have a true reference microphone).

In that case, you would fall back to a Piston Phone and do a single
point calibration. The method is as old as the hills, the math is
extremely simple volumetrics, but the implementation (construction of
the calibration unit) is not something for the faint of heart. You
will need a precision lathe. Again, google using Brüel & Kjær as a
jump-off point.

Once you do the single point calibration, then you can proceed to a
swept frequency analysis. Unfortunately this returns us to the
necessity of a reference microphone. However, as relative frequency
response is more available (from expensive retail models), you might
have a chance.

73's
Richard Clark, KB7QHC

Richard Clark wrote:

On Tue, 26 Apr 2005 20:28:01 -0500, Tom Ring
wrote:


Any references on microphone calibration? Maybe a short tutorial? That
is something I have a need to do.


Hi Tom,

As a second thought, you may not be in the market for the reciprocity
technique (it does require that you have a true reference microphone).

In that case, you would fall back to a Piston Phone and do a single
point calibration. The method is as old as the hills, the math is
extremely simple volumetrics, but the implementation (construction of
the calibration unit) is not something for the faint of heart. You
will need a precision lathe. Again, google using Brüel & Kjær as a
jump-off point.

Once you do the single point calibration, then you can proceed to a
swept frequency analysis. Unfortunately this returns us to the
necessity of a reference microphone. However, as relative frequency
response is more available (from expensive retail models), you might
have a chance.

73's
Richard Clark, KB7QHC

And thanks for this also.

I had a nicely useful, yellow, roughly 11x14, hardcover book that was a
handy audio manual that was lost during moving a couple decades ago. It
covered beginning through midrange complexity, and had a decent tutorial
on bi-amp. Also had a description of Indy Speedway Pit announcement
system, high sound pressure level, baseball stadium sound system, and
R&R, may have been The Grateful Dead. I think it may have been a
husband and wife team that wrote it. Ring any bells? I'd like to order
a copy of it.

tom
K0TAR

On Wed, 27 Apr 2005 20:26:10 -0500, Tom Ring
wrote:

Ring any bells? I'd like to order
a copy of it.

Hi Tom,

Sorry, no bells not even decibells.

73's
Richard Clark, KB7QHC