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 Sun, 24 Apr 2005 05:44:34 +0000 (UTC), "Reg Edwards"
wrote:

All electrical calibration and testing laboratories issue tables of
claimed accuracies of measurements. Measurement uncertainties stated
on calibration certificates are legally binding. All stated
measurement results must be traceable to International Standards or a
laboratory or testing station loses its status.

Consequently there is no incentive for a laboratory to overstate its
capabilities in its sales literature. Indeed, it is dangerous,
illegal even!

Naturally, laboratories can differ widely, one from another.

It would be interesting to compare laboratory uncertainties with
performance figures claimed by antenna manufacturers. Or anyone else.

Does anyone have typical examples of measurement uncertainties claimed
by antenna testing stations? Answers in decibels please.

A reply from a testing station, at HF or VHF, would be specially
appreciated.

Reg propped up this tar baby and everyone's taken a punch at it.

Perhaps it is time to check in and see if you have your answer yet
Reg.

Reg propped up this tar baby and everyone's taken a punch at it.

Perhaps it is time to check in and see if you have your answer yet
Reg.

==========================================
Wes,
Not everybody has yet taken a punch at it. There are several regular
names who are missing.

All I want is a number, eg., of decibels, preferably from a standards
lab.

But it has only been been demonstrated "Measurements" is not a
"Science" - it is an "Art". Perhaps I can clarify my question.

Suppose a customer, perhaps an antenna manufacturer, walks into the
lab wheeling behind him a weird contraption (we've heard of them) and
asks for the forward and reverse gains to be determined and for a
calibration certificate to be issued.

For present purposes actual forward and reverse figures don't matter.

But for the two figures to be of value the uncertainties in the
determination should be stated on the certificate (a legal document).

What are TYPICAL uncertainties, in dB, which appear above the Head of
the Laboratory's signature.

A laboratory or ex-member should be able to put me in the right
ballpark even if it is only for one typical case. For TRUE antenna
performance measurements the best source of information is from a
standards lab. There is no incentive to overstate performance. If
discovered, exaggeration of a laboratory's capabilities results in
loss of reputation.

In the UK, Standards Laboratories were regularly monitored for
performance by the National Physical Laboratory (NPL), in effect
Government controlled. I have been out of touch for 20 years with what
happens these days.

In the 1970's I was a Government Approved Head of Laboratory. I
personally set up the lab from scratch begining with a 30 x 40 feet
empty room. All our own standards were traceable directly to the
National Measurement Standards at the NPL.

An offshoot of the lab, also under my control, was a central
calibration service for instruments used nationally by field engineers
for investigation of radio interference complaints by the general
public and other parties. Many of the instruments were of Eddystone
manufacture whose factory was in Birmingham a few miles from the
Standards Lab. In between Eddystone's works and the lab lay B'ham
University from which the very first 3000 Mhz magnetron appeared
during the WW2 air raids on the city. Just in time to defeat the
U-Boats which were sinking a 10,000 ton cargo ship every day in the
horrible Battle of the North Atlantic. More than 100,000 merchant
seamen and suicidal iron-cross submarine crews still lie sleeping in
Davy Jones' vast locker.

That's quite enough variation for one paragraph. To return to
normal -

Although we had a small screened room to calibrate RFI instruments,
the laboratory's capabilities did not include measurement of antenna
gains and losses. Hence my modern enquiry about uncertainties.

Note: Uncertainties are best considered because they arise from a
multplicity of sources. Therefore they accumulate arithmetically -
whereas accuracies do not and are more inconvenient!
----
Reg, G4FGQ. Alias Brer Rabbit or Punchinello.

On Tue, 26 Apr 2005 19:59:59 +0000 (UTC), "Reg Edwards"
wrote:

But for the two figures to be of value the uncertainties in the
determination should be stated on the certificate (a legal document).

What are TYPICAL uncertainties, in dB, which appear above the Head of
the Laboratory's signature.

Hi Reg,

I thought Wes' link was quite specific to the matter:
Measurement Mismatch Correction Error 0.04
Noise Power of Power Sensor 0.00
Zero error of Power Sensor 0.00
Power Meter Linearity 0.04
Space Loss Measurement Error 0.01
Multipath Curve Fitting Random Error 0.04
Proximity Effect Correction Error 0.05
All expressed in dB and may be combined using the usual methods of
RMS, RSS, or worst case simple sum.

73's
Richard Clark, KB7QHC

Richard, I can agree with that
Wes obviously paid close attention to Reggies initial post
and replied in a way that was very informativeI and reflected
his knoweledge in that particular field
Nobody else came even close
to identifying Reggies needs and responded
in such a professional manner.

He should be congratulated
Regards
Art


"Richard Clark" wrote in message
...
On Tue, 26 Apr 2005 19:59:59 +0000 (UTC), "Reg Edwards"
wrote:

But for the two figures to be of value the uncertainties in the
determination should be stated on the certificate (a legal document).

What are TYPICAL uncertainties, in dB, which appear above the Head of
the Laboratory's signature.

Hi Reg,

I thought Wes' link was quite specific to the matter:
Measurement Mismatch Correction Error 0.04
Noise Power of Power Sensor 0.00
Zero error of Power Sensor 0.00
Power Meter Linearity 0.04
Space Loss Measurement Error 0.01
Multipath Curve Fitting Random Error 0.04
Proximity Effect Correction Error 0.05
All expressed in dB and may be combined using the usual methods of
RMS, RSS, or worst case simple sum.

73's
Richard Clark, KB7QHC

Richard Clark says -
Hi Reg,
I thought Wes' link was quite specific to the matter:
=====================================

Hi Richard,
I originally wrote -

"Does anyone have typical examples of measurement uncertainties
claimed
by antenna testing stations? Answers in decibels please."

The two links to papers, kindly found by Wes, are both devoted to
microwave horns and dishes. Very interesting and directly related to
the subject.

But in anticipation of the sort of replies I would receive, and in
fact did receive, I specifically asked -

"A reply from a testing station, at HF or VHF, would be specially
appreciated."

It appears that at microwaves a worst-case uncertainty of 0.2 dB, that
is a range of nearly half dB, is achievable in the National Physical
Laboratory at Teddington on Thames, London. Which is a little hard for
an Old Timer like me to believe. But at HF and VHF, at which amateurs
are mostly interested, the uncertainty on a typical open-air range is
sure to be greater. If only because great accuracy of rocket
technology at the lower frequencies is not needed.

It nearly always occurs that technical enquries at LF and HF get lost
in the elevated mysteries of microwaves, circulators and
scattering-parameters.

I am unfamiliar with precision antenna test and measurement methods. I
don't particularly wish to know. But if you, as an employee of a
reputable laboratory, were given the job of determining the forward
and reverse gains of fractal or other weird antennas, at 7 MHz and
144 MHz, what uncertainties would you state? I'd believe you.
----
Reg, G4FGQ.

On Wed, 27 Apr 2005 03:01:20 +0000 (UTC), "Reg Edwards"
wrote:

But if you, as an employee of a
reputable laboratory, were given the job of determining the forward
and reverse gains of fractal or other weird antennas, at 7 MHz and
144 MHz, what uncertainties would you state? I'd believe you.

Hi Reggie,

Measurement Mismatch Correction Error 0.04
Noise Power of Power Sensor 0.00
Zero error of Power Sensor 0.00
Power Meter Linearity 0.04
Space Loss Measurement Error 0.01
Multipath Curve Fitting Random Error 0.04
Proximity Effect Correction Error 0.05
The errors remain across all applications, only the assigned values
change. If I arbitrarily scaled all values by 25, few could challenge
the numbers.

At 7MHz we can all agree that the errors are going to be inversely
proportional to the astronomical cost to determine. No one is going
to perform it at HF when they can only afford 1/100th scale models
that offer the accuracies implied above. What would spending more
money buy them anyway?

73's
Richard Clark, KB7QHC