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'[OT]:Sound meter calibration'
2004\04\26@121017 by John Ferrell

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I use a Radio Shack DB Meter for sound measurement of competition model airplanes. It works pretty well for relative measurements but seems occasionally be off by as much as 3db from the "official sources". A commercial calibration device is available, but is beyond what I am willing to spend.
The range of interest is about 90-100db, A weighted.
I would appreciate any thoughts on construction of such a device. The PIC relationship is optional!

John Ferrell    http://DixieNC.US

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2004\04\26@121425 by Tom

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How have you determined that your meter readings are 3db off?
Tom

At 12:10 PM 4/26/04 -0400, you wrote:
>I use a Radio Shack DB Meter for sound measurement of competition model
airplanes. It works pretty well for relative measurements but seems
occasionally be off by as much as 3db from the "official sources". A
commercial calibration device is available, but is beyond what I am willing
to spend.
>
>The range of interest is about 90-100db, A weighted.
>
>I would appreciate any thoughts on construction of such a device. The PIC
relationship is optional!
>
>John Ferrell

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2004\04\26@122049 by John Ferrell

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Comparing multiple meters on the same test subject.
At this time I am not certain the "official measurement" at the contest is
good, but it is what is taken to be standard. I find the RS meter to be a
very good tool, and at a very good price.

John Ferrell
http://DixieNC.US

{Original Message removed}

2004\04\26@124339 by Bob Blick

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> I use a Radio Shack DB Meter for sound measurement of competition model
> airplanes. It works pretty well for relative measurements but seems
> occasionally be off by as much as 3db from the "official sources". A
> commercial calibration device is available, but is beyond what I am
> willing to spend.

Using a mechanical piston/cylinder is the only official method I know of
for microphone calibration, that shouldn't be too hard to come up with.
You could use an old weed-eater motor as a basis, subtitute something else
for the cylinder head, use an electric motor(speed controlled by a PIC!)
to spin it.

Cheers,

Bob

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2004\04\26@125421 by Tom

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A friend of mine used to fly glow-plug RC a lot and when the topic of noise
came up they bought the RS meter.  It's difficult to say how good it is on
an absolute basis but it was very good at relative measurements.  Get a
reading, modify an exhaust system, get another reading.  It was good at
telling you if your efforts were working or not.
Tom

At 12:21 PM 4/26/04 -0400, you wrote:
>Comparing multiple meters on the same test subject.
>At this time I am not certain the "official measurement" at the contest is
>good, but it is what is taken to be standard. I find the RS meter to be a
>very good tool, and at a very good price.
>
>John Ferrell

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2004\04\26@132533 by Robert Rolf

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John Ferrell wrote:
>
> Comparing multiple meters on the same test subject.

With their microphones at EXACTLY the SAME point in space
relative to the source? If not, then you may have experienced
ground reflection effects or other phase cancellation effects
(think interferometer).

> At this time I am not certain the "official measurement" at the contest is
> good, but it is what is taken to be standard. I find the RS meter to be a
> very good tool, and at a very good price.

And there is a calibration pot inside the unit, but I would
only tweak it if I had a recently calibrated professional
unit and had the two microphones within a cm of each other.

Are the official level measurements not made by finding the
'peak level' while moving the microphone at a fixed radius
from the source? Otherwise you run the risk of measuring
a null point for a specific frequency component caused
by reflections.

The proof is easy. Have a speaker produce a pure tone
and then measure with the same physical arrangement
as you use for the aircraft. My bet is ground reflections
will cause at least 6db amplitude change as you elevate
the microphone a few inches. I'm assuming the measurement
is made 'hand held' so height is not fixed.

> {Original Message removed}

2004\04\26@133606 by Edward Gisske

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John,
Welcome to the arcane world of sound measurement, which is only exceeded in
arcanity by light level measurement.

As Bob pointed out: The "official" method of calibrating consists of
generating a known sound pressure level (SPL) with a piston/cylinder tightly
coupled to the sound meter microphone. A converted weed whacker motor would
likely turn the sound meter into an air-propelled projectile, however,
rather than calibrating it. The old B&K (Bruel and Kjaer, not the cheesy TV
service equip manufacturer)"Gold Standard" calibrator consisted of a roughly
1" diameter piston with about a 1/6" stroke powered by a little battery
fired Mabuchi motor. The "cylinder" is about 1/2" long. You could probably
fake the mechanical stuff by using a little speaker coupled to a little tube
driven by a multivibrator. Make sure the sound level meter mike is tightly
sealed to the "cylinder" The calibrating of the calibrator is the hard part.
I suggest that you do a transfer calibration from a "known good" meter to
avoid all the mathematical intricacies.

Edward Gisske, P.E.
Gisske Engineering
608-523-1900
spam_OUTgisskeTakeThisOuTspamoffex.com


{Quote hidden}

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2004\04\26@135524 by John Ferrell

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The International organization is very specific in the measurement protocol.
There is no need to mechanically adjust the RS meter, just knowing the
deviation in the desired range would serve the purpose. The meters now in
official use require at least hourly checks & adjustments to a standard.

Since I don't understand the technology, it appears akin to witchcraft to
me. There are tons of Google information, but so far I have found little to
be useful.

I should add that this is not a high priority issue with me, mostly just
curiosity.
John Ferrell
http://DixieNC.US

----- Original Message -----
From: "Robert Rolf" <.....Robert.RolfKILLspamspam@spam@UALBERTA.CA>
To: <PICLISTspamKILLspamMITVMA.MIT.EDU>
Sent: Monday, April 26, 2004 1:24 PM
Subject: Re: [OT]:Sound meter calibration

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2004\04\26@143101 by John Ferrell

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I will pursue this further!
I do have the resources in my shop to build such a device and the mechanical
nature would probably be more stable in the long run. Thanks for the tip...

John Ferrell
http://DixieNC.US

{Original Message removed}

2004\04\26@144759 by Edward Gisske

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Oops,

Make that a 1/16" stroke.

One other tip is make the motor spin at a frequency comfortably in the
middle of the meter passband. A-weighting rolls off both ends of the
spectrum to approximate the human ear response. If you get to close to the
edges of the passband, you may have more of a problem with correlation
between meters.

Edward Gisske, P.E.
Gisske Engineering
608-523-1900
.....gisskeKILLspamspam.....offex.com

{Original Message removed}

2004\04\26@160309 by Andrew Kilpatrick

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You know, 3dB isn't really all that bad. Most audio equipment
such as speakers and microphones have peaks and valleys far
greater than that. Depending on your needs, I would say just
to live with it.

Andrew

On Mon, Apr 26, 2004 at 12:10:33PM -0400, John Ferrell wrote:
{Quote hidden}

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2004\04\26@161522 by Richard.Prosser

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I think there used to be a German DIN standard for this. IIRC it involved
dropping a mass of ball bearings onto a steel sheet (or bowl or something).
It was used as a sensitivity test for microphones.
Richard P




I use a Radio Shack DB Meter for sound measurement of competition model
airplanes. It works pretty well for relative measurements but seems
occasionally be off by as much as 3db from the "official sources". A
commercial calibration device is available, but is beyond what I am willing
to spend.

The range of interest is about 90-100db, A weighted.

I would appreciate any thoughts on construction of such a device. The PIC
relationship is optional!

John Ferrell
http://DixieNC.US

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2004\04\26@182200 by John Ferrell

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I am inclined to agree with you. In time I expect the rules to be changed.
For the moment, meeting th rule can be a problem.

BTW, I cannot hear the difference between 94 and 97 db with my ears at the
prescrebed 10 meters from the source. I have not tried it since the new
hearing aids though...


John Ferrell
http://DixieNC.US

{Original Message removed}

2004\04\27@054749 by Peter L. Peres

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I think that you can set up something using a good quality speaker with
known acoustic efficiency and play back A-weighted noise through it, from
a computer or compact disc or MD. It should produce the specified dBs at
the standard distance (1meter). The efficiency plots of studio monitor
speakers are more accurate than 3dB.  You could check the setup by
temporarily renting an accurate sonometer from an equipment supply.
Temperature and humidity has an influence on the speakers (esp. on the
magnets), and on sound measurements. Could this be what is causing you
trouble ? Also other problems, e.g. wet soggy ground reflecting not at all
and the next day the same thing frozen, reflecting like a mirror ?

Peter

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2004\04\27@061822 by Peter L. Peres

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>Since I don't understand the technology, it appears akin to witchcraft to
>me. There are tons of Google information, but so far I have found little
>to be useful.

The calibration is in dB spl (sound pressure level) referenced to an
absolute spl, for single tone sine wave. An A-weighted sound is actually
measured as a rms value over amplitude and frequency, giving an equivalent
single-tune spl for that energy level.

The moving piston calibration device has a known displacement and causes a
known pressure variation across the surface of the microphone membrane
given a certain constant volume of the coupling tube (it is assumed that
the microphone membrane does not move or that its movement is negligible
wrt the pressure variation in the tube). You could calibrate such a
homemade piston device using a water manometer (U tube) at low speed. In
fact, you can substitute a pressure-capable speaker (e.g. a miniwoofer)
and calibrate as above. A means to measure the physical displacement of
the piston or woofer at the frequency of interest would be needed. But
your calibration would not be traceable to standards. Renting a known good
instrument and comparing is the way to go imho.

Peter

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