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SPL meters...

152.16.213.173

Posted by RichardH on March 7, 2001 at 13:19:45

In Reply to: Hey Sean ... posted by Stephæn on March 7, 2001 at 13:04:12:

As I understand it, not all SPL meters will give the same measurements, but they are supposed to be within a dB or two to any other meter. Don't know if this is really the case as I only have one and I don't have acess to an RTA. Anyway, here's some corrections I saw on another site, don't know if this duplicating something that's already been posted....

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The Radio Shack meter is a wonderful unit. About a two and a half
years ago, I did a calibration curve for the RS meter using lab
equipment, with Eric Busch from DLC Design adding the low bass down
to 10 Hz. This was published in PSACS Sound Bytes in two issues.
These are the corrections that should be added to the meter readout
in order to achieve the correct SPL. These corrections are only
valid for the meter set to C weighting, using 1/3 octave pink noise
(easily available from various CDs), with the mic pointed at the
speaker.
Both my analog meters and my digital meter measured the same in
October, 1996.These are corrections, they are to be added to the
meter readout for the correct response in dB SPL.

10Hz +20.5
12.5Hz +16.5
16Hz +11.5
20Hz +7.5
25Hz +5
31.5Hz +3
40Hz +2.5
50Hz +1.5
63Hz +1.5
80Hz +1.5
100Hz +2

125Hz +0.5
160Hz -0.5
200Hz -0.5
250Hz +0.5
315Hz -0.5
400Hz 0
500Hz -0.5
630Hz 0
800Hz 0
1KHz 0

1.25Khz 0
1.6KHz -0.5
2Khz -1.5
2.5Khz -1.5
3.15Khz -1.5
4KHz -2
5KHz -2
6.3KHz -2
8KHz -2
10Khz -1

12.5KHz +0.5
16KHz 0
20KHz +1

To measure the in-room response of your speakers with the SLM, find
a quiet and undistracted time, obtain a 1/3 octave pink noise CD,
mic stand, and worksheet. With the speakers in their normal
positions and using a mic stand for the SLM (I've had good results
just holding the meter too, pink noise is forgiving), place its
microphone where your ear would be at your favorite position or
"sweet spot". Set the meter to "C" and "slow". Play the 1 Khz, 1/3
octave pink band and set the level on the amp or preamp, and the
meter range, so the meter reads 80 dB at 0 dB on the meter. Higher
levels might cause driver compression in the frequency extremes,
rolling off the response. Now go back to the first ISO center 1/3
octave band on your CD (25 Hz on mine) and record the response:
3.5, or -1.0 , or -5.0, or whatever it is. Now step through the
pink noise bands, recording the meter level each time. This is the
raw data.
Keeping the meter in the same measuring position, using an accurate
CD, having a quiet room, repeating the measurements for accuracy,
understanding what you are measuring, fresh batteries, not talking
while measuring, having your meter calibrated for overall level
(relative level comparison is unaffected), and doing only one
speaker at a time if possible to avoid comb filtering (variations
of plus or minus 2 dB are possible) are all important factors.
Sometimes if I have to measure two speakers at a time, I move the
meter around in a figure eight pattern, slowly, and try to obtain
an average reading, as the microphone moves in and out of the
combing peaks and troughs. Best accuracy would be obtained from
taking several measurements at different listening positions, if
one has the time and patience.
Now take the raw data and make the corrections on the work sheet,
entering the new values in the appropriate column. Time to either
moan or marvel, since this is the actual in-room 1/3 octave pink
noise frequency response of your speaker/system at that listening
position. Plotting the results on graph paper in different colors
for left, right, center, etc. makes it look cool.

Yes, this way is tedious, but it is very inexpensive. And very
accurate. For the third octave pink noise, I use Carver's Amazing
Bytes CD, GRP Z-9907; other CD's with 1/3 octave pink noise ISO
centers are:

My Disc, Sheffield 10045-2-T
Sheffield/Coustic Test and Demonstration Disc, Sheffield 10040-2
Autosound 2000 CD #103, $18, 800-795-1830
IASCA Setup and Test CD

HI-FI News and Record Review "CD-II", $30 Japan Audio Society Audio
Test CD-1, YDDS-2
--- these last two available from DB Systems, 603-899-6415.

For the others:
http://www.audioxpress.com/
http://www.mcmelectronics.com/main.html
http://www.parts-express.com/

There are other CD's that have warble tones on them, but I am not
comfortable using them. I also extensively use track 19, disc 2,
from Delos "Surround Spectacular", DE 3179.
This has a slow sine sweep from 160Hz to 20Hz with the voice of
David Ranada announcing the frequencies as they
descend. A fantastic help when used with the Radio Shack SLM, you
can easily tell where the room peaks and dips are.

http://www.delosmus.com/

You have permission to copy and distribute this information freely,
as long as no commercial gain is involved.

Radio Shack is your friend.

Here is how I did my calibration:

The $34.99 Radio Shack 33-2050 analog sound level meter has been
around for over 25 years. Its predecessor, the 33-1028, was
reviewed favorably in Stereo Review, (Julian Hirsch, "Equipment
Test Reports", Stereo Review, August 1972). It has a much
different curve than the ones I tested.
To verify the accuracy of the newer version, I compared it to an
Audio Control 3050 RTA, the same one Tom Nousaine used for years in
his test reports for Car Stereo Review until he bought MLSSA. The
overall SPL accuracy of all three of my Radio Shack sound level
meters -- 2 analog and 1 digital 33- 2055-- were within 1 dB of 75
dBC SPL compared to the Audio Control. I then checked the
frequency response, comparing it while set to C weighting and slow,
with pink noise, 1/3 octave band by 1/3 octave band, to the Audio
Control RTA in the SPL mode. Using the same official PSACS
calibrated PSB loudspeaker and a pink noise CD, I made a
calibration curve that can be subtracted from the results obtained
by the Radio Shack in your living room to obtain accurate,
repeatable measurements for about $60, including pink noise CD.
Make sure your meter is set to C weighting. The digital meter and
my second analog meter (6 years newer than the test unit in 1996)
were the same as the test SLM. Response below 25 Hz done by Eric
Busch with sine waves and B&K equipment at Dave Clark's DLC Design
in Michigan.

Michael Sims
Prairie State Audio Construction Society

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RichardH

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