Unmodified Radio Shack SPL Meter-Useful For Readings In Midrange And Bass?

[kelticwizard]

Eric Wallin, author of Speaker Workshop, has written extensively on the Radio Shack SPL meter.
http://www.gti.net/wallin/audio/audio.html
http://www.gti.net/wallin/audio/rsme...0/33-2050.html

Basically, he examines the internal electronics of it and makes several modifications. He also suggests a replacement for the stock microphone element.

He also supplies equalization curves. He says the stock microphone is good throughout the bass, it is the treble where things go bad, largely because of the protective cup the Meter has surrounding the microphone element.

Looking at the Radio Shack catalog and several internet sites, it seems to me that most electret mic elements have little trouble through the bass and midrange region. Very smooth there. Except for the Panasonic WM 60 series, though, most of them have a peak somewhere in the 4K Hz thru 8K Hz region.

Here are the equalization curves:
20HZ =6.2DB 25HZ =4.4DB 31.5HZ =3DB 40HZ =2DB 50 =1.3DB
63HZ =.8DB 80 HZ =.5DB 100HZ =.3DB 125HZ =.2DB 160 =.1DB
200HZ to 1250HZ Flat
1600HZ = .1DB 2000HZ =.2DB 2500HZ =.3DB 3150HZ =.5DB
4KHZ =.8DB 5KHZ =1.3DB 6.3KHZ =2DB 8KHZ =3DB 10KHZ =4.4DB
12.5KHZ =6.2DB 16KHZ =8.5DB 20KHZ =11.2DB



Since the Radio Shack meter has an electret element, is it safe to assume that the unmodified meter is accurate up to 5K Hz or so? For amateur pruposes, I mean. Wallin seems to imply it, and electret responses for various models seem to confirm it. I'm just wondering if anyone used a Radio Shack meter, then bought a better microphone and saw big discrepancies in the midrange and bass from the Radio shack meter and the better setup.

Finally, Radio Shack has a new model of SPL meter, the 4050 instead of the 2050. Yet the model uses the same equalizqtion chart in the manual as the old one. Are we safe to assume that through the midrange and bass, with the new model, we can use the same equalization charts and get the same accurate results we got with the old 2050, (assuming the 2050 was in fact accurate)?

Bear in mind that this is for amateur, fun purposes. Fractions of a dB don't mean that much. I mean is the Radio Shack meter, with the equalization curve, accurate from the midrange on down within, say, +/- 1 dB? Or +/- 2 dB? (The latter is getting a little loose).

[GM]

FWIW, here's two charts posted on the basslist some years ago. Mine is a very early unit like TD's and needs about the same correction according to the Westinghouse instrumentation calibration lab I took it to back when it was still fairly new, while MS's is a much later unit AFAIK, though I don't recall if it was the analog or digital one:

> Freq Danley Sims
> 10 +20.5
> 12.5 +16.5
> 16 +11.5
> 20 +6.2 +7.5
> 25 +4.4 +5
> 31.5 +3 +3
> 40 +2 +2.5
> 50 +1.3 +1.5
> 63 +0.8 +1.5
> 80 +0.5 +1.5
> 100 +0.3 +2
> 125 +0.2 +0.5
> 160 +0.1 -0.5
> 200 0 -0.5
> 250 0 +0.5
> 315 0 -0.5
> 400 0 0
> 500 0 -0.5
> 630 0 0
> 800 0 0
> 1000 0 0
> 1250 0 0
> 1600 +0.1 -0.5
> 2000 +0.2 -1.5
> 2500 +0.3 -1.5
> 3150 +0.5 -1.5
> 4000 +0.8 -2
> 5000 +1.3 -2
> 6300 +2 -2
> 8000 +3 -2
> 10000 +4.4 -1
> 12500 +6.2 +0.5
> 16000 +8.5 0
> 20000 +11.2 +1

So it appears that EW is using the 'early' correction curve and doing the mods to a later unit based on it may be off quite a bit overall.

GM

[kelticwizard]

GM:

Thank you very much for posting this valuable chart. I think it clears things up quite a bit.

I think what is being made clear here is that the Radio Shack meter is very good for amateur purposes from the midrange down through the bass, and the treble, from 2000 on up is progressively more uneven poatly due ot the fact that they use an electret, unlike the Panasonic series, which has a hump in the response, and the effect of the cup surrounding the unit.

I notice that when Radio Shack went to thge 3 terminal electret, they changed the upper equalization, but not the lower.

Why they roll the electronics off at the low end I dunno, but the meter was first carried by Radio Shack back when few speakers went as low as 30 Hz, so maybe that had something to do with it.

At any rate, thanks for the chart. I think it will be very valuable to people just getting into speakerbuilding who want readings on their midrange/bass. With your correction chart, I would think that a sub builder will get useful readings down to 16 Hz, (if he is willing to exprapolate).

[GM]

You're welcome!

Ugh! The chart looked fine in the preview so didn't look at it once I posted it. The 10/12.5/16Hz correction is from MS's calibration, TD's only went down to 20Hz.

Yeah, don't know about later units, but mine is rated 35-7kHz +/-3dB, which both mine and TD's are, and -7dB/20kHz, which is optimistic for both.

Right, back when I bought mine, speakers rated down to 35Hz were actually 10-15dB down at this point, so putting them up against the wall was standard. At the other end they were rolling off by 10-12kHz and gone by 15kHz, so the 20kHz (or Altec's 22kHz!) rating was as bogus as many of today's performance claims we joke about now if a high DF amp was used.

GM

[andrikos]

If it helps, I have the chart here too:

http://www.audiocircle.com/circles/m...view_photo.php

[kelticwizard]

Andrikos, you have the same chart as GM's second chart. He does not know if his second chart is for the Radio Shack analog meter or digital meter.

Is your chart for the analog or the digital meter?

[Ron E]

I think I posted a comparison of my Behringer ECM 8000 vs. my Radio Shack analog meter here a few months ago (Dec or Jan). If you can't find it, I can repost. For what it is worth, it looks very much like the graph in the meter manual and is within a dB or so of the Cal file plot posted at the ETF Acoustic site.

My file is from ~100Hz up and I haven't done a check on the bass.... I think the RS SPL meter is fine for ~50Hz to ~1kHz... I show it reads 6dB high at ~5kHz and ~12+dB low at 20kHz.

[andrikos]

Originally posted by kelticwizard
Andrikos, you have the same chart as GM's second chart. He does not know if his second chart is for the Radio Shack analog meter or digital meter.

Is your chart for the analog or the digital meter?

Kelticwizard.
My chart is definitely for the analog meter.