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I got to searching around a little deeper on the Radio Shack correction values and ran across a few pieces of interesting info.

I wonder if this guy was the first to offer up the original correction values:
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

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

Some of you may have seen this mod info before, but for those who have not... here is how you can supposedly make your RS Meter ruler flat and not have to worry with correction values. Full article here. Unfortunately all of his links in the article appear to be broken.
Useful Modifications to the RadioShack Analog SPL Meter

Tom Mallin

29 March 2002

So you think a review of a DIY project involving about $12 worth of parts for a RadioShack SPL Meter listing for just $40 is not esoteric enough for StereoTimes?

Think again.

It's easy for audiophiles to be fooled by inaccurate bass response. Some may classify a given speaker's bass response as "tight as a drum," when, in fact, measurements reveal a significant roll off beginning at 80 Hz or so. Other speakers may be praised for their subjectively rich and powerful deep bass response, but the meter shows a peak of 5 to 10 dB between 80 and 120 Hz, with a slow roll off below 60 Hz or so.

For some listeners, an overall downward slope of more than 10 dB in frequency response from the low bass to the high treble sounds authoritative, accurate, and pleasing, particularly on large-scale classical works, and especially when combined with a midrange suck-out, which adds spaciousness. Admittedly it takes extraordinary air-moving ability in the bass to be able to both produce this kind of bass and play at high volumes, usually meaning a large enclosure or bass towers. Anyone remember Robert Harley's praise for the Genesis II.5 and wonder how the measured frequency response (see Stereophile, Volume 18, No. 1, January 1995, page 88) of such a highly rated speaker could slope down 15 dB from 20 Hz to 10 kHz? Harley justified his admiration for this type of balance by saying: "The response is smooth and flat, but with an overall tendency toward an uptilted bass and a downtilted treble [plus a measured suck-out along this slope of 5 dB between 400 Hz and 2.5 kHz]. Loudspeakers that measure flat tend to be too bright, in my experience. The curve could be summed up as 'flat with lots of bass,' which corresponds to my overall impression of the II.5."

Audiophiles who really want to get the best bass (and other frequency) response from their systems and who want to train their ears to recognize good versus anomalous frequency response need a fairly accurate and convenient way to measure the frequency response of their audio systems in their listening room from their listening position.

It's even more important to me since my Legacy Audio Whispers are designed to need electronic low-frequency equalization for flat bass response. One convenient measuring device that some audiophiles, including me, have been using for years is the venerable RadioShack Analog-Display Sound-Level Meter, Catalog Number 33-2050.

The Controversy

But is the RS meter accurate enough to use for this purpose? Here there is great debate documented on the Internet--just use "Radio Shack SPL meter" as an exact phrase search term in Google's Advanced Search mode to get a feel for the controversy. Few seem to dispute the basic accuracy of the meter from mid-bass to mid-treble, and some sources claim even better performance. For example, one test compared the RadioShack to two B&K (the calibration microphone folks) reference instruments over the 125 Hz to 20 kHz range and the RadioShack held its own quite nicely over this range. At least one source, however, claims the meter is wildly inaccurate above 1 kHz (scroll down to near the end). Others mention "correction factors" which should be applied, but there is some disagreement about the size of such correction factors, as well. Compare the values in the above link with the ones mentioned in the Web instructions for the modification under review.

The audiophile literature I respect, such as Robert E. Greene's comments in The Absolute Sound, suggests that while it takes considerable expertise to correlate subjective listening tests with measured results in the midrange and treble, in the bass, what you measure with a good meter is basically what you hear. Thus, using a good meter is a much quicker and more accurate way to determine the flatness of bass response than listening to a wide variety of recorded performances, especially since the relative weight and extension of bass response varies considerably even among otherwise fine recordings.

My primary focus in using the RadioShack meter has usually been to measure the bass response of the system, given particular loudspeaker placement and listener position. Unfortunately, this is the area where there is most disagreement about the accuracy of the inexpensive RadioShack meter. While I have always thought I heard good correspondence between the RadioShack meter readings and my subjective impression of bass weight from a system compared to my aural memory of live unamplified bass in orchestral concert venues, others obviously disagree.

The Modifications

Thus I read with great interest Eric Wallin's Internet discussion of modifications he developed for the RadioShack meter which he claims make the bass response ruler flat down to a very few Hz -- certainly low enough for complete confidence in main loudspeaker and subwoofer setup. He also describes modifications to make the meter response very flat out to the limits of hearing and beyond.

This is the only DIY project I have seen documented on the Web for correcting the meter's supposed high and low frequency deficiencies. The low-frequency modifications seemed simple enough to perform, even for a "half-thumbs" (as opposed to "all thumbs") solder-slinger like me.

I decided not to attempt most of the high-frequency modifications (I did replace capacitor C12). They are a bit more complex, involving replacing the microphone element in the meter, and I have never used the meter to adjust high frequency response at the listening position anyway.

The low-frequency modification only involved replacing eight capacitors with new ones of different values. And, if I didn't like the results, what would I lose? Only an hour of two of my time and $52 or so for the parts and a replacement meter. I decided to proceed.

Cutting to the chase, the low-frequency mod is, in my opinion, quite worthwhile. Quite.

I won't belabor the modification details. They are available at the referenced link and are very detailed and accurate, as such things go. So is the description of the more skittish operation of the meter following the mod. The only modification details I would add are:

1. Unless your spatial relations are a lot better than mine, I strongly recommend removing the stock capacitors and replacing them with the new ones on the top side of the circuit board. This may take longer, but it is easier to see what you are doing this way and easier to be sure you are connecting the new capacitors into the proper holes in the circuit board. The top of the board is nicely labeled with the capacitor numbers referred to in the instructions and plus and minus polarity markings which match the plus and minus markings on the caps.

2. It was difficult for me to determine which board connections to desolder when looking at the board from the bottom -- there are a lot of connections spaced closely together and nothing is labeled. Unfortunately, all soldering and desoldering requires working on the bottom of the board. Once I determined which connection probably needed to be desoldered next by repeatedly comparing the top and bottom of the circuit board, I used a 30-watt soldering iron together with desoldering tape to remove the solder from that connection on the bottom of the board. Once that hole was clean of solder, the wire lead of the stock capacitor could be pulled out of the hole from the top of the board. Once both leads of that capacitor were free, that cap could be removed and replaced with the new value.

3. When inserting the wire leads of the new capacitors into the circuit board holes, be careful to match the labeled plus and minus polarity on the cap with the plus and minus labels on the board holes for that cap.

4. Soldering the new caps into place was easier than desoldering the old ones since I could see the new capacitor's lead wires poking through the board holes when looking at the bottom of the board and thus knew at a glance exactly where to apply heat and solder.

5. Working carefully, I took almost two hours to complete the project. But then, as I said, I am somewhat ham-handed at this sort of thing, and that time included correcting one cold-solder joint I located after the meter didn't work at all following my initial reassembly.

6. The new caps are somewhat larger than the stock ones and I had a bit of trouble getting the case back together because of lack of clearance of the new parts on the top side of the circuit board. I corrected this by carefully bending the wire leads on a couple of the new caps so the caps leaned over a bit allowing the case to again fit around the now-more-stuffed board.

Here's more info from another source on the mod...
In order to correct the low frequency roll-off, you can do the following modifications to your meter. This will make the meter FAR more sensitive to low frequencies and allow measurements with very good accuracy to well below 20 Hz. Due to the increased sensitivity at very low frequencies, it is possible for low frequency "thumps" to slam the meter if using a very low SPL setting. In order to prevent meter movement damage, take precaution not to peg the meter off scale on a regular basis. If this is occuring regularly, you either need to move up to a higher SPL range on the meter or take more caution as to how your performing your tests.

All of the following capacitors must be rated for AT LEAST 15 volts or so. Size does matter, so try to use the smallest package possible.

C1 & C2 are changed from 1 uF to 10 uF
C3 & C4 are changed from 1 uF to 47 uF
C7* is changed from 10 uF to 220 uF
C8 is changed from 100 uF to 470 uF
C9 is changed from 22 uF to 220 uF
C15 is changed from 100 uF 220 uF

While all of the above parts are spec'd as microfarads, the following is in picofarads. Do not confuse the two values or the meter will not work very well at all. This last change helps minimize high frequency roll-off that is inherit in the stock microphone

C12 is changed from 33 pF to 12 pF.

Please note that all of the above parts can be soldered directly in place of the originals EXCEPT C7. Due to its location, a "normal sized" 220 uF cap will be too big to allow the case to close correctly. In order to get around this, simply solder it on the "solder side" of the board instead of on the "parts side" of the board. Pay special attention to the POLARITY of ALL of the caps as you pull them out to replace them. For this reason, i recommend pulling and replacing the caps one at a time to minimize confusion. Once all of these mods are done, the meter is more than accurate enough for anything that a home audio enthusiast would ever need use of. If you really want to "get crazy", you can remove the factory installed mic and either remotely mount it on a "wand" or make use of a calibrated mic like those available from Old Colony.

I hope this helps some of you out and sorry it took so long to dig all of this up.


11 Posts
The old analog Radio Shack SPL meter (33-2050) and the newer digital display meter (33-2055) have different capacitor designations, so it's a bit confusing when looking at modification. They both share (IMHO) woefully inadequate coupling capacitors, relying mostly on 1 uF electrolytics (at least they're not tantalums). Crosschecking the schematics yields this table:
33-2050 33-2055 Original Value Suggested Value
C1 C2 1 uF 10uF
C2 C5 1 uF 10uF
C3 C6 1 uF 47uF
C4 C7 1 uF 47uF
C7 C13 10uF 220uF
C9 ---- 22uF 220uF
C8 C3 100uF 470uF
C15 C1 100uF 220uF
C12 C12 33pF/27pF 12pF
---- C11 1uF 10uF
---- C17 1uF 10uF
Make sure to buy the absolute tiniest caps you can--they ain't much elbow room!!
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