Strange RS SPL meter calibration results

[Ilkka]

A friend of mine wanted to have his newer type analog Radio Shack SPL meter calibrated, so he sent it to me. I noticed some strange behaviour while calibrating it against to my professionally calibrated microphone. The amount of correction needed wasn't anywhere near to what Sonnie's correction file suggest for this type of meter. I believe a few graphs explain it better. All the measurements were done with the REW (4.00), I merely used the TrueRTA to show them.

The first graph shows the difference between the correction files for my (i.e. my friend's) new analog meter and Sonnie's new analog meter. Quite a difference!



The second graph shows the difference between the correction files for my new analog meter, my old analog meter, and Sonnie's old analog meter. As you can see, they all match reasonably well (within ~3 dB). My new analog meter seems to be only a little bit better than the old ones.



So what does this all mean? Assuming both Sonnie and I took accurate measurements, some of the new analog meters are better then the others. The worse ones aren't much better than the old analog ones. This will definitely cause some strange measurement results when all the people having the new analog meter are using the same correction file. Their frequency response can look like having much more low frequency roll-off than it in reality has.

So it looks like some of the new analog meters are still having the insides of the old analog meter. How do we separate them? I took a picture of the QC stamp, which reads "10A05". Could people post their QC codes for all the RS meters they have?

Here's the list:

New analog (soft-look)
04A06 (tatkinson)
12A05 (Chrisbee, toecheese)
10A05 (Ilkka's friend)
06A05 (WillyD, Average Joe)
4A05 (norpus)
02A04 (JRace)

Old analog (boxy)
07A06 (Jeffeory)
11A05 (daxie)
12A03 (Ilkka)
10A02 (Peter De Smidt)
09A01 (snatcher)
9A4 (Chrisbee)

[brucek]

So it looks like some of the new analog meters are still having the insides of the old analog meter.

I can't see any other explaination. Granted, Sonnie only tested a few when the calibrations were done, but a random pick of two that reveal the same response ain't bad....

I seem to remember when Chrisbee said his new analog didn't give him anywhere near as good a response as he was measuring with the old meter. I wondered at the time why they weren't the same... This could be it.

I do have to agree with something you've commented on before, and that's if you want to test down to 10hz it is probably a better idea to get a good microphone like many of us have resorted to..

brucek

[Ilkka]

brucek wrote:

I can't see any other explaination. Granted, Sonnie only tested a few when the calibrations were done, but a random pick of two that reveal the same response ain't bad....

I believe RS made some changes (different mic capsule etc.) to their analog meters, but not necessarily at the same time they changed the outlook. It is more than likely that both Sonnie's meters were these "upgraded" ones, while my friend's isn't. Eventhough they all look the same.

I do have to agree with something you've commented on before, and that's if you want to test down to 10hz it is probably a better idea to get a good microphone like many of us have resorted to..

brucek

I agree 100%.

[Chrisbee]

Recently purchased soft-look analogue: QC 12A05 TEC

12 year old (?) boxy shaped analogue: QC 9A4 TEC

I shall do an REW sub response curve for each meter without loading any meter calibration files tomorrow.

[brucek]

I shall do an REW sub response curve for each meter without loading any meter calibration files tomorrow.

I would do it near field to remove as many spikes etc from the room that makes comparisons difficult.

I'd tape the two mics together.....

brucek

[Sonnie]

This could definitely throw a kink in things, especially for those purchasing newer analog meters.

I do have the $99 Galaxy CM-140 in hand, but haven't done any measurements with it. It may be that we simply need to advise people that they will need to either have someone check their RS meter for accuracy or purchase one of the CM-140's, if it turns out to be as accurate as we suspect it will be.

[Chrisbee]

Okay. Nearfield measurements at 4" from the dustcap of bottom driver of my IB.

Both meters carefully calibrated individually to 80dB prior to measurement.

Meter calibration file in REW cleared prior to each measurement to ensure no cal file loaded.

Here's the old analogue meter.



Here's the new analogue meter.



Like two peas in a pod!!!

[brucek]

Yep...... I guess you better use the cal file for the old meter.

Ilkka has made an interesting discovery?

What can we do about it?

brucek

[Chrisbee]

Those of us with two meters at least have a control (of sorts).

The chances of two meters being so alike is er.... very unlikely.

It certainly explains why my new meter behaves exactly like old meter when using the old RS analogue cal file.

But can we trust Sonnie to have got the old RS analogue cal file right?

How many were in his sample?

Does this mean I can now have my bass back which Sonnie stole by giving us the new rs analogue file?

[Ilkka]

Chrisbee wrote:

Okay. Nearfield measurements at 4" from the dustcap of bottom driver of my IB.

Both meters carefully calibrated individually to 80dB prior to measurement.

Meter calibration file in REW cleared prior to each measurement to ensure no cal file loaded.

Like two peas in a pod!!!

Thank you! Just what I thought. You should definitely use Sonnie's old analog correction file for both of them.

And might I say, that's a fine looking NF response. When you put the correction file in, you'll have loads of low frequency gain! How does it look at the LP?