Tenma 72-860 SPL Meter .cal File

[BigPines]

I saw the calibration files for the Radio Shack meters. I have the Tenma 72-860 meter and I'm now thinking I probably need a cal file as well. Does anyone have any experience with this meter or know if it is any good? I paid about $125 for it as I recall so it is not the cheapest or the most expensive meter on the market.

How do I know if I need a .cal file? Where do I get one or (gulp) how do I make one? I may be in over my head here.

Thanks in advance.

Mike

[brucek]

You need to have the microphone calibrated at a place such as WCCL, or alternately (and less accurate) you need to borrow a cal file from someone who has had their microphone calibrated. Then you just make a file that can be read by REW.

The problem with borrowing a cal file is that you don't know if the mic you own is consistent in its response between the same model. We found the ECM8000 and the Galaxy is fairly close and offer the cal files on our download page. It's not as accurate as a real calibration, but certainly better than not using a file.

Most microphones won't measure flat down to 15Hz as required for subwoofer measuring, and so usually a cal file is needed.

brucek

[BigPines]

Thanks Bruce,

I looked up the specs to my meter and found the following:

"Typical Instrument frequency range: 30Hz ~ 12KHz"

What!? This sounds like a problem to me. Can this even be compensated for with calibration or do I need a new meter?

Supposing I can get the meter calibrated, could I make the .cal file with REW?

Sorry, I'm a bit of a newbie to this.

Mike

[brucek]

Supposing I can get the meter calibrated, could I make the .cal file with REW?

Sure, it's very easy once you have a calibration file to make it comply with REW.

REW cal files are actually simple .txt files with the first entry being the frequency and then followed by how much compensation is required. For example here's an extract from the newecm.cal for the ECM8000 meter off our downloads page. It reads that at 10Hz the mic is down -9.23dB and needs to be compensated by that much in REW, etc, etc.
10.00 -9.23
12.50 -6.64
16.00 -4.68
19.95 -3.09
25.12 -2.08

You can create a calibration file of your own and then rename it to a .cal extension from a .txt extension that notepad uses. You can also download any of our .cal files on the downloads page and rename them and look at the contents. Some people even make their own files by taking readings off a graph they have for their microphone.

One problem is that it is usually at least $100 to have a microphone calibrated. Then you also need an SPL meter to set the approximate level in the room when measuring with REW, so that's another expense.

The easiest and fairly accurate method for the least cost would be to buy a Galaxy CM-140 SPL meter and use the .cal file from our site and you've got all you need. The Galaxy is a mic and meter all in one with a fairly accurate calibration file.

brucek

[BigPines]

I checked out the Galaxy-Audio CM-140 and can get it for about $100 which is not bad. However, I decided to see if this meter could handle more of the frequency spectrum than my Tenma and I found this:

"Frequency Response 31.5 Hz - 8 kHz"

This is even worse than my Tenma. Sorry if these are stupid questions but should we be alarmed by this? We really can't measure above 8 KHz or below 31.5 Hz? What is the point of calibrating the SPL if it has such sever limitations in frequency response?

I guess then my choice would be to either pay $100 to have my Tenma calibrated or pay $100 to get a Galaxy-Audio and use the existing .cal file - or pay $200 to buy a Galaxy-Audio and have it calibrated. Hmmm...

Mike

[brucek]

This is even worse than my Tenma. Sorry if these are stupid questions but should we be alarmed by this? We really can't measure above 8 KHz or below 31.5 Hz? What is the point of calibrating the SPL if it has such sever limitations in frequency response?

When a response spec is given for a device, it often comes with an indication of the deviation. A typical response may be stated as (20Hz - 10KHz +/-3dB). This gives us an indication of the limits over the bandwidth specified. The deviation (when missing) in the spec implies the device is quite useable within the range specified. This doesn't mean that it doesn't respond to the frequencies below the range given. In fact, the response simply trails off and can be compensated for by simply adding the deficiency onto the response at that specific frequency.

So, let's look at the Galaxy meter from 10Hz to 35Hz. You can see that the meter doesn't hit a brick wall at 35Hz, it simply trails off with a lowering output.

10.00 -10.62
11.00 -9.86
12.00 -9.03
13.00 -8.40
14.00 -7.95
15.00 -7.40
16.00 -7.04
17.00 -6.56
18.00 -6.28
19.00 -5.90
20.00 -5.49
21.00 -5.10
22.00 -4.75
23.00 -4.35
24.00 -4.03
25.00 -3.73
26.00 -3.53
27.00 -3.25
28.00 -3.00
29.00 -2.80
30.00 -2.70
31.00 -2.60
32.00 -2.42
33.00 -2.34
34.00 -2.22
35.00 -2.20

At 35Hz the level is -2.2dB low. If we add that value to the reading we take at 35Hz, then we're back to zero. If we do this to all the readings we take down to 10Hz (where we have to add +10.62dB), we have a perfectly flat meter.

Get it?

$100 to get a Galaxy-Audio and use the existing .cal file

Remember that with a microphone, you also need a meter to use REW correctly. The Galaxy is a microphone and meter in one. They are very consistent between units, so the cal file on our site is quite close.

brucek

[BigPines]

Thanks, that helps a lot!

So the question is, how do you figure out where the meter trails off? Do they give you this information when they calibrate the device? If so, this all makes perfect sense.

Mike

[brucek]

So the question is, how do you figure out where the meter trails off?

You can either send the meter in for calibration or you can use a (less accurate) comparison method to derive the file from a known calibrated microphone.

We sent an ECM8000 to be professionally calibrated, then we did some rather painstacking near-field comparison measurements of that ECM8000 and a Galaxy CM140 (actually a whole lot of Galaxy CM140's) to derive the CM140 file we have on our site.

This was also done with the three Radio Shack models of SPL meter.

We feel they're all within a few dB. Close enough when measuring a sub.

Here's 3 ECM8000 and 3 Galaxy CM-140 meters compared using the cal files from our site.

If you feel you need it closer than this, you should have your own calibrated.

3ecm and 3galaxy.jpg

brucek

[BigPines]

Wow, those are incredibly close! Close enough for my needs I would imagine. Is one of those lines the actual meter that was calibrated or are they all off-the-shelf meters that are using the .cal file?

Mike

[brucek]

Yeah, one of them is the calibrated one, with 2 off the shelf ECM's and 3 off the shelf Galaxy.........

[BigPines]

Nice. I'm sold!

Thank you for the education brucek.

Looks like I have a new meter in my future...

Mike