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Discussion Starter · #1 ·
Hi all,

I'm considering using REW to optimise my stereo setup and future sub setup but its hard to get the Galaxy or Radioshack SPL meters in Australia - but there are lots of cheap Chinese ones from ebay

http://www.ebay.com.au/sch/i.html?_...ound+pressure+level&_sacat=See-All-Categories

Are these any good for using with REW? or are they generally junk?

Possible dumb question coming up...but I couldnt seem to find the answer in the help files: Do SPL meters need to be calibrated by the new owner using a known sound volume before use? or are they pre-calibrated out-of-the-box? If the owner needs to calibrate, how is it possible to generate a known sound volume?
 

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Hey Billy,

First you have to understand that “calibration” in the world of SPL meters is not the same thing as “calibration” for REW. With REW we want to measure (among other things) frequency response. However, sound level meters are not specifically designed for that. They’re designed for measuring noise levels, and are calibrated as such. Budget meters are Class 3 devices and should be accurate ±1.5 dB when measuring a noise source, if they’re up to spec.

But again, this isn’t especially relevant for REW. For REW we’re interested in the meter’s frequency response. By design Class 3 meters do not have flat response, but filter response as either A or C weighting, depending on the setting you employ. As you can see from the graphs below, C weighting is much flatter but begins rolling out below ~50 Hz.


Text Line Design Plot Pattern

A Weighting Curve

Text Line Parallel

C Weighting Curve


The Shack has a generic calibration file for the Radio Shack meter that compensates for the meter’s C-weighted response, and is reasonably accurate up to about 3 kHz. The reason for the upper-limit figure is, we determined that there was too much variation from one sample to the next for a generic calibration file to be accurate beyond that point for any given meter.

With that background we can answer your question about the cheap meter’s usefulness for REW. Since our calibration file is essentially compensating for C-weighing, it should be useful for other meters that hopefully track the C-weighing curve reasonably accurately. So, those meters should be useful at least for subwoofer measurements with REW.

But – and this is critical – make sure any meter you get has a line output. Without the line output it can’t be used for REW – at least not for taking measurements.

Regards,
Wayne
 

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Discussion Starter · #6 ·
Hi all,

Still considering, considering, considering... :R

I watched the REW Youtube video and where they used the Behringer ECM8000 rather than a SPL Meter, and the question occurred to me why would anyone buy a SPL when the Behringer ECM8000 microphone is half the price?

Its particularly relevant for me living in Australia, since I'd need to import a Galaxy SPL and waste time and money on postage, whereas the Behringer mike is easily available.

Is the Galaxy SPL meter better than the ECM8000 for use with REW?
 

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I may not be entirely clear on this, but I believe the whole Radio Shack SPL meter thing with REW came about because a lot (if not most) of the home theater enthusiasts who frequent forums like this already have a meter. So it was an easy way for them to use REW for measuring and equalizing subwoofers (which was the main interest of most early REW users) with only the addition of an inexpensive sound card.

The sound level meter is still useful for accomplishing REW’s SPL calibration routine. However, if you don’t have a meter you can still use REW. You just have to keep in mind that the vertical-axis SPL calibrations on the graphs are meaningless in absolute terms.

One benefit a meter has over the mic, at least for measuring subwoofers, is that its frequency response is designed to track the C-weighting curve. It’s a requirement for meeting the specifications as a Class 1, 2, or 3 device. So, with a calibration file (like we provide) to compensate for the intentional rollout of the lower frequencies inherent with C-weighting protocol, most meters should be able to get reasonably accurate low frequency measurements. Naturally, if you're interested in full-range measurements, other options must be explored.

While the basic Behringer mic is cheaper, you can see from this graph that response from one sample to the next is all over the map.




Herb at Cross Spectrum also reports in this post that the quality of the ECM mics on the whole is sliding.

Thus, it appears that the Galaxy meter is overall a better deal. I'd suggest getting in touch with Herb. As I noted in this post, you might be able to get a fully-calibrated Galaxy for not much more than the price of a generic one.

Regards,
Wayne
 
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