Can I use Radio Shack SPL meter with REW for our needs?

[mbira]

Hi folks,
I am just diving in to REW to try and get an idea of what is happening in my room. In their literature, they say you can use the Radio Shack SPL meter but that it is really only good for the low end. I have that meter, and I don't have one of those specific types of mics. Can I use the Radio Shack meter, or do I need to get the mic?

Also, are there are tutorials specifically on setting REW up for our studio needs as it seems like there is conflicting information in what they recommend (a few examples is calibrating to 75 dbu and putting the mic straight up vertically instead of pointing at the speakers, etc).

Thanks!

[Soundman2020]

You probably could use the RS meter, but to be honest, I'd just go ahead and buy a proper acoustic measurement mic. For 100 bucks or so, you can get a decent one. Not expensive at all. You'll need both the meter and the mic to calibrate REW properly.

Also, are there are tutorials specifically on setting REW up for our studio needs as it seems like there is conflicting information in what they recommend (a few examples is calibrating to 75 dbu and putting the mic straight up vertically instead of pointing at the speakers, etc)

The REW manual and tutorials are aimed mostly at home theater builders, not recording studio builders. I'm not sure why, but they seem to prefer 75 dB as their reference point. However, the "standard" level for calibrating studios and cinemas is 86 dBC. You get that by setting each of your speakers to 80 dB if you have a 2.0 system, or 76 dB each if you have a 2.1 system, or 72 dB each if you have a 7.1 system, or 71 dB each if you have a 7.1 system, etc. That way, with all speakers on and playing the same coherent signal, the total level will b 86 dB. That's loud enough to ensure that you trigger all of the important mods, and that you have a good signal-to-noise range in your measurements.

Setting the mic vertically is, indeed, an accepted method for getting a valid reading. However, I feel that it over-emphasizes ceiling reflections in small rooms with low ceilings, so I prefer to angle the mic upwards at about 60°. You don't want it pointing straight at the speakers (except for a couple of specialized measurements, in some circumstances), as that tends to over-emphasize the speaker instead of the room. 60° up seems to be a good compromise between those two extremes.

I have developed a REW calibration and testing procedure that I use with all of my paying customers when I'm tuning their rooms. If you'd like a copy of that, then PM me, and I'd be happy to send it to you.


- Stuart -

[mbira]

Thank you for the detailed reply. I will get a mic to do the job right. Also PMing you now....thanks again!

[Soundman2020]

Also PMing you now....thanks again!

I got your PM and responded to it weeks ago, but I just noticed that you never accepted that response....


- Stuart -

[DanDan]

Some REW thoughts. I have had trouble with those RS meters. Mics are readily available now which come with Calibration Files. Some include an absolute level line at the start.
REW will recognise such files and the inbuilt SPL Meter will become Level Calibrated, while the Frequency Response is in effect flattened.
Cal File are typically 0 degrees, but many now included a 90 degree also.
Furthermore such Mics are available with a USB interface built in. This eliminates many many issues with interfaces and drivers etc.
I have found the Cal File Corrected UMIK-1 (two samples) to be very consistent with my B&K in terms of FR, within a dB across the spectrum. But the absolute level was about 1.8dB off. I corrected this myself by writing into the Cal File which is simply text.
Dayton, Cross Spectrum Labs, supply Mics with Cal Files. Here's an EU supplier of an in between range of mics. Classy but without the vast cost of the Bruel and Kjaer and such.
http://www.isemcon.net/en/download.htm
John, REW creator, has clued me in to the 75dB thing. It is about hearing and speaker protection. The sine sweep plus resonance can get quite hot. But more importantly REW 'hears' down due to the single frequency at any moment nature of it. Thus the S/N Ratio is a good 15-20dB better than one might seem apparent.

DD

[mark77]

John, REW creator, has clued me in to the 75dB thing. It is about hearing and speaker protection. The sine sweep plus resonance can get quite hot. But more importantly REW 'hears' down due to the single frequency at any moment nature of it. Thus the S/N Ratio is a good 15-20dB better than one might seem apparent.

Thanks for posting info on ref mikes and your experience with calibration files - very useful.

In respect of the 75dB level used by HT users of REW, I don't quite follow. The loudness angle makes sense (hearing and speaker protection), but I don't get how the "single frequency" is related to getting a 20dB better S/N ratio.

Any chance you could briefly explain the point you're making there?

[DanDan]

I have had quite a few to and fro communications with John Mulcahy, sort of Beta Testing for him sometimes. So I can't remember how much of his explanation was on GS or in PM, or a mixture of both.
I am not into some of the theoretical principles behind these systems, so my explanation can only be that of the driver rather than the designer of the vehicle. If you are into the deep stuff, JohnPM is extremely helpful.
So fwiw, a sweeping sine is playing only one frequency at any particular moment in time. Afterwards when the analysis process is in action, convolution perhaps, the system tracks what happened, it only needs to listen to one frequency at any moment. Not full range noise. That may be complete nonsense in terms of what is actually going on, but it works for me. In any case I tested it roughly. I readily saw 15dB and more greater improvements between sweep and broadband techniques.
EDIT, there are also issues relating the level of a Sine Wave to Pink Noise, e.g. RMS or Peak.
DD