How accurate is SPL meter feature?

[Scrubsphobia]

I'm currently using UMIK-1 with calibration file and im wondering on how accurate this really is? If its as accurate as any hand held SPL meters im considering using this feature instead of a hand held one.

 

[Robert B]

Being calibrated it will be much more accurate than most consumer level SPL meters. I can't say precisely how much more accurate.

 

[Todd Anderson]

I'd agree with Robert on that one.

 

[AudiocRaver]

I agree with Robert as well. Extreme absolute accuracy is not all that important for most of what we do. Relative accuracy between measurements is very important, and would be very high with REW and a UMIK-1.

 

[Scrubsphobia]

Thanks for all the replies! This is the curve after EQ'ing a bit tonight. found out that my room can't handle 16hz tuning on a SVS PC12-plus so. currently running 20hz. and a XTZ 12.16 in the opposite corner. https://imgur.com/a/2RDfv

 

[Matthew J Poes]

Its accuracy is more dependent on your calibration and microphone than on the software. The software can be basically accurate as anything out there. With the right mic it will easily meet class 1 standards, and in fact, no reason it can't meet class 0 lab standards. If you are using any of the inexpensive mics it easily meets class 1 standards. If its a USB mic that automatically sets sensitivity it comes very close to what my calibrator says it should be. In fact, for all I know it is right and I just don't have a good fit with my calibrator.

With most mics it will give accurate results down to the noise floor of the room or 35dbs, whichever is higher, and up to about 125-130db's or so. With better mics and electronics it can do whatever you feed it. With a Lab grade low noise measurement mic and preamp I see no reason it won't go down to the actual noise floor of the room. I've personally read as low as 20db's in a scenario where the mic was calibrated and I had reason to believe that was an accurate measurement.

 

[Wayne A. Pflughaupt]

All good answers here.

Anytime I see a post with someone asking about the “accuracy” of a SPL meter, I immediately question what kind of “accuracy” they’re talking about. Most often people on home theater forums are inquiring about frequency response, but the fact is, SPL meters are typically not designed for flat frequency response, at least not stand-alone meters.

In short, SPL meters are designed to register noise levels, and as such “accuracy” means: If the noise source is generating an actual 83 dB, how close to that is the meter showing?

Most affordable meters have a pretty wide variable. For instance, Class 3 meters (the most affordable) typically are have a +/-1.5 dB tolerance, which means they can be as much as 3 db off. Class 2 meters, such as the popular Galaxy CM140, have a +/-1 dB tolerance.

If you want more accuracy that that, it starts getting expensive – often hundreds of dollars.

Some higher-end meters do feature flat frequency response, but you’re usually talking over $1000 for a meter like that.

The point is, with REW and a $100 UMIK mic, you have not only a great platform for frequency response and acoustics measurements, but a laboratory-grade Class 0 SPL meter.

My only concern with the REW SPL measurements, if one really needs highly-accurate readings, is that all commercial-grade meters come with outboard calibration devices, and are supposed to be calibrated prior to each use. I’m not sure how that squares with REW.

Regards,
Wayne

 

[Matthew J Poes]

All good answers here.

Anytime I see a post with someone asking about the “accuracy” of a SPL meter, I immediately question what kind of “accuracy” they’re talking about. Most often people on home theater forums are inquiring about frequency response, but the fact is, SPL meters are typically not designed for flat frequency response, at least not stand-alone meters.

In short, SPL meters are designed to register noise levels, and as such “accuracy” means: If the noise source is generating an actual 83 dB, how close to that is the meter showing?

Most affordable meters have a pretty wide variable. For instance, Class 3 meters (the most affordable) typically are have a +/-1.5 dB tolerance, which means they can be as much as 3 db off. Class 2 meters, such as the popular Galaxy CM140, have a +/-1 dB tolerance.

If you want more accuracy that that, it starts getting expensive – often hundreds of dollars.

Some higher-end meters do feature flat frequency response, but you’re usually talking over $1000 for a meter like that.

The point is, with REW and a $100 UMIK mic, you have not only a great platform for frequency response and acoustics measurements, but a laboratory-grade Class 0 SPL meter.

My only concern with the REW SPL measurements, if one really needs highly-accurate readings, is that all commercial-grade meters come with outboard calibration devices, and are supposed to be calibrated prior to each use. I’m not sure how that squares with REW.

Regards,
Wayne

Hey Wayne,

I am not sure if you were asking seriously, but I can probably answer this anyway. REW needs to be calibrated with most microphones. However, because a USB mic has a fixed gain level and thus a fixed output for a given input, it can be set automatically in REW. The sensitivity data is then included in the correction file so that REW can create a calibrated spl level. The accuracy of this data in the correction file I suppose would effect the accuracy of this. I used to have, then sold, now have again a calibrator. Nothing expensive, but still a lot more accurate than what most people have. I used the Dayton USB mic and this calibrator recently to do a test. The calibrator is 94db's +/- .5 db's I believe. I measured closer to 95db's when I tested and so I found it a little off. Of course, for what we do, that is VERY accurate. I think if we are within a db or so that is more than good enough for setting up a room. In fact, the actual accuracy is totally unimportant if its reliable, i.e. relatively accurate.

NIOSH has an SPL meter app for smart phones called the NIOSH SLM app. If you have an iPhone its accurate as a class 2 meter per NIOSH testing, and if you add an inexpensive external mic its class 1. If you have an android they found inconsistency in mic quality and couldn't recommend but its fine if you use an external mic. I've heard that some of the new androids are even better than the iPhone. However price is not an indicator, the new Pixel phone distorts heavily at modest ~90db spl levels and has very limited frequency range. In any case, anyone with an iPhone has access to a pretty accurate SPL meter. I can't use a calibrator with it to test, but I did use a calibrator with the Dayton external mic and its very accurate as is. It didn't need any calibration. 94db's on the nose. It will measure up to about 130db's without issue. I find that impressive!

 

[Wayne A. Pflughaupt]

Yeah, probably should have been a bit clearer - a software platform certainly needs no “calibration,” but a transducer (like the measurement mic) can have its physical (and therefore electrical) properties altered by temperature and humidity. I assume that the mic is what’s being calibrated with commercial SPL meters, but I could be wrong about that...

Regards,
Wayne

 

[Matthew J Poes]

I guess I'm not sure what you are suggesting so I may be misunderstanding. I apologize if that is the case. A microphone capsule can't be modified or calibrated. The electrical signal is modified, the little set screw on an SPL mic is just a gain adjustment basically. You do the same thing digitally with REW, adjust it so that the voltage it gets (in this case a digital voltage level) is calibrated to a particular spl level. If the digital voltage output of a device is fixed (for a given input), as in a USB mic, then as long as you know the sensitivity values, you can include that information in the file. In the REW TXT file I believe its the SensFactor that gives the sensitivity value that REW uses for its calibration.

That approach isn't possible with any normal measurement mic where you have the mic, an XLR cable, a mic preamp, an A/D converter, etc. You would have to know the mics sensitivity, the gain level of the preamp, A/D converter, etc. It's easier just to use a calibrator. But as mentioned, for our purposes, anyone using a mic setup like that should just buy a cheap type 2 SPL meter or use their phone. It's more than good enough to get them in the ballpark.

 

[Wayne A. Pflughaupt]

No, of course the mic can’t itself be calibrated. However, its properties will be altered with changes in temperature and humidity, as is the case with any transducer, and that's what the calibration is for.

Below is a graph from a little experiment I did a number of years back. It was a cold winter day (for Houston at least), and it was 73˚ inside the house. After taking a measurement, I set the mic on the floor where sunlight was streaming in a window. The thermometer registered 86˚ in the sun. After a half hour or so I took another reading. The red trace is the reading from the warmed-up mic. As you can see, response is reading 1 dB or so higher across the board for the 86˚ sweep.

Two REW readings, mic element at 73 vs. 86 degrees​

Naturally, this is mainly only relevant for industrial or commercial noise surveys, where measurements might be taken in the dead of a Minnesota winter or a south Texas summer, not so much for home theater applications. All the on-site calibration does (via the outboard signal-generating calibrator device) is compensate for any deficiencies in performance the mic might display as a result of immediate ambient conditions, so that an accurate SPL reading can be acquired.

Regards,
Wayne

 

[Matthew J Poes]

No, of course the mic can’t itself be calibrated. However, its properties will be altered with changes in temperature and humidity, as is the case with any transducer, and that's what the calibration is for.

Below is a graph from a little experiment I did a number of years back. It was a cold winter day (for Houston at least), and it was 73˚ inside the house. After taking a measurement, I set the mic on the floor where sunlight was streaming in a window. The thermometer registered 86˚ in the sun. After a half hour or so I took another reading. The red trace is the reading from the warmed-up mic. As you can see, response is reading 1 dB or so higher across the board for the 86˚ sweep.

Two REW readings, mic element at 73 vs. 86 degrees​

Naturally, this is mainly only relevant for industrial or commercial noise surveys, where measurements might be taken in the dead of a Minnesota winter or a south Texas summer, not so much for home theater applications. All the on-site calibration does (via the outboard signal-generating calibrator device) is compensate for any deficiencies in performance the mic might display as a result of immediate ambient conditions, so that an accurate SPL reading can be acquired.

Regards,
Wayne

Ah I see, Fun experiment.

I'm actually working with a friend to conduct a CEA-2010 subwoofer measurement in 40 degree weather. Should be complete one of these weekends. It will be below the official minimum temp for such a test. My friend has already completed a high temp version in weather over 100 degrees. We hope to be able to show the effect of temperature on this standardized test procedure, which may help give context to published CEA-2010 subwoofer tests. In this case we are testing the effect of temperature on three factors. Air density, the subwoofer transducer, and the mic transducer. It is likely that the current temperature bandwidth for this test is too wide, that in fact measurements at one end are substantially different from measurements at the other end, and so someone who lives in a temperate zone like I do where we have seasons who is taking measurements for research or magazines under varied conditions could actually be giving inaccurate results (in the sense that they would be compared against each other even though they aren't comparable).