How Accurate Are These Smartphone Sound Measurement Apps?
blogs.cdc.gov
blogs.cdc.gov
I have a side business that does audiovisual services, and I've been meaning to get the other folks in the crew to test their phone with one of these apps against my SPL meter (which is a ~$80 unit that I believe is pretty accurate in most circumstances), so they'd have a ballpark idea of how loud they're running the gear (there are noise ordinances that we sometimes have to take responsibility for following).
I'd be curious how well a phone could act as an RTA, as well. I used to use my Amiga to do analysis (non-realtime, but reasonably useful in a time when having a high resolution spectrum analyzer was amazing), and standard PCs have been able to act as RTAs for at least 15 years, so I'm absolutely certain the processing power needed is there in modern phones and tablets, but the mic and audio interface would be a limiting factor.
I think if I was doing any sound work anymore I would quickly pick up an external mic and just always use pink noise and the meter to rough in a system.
The problem that remains is that there are differences in microphones between iOS devices. SPLnFFT author should probably calibrate the app on every device that is out there on the market. I'm not sure if that's at all feasible.
https://twitter.com/NIOSHNoise/status/522456711882629120
I expect apps would be even more accurate with external mics
That said, the JASA paper give this very huge caveat near the end: Furthermore, this study examined these apps in a controlled noise environment. Field measurement results may vary greatly due to the effect of temperature, humidity, long-term use, object interference, and overall stability of the microphone and electronics in these devices.
Furthermore, they tested the devices with relatively high SPL signals (65 to 95 dB), the only tested overall noise levels (no frequency response testing) and in a reverberant field so these results may not be applicable to many other situations (temperature extremes, lower sound levels, etc)
The week point of using a smartphone as a sound meter is going to be the microphone. If you can pair the smartphone with a precision microphone (using something like Studio Six Digitals iAudiointerface [1] for example) you can have the equivalent of a IEC Class 1/ANSI Type 1 microphone (I and colleagues have used this setup for sound measurement projects involving litigation). But if you're depending on the device's internal microphone, be very careful at how you apply those results, especially given the vacates listed in the paper.
That said a smartphone sound meter apps will get you in the right ball park, and as alluded to by SwellJoe they can be at least as accurate (if not more accurate) than some of the $50-$100 Type 2 meters on the market. Just don't rush to any conclusions based solely on data obtained from those setups.
[0] http://www.bksv.com/Products/handheld-instruments/sound-leve...
[1] http://www.studiosixdigital.com/audio-hardware/iaudiointerfa...
"NIOSH does not recommend nor endorse a particular commercial product. The data from our study “Evaluation of smartphone sound measurement applications” is available in the journal article referenced at http://scitation.aip.org/content/asa/journal/jasa/135/4/10.1... . The discussion section (page 189) highlights two apps — SoundMeter and SPLnFFT — that had the best accuracy over our testing range. There were two other apps — Noise Hunter and NoiSee — that were within ± 2dB of the reference sound source. Once again, note that the data results in the study do not constitute as NIOSH endorsement of any of these products."
> A total of 62 Android apps were examined and downloaded, however, only 4 apps partially met our criteria and were selected for additional testing.
> Android-based apps lacked the features and functionalities found in iOS apps. This is likely due to the iOS advanced audio capabilities compared to other platforms, the open ecosystem of the Android platform, and having so many different Android device manufacturers using different suppliers and components.
The hardware is at least as important as the software (I'm amazed to see so much scatter between apps), so the homogeneity of the iOS ecosystem is a boon to this sort of measurement. Figure three in their paper shows that the Apple hardware is quite consistent across all models tested.
They did test Android phones and apps; the final paragraph from their results section makes a general statement:
A similar examination of Android apps and devices was not performed because of the low number of apps with similar functionality, and the lack of conformity of fea- tures between devices. Testing conducted with the four Android smartphones also revealed a high variance in measurements of similar apps between different devices.
Thanks for pointing the paragraphs out.
It was a boon to the researchers because it made their job more simple. Given equivalent or better hardware quality, the same measurements could have been taken without regard to which OEM produced the phone; albeit with more effort on the part of the researchers.
I have an app (Sound Meter) which apparently downloads calibration data for a number of popular phone models. This is probably the only realistic solution when you're faced with such a wide variety of hardware. I have no idea how accurate the calibration is though.
It's a byproduct of Apples total ecosystem control that developers have a very good idea of what hardware users have. This has upsides and downsides - with other phone ecosystems you lose accurate sound meters, but you gain a wider variety of devices running the same operating system.