Scott Wilkinson tries out his new audio-measurement app and microphone at the movies. How does it measure up to his professional SPL meter?
I'm a strong advocate for hearing protection—plus, I'm an audio geek by nature—so I often measure sound levels in loud environments, especially commercial cinemas, concerts, etc. Normally, I use a Larson Davis Model 720 professional SPL (sound pressure level) meter, which measures the instantaneous level as it changes over time and calculates various metrics, such as Leq (equivalent continuous level over the entire duration of the measurement; i.e., the level of continuous, steady-state noise that would result in the same exposure as the measured sound), Lmax (the maximum RMS level in any 1-second interval during the measurement), Lmin (the minimum RMS level in any 1-second interval during the measurement), L10 (the level the measured sound stayed above 10 percent of the time), L50 (the level the measured sound stayed above 50 percent of the time), and L90 (the level the measured sound stayed above 90 percent of the time). It also calculates the noise-exposure dosage as a percentage of the maximum daily dosage based on various criteria.
Recently, I found an app called AudioTools for iOS that does much the same thing along with many other functions. Offered by Studio Six Digital
, AudioTools is not inexpensive for an app—$20, but on sale for $10 until July 25—and that's only for the basic tools, which are nevertheless quite useful. These include a basic SPL meter that looks just like the RadioShack analog meter, an RTA (real-time analyzer), an FFT (Fast Fourier Transform) display, line level and frequency measurement, a dual-trace audio oscilloscope, a speaker-polarity tester, some system-design and installation tools, a signal generator, microphone monitor, a very handy audio calculator, and an audio recorder. Measurements and recordings can be saved and exported to a computer.
Among the many available add-on modules, I was most interested in SPL Graph, which is another $10. This module measures and records SPL over time and provides the metrics I'm used to from the Larson Davis meter—well, most of them. It does not calculate L33 or dosage, but it does provide Leq, Lmax, Lmin, L10, L50,and L90. It also graphs the level over time, and you can use standard finger-swiping gestures to zoom in and out of the graph and place the cursor at any point in the measurement. It also lets you store and recall each measurement, and it even lets you record the audio so you can go back and determine exactly what caused, say, any peak.
After I bought AudioTools and SPL Graph, I tried them out on my iPhone 4S using the phone's internal microphone. One of the first measurements I took was at the Sonos party at CEDIA 2013, at which a live rock band was playing. The display soon indicated an overload, which wasn't all that surprising—the band was really loud!—but I was disappointed that the iPhone's internal mic didn't have more dynamic range. Mark Henninger (imagic) was with me, and the mic in his Samsung Galaxy S4 Android phone didn't overload.
That experience led me to look for a better external mic to use with AudioTools—which Studio Six Digital just happens to offer for $229 ($200 from its US distributor, AudioControl). The iTestMic connects directly to an iDevice using a 30-pin connector, and you can use an adaptor to connect to a newer iDevice with the Lightning connector. It is powered from the iDevice, drawing less than 50 mA, or it can be connected to an AC charger from its mini USB port and charge the iDevice while running for long periods. I've made measurements over three hours starting with a full charge and the battery was drained only about 50%.
The omnidirectional iTestMic is factory-calibrated, and it downloads its calibration to AudioTools when you connect it. The frequency response is spec'd from 20 Hz to 20 kHz (±3 dB), and it has two selectable gain ranges: 27-105 dBA and 48-120 dBA. I'm not generally interested in low-level sounds—at least, not yet—so I usually select the high range, which is set in the AudioTools app. (BTW, the iTestMic will work with other audio apps; see the Studio Six digital website for more info.)
The only thing I don't like about the iTestMic is its short cable, which is about 17 inches long. According to Studio Six Digital, this is due to the limitations of data communication and power transfer, and an extension cable is not recommended. If you need to set up the mic far from your position, the company offers the AudioTools Wireless app ($25), which sends the mic data to another iDevice running AudioTools over any local WiFi network. The wireless app also has its own SPL meter and remote signal generator.
For now, I'll make do with the iTestMic's short cable. My most common application is measuring movie levels, so I take my iPhone, iTestMic, and a cane to the theater. I tape the mic to the top of the cane using blue painter's tape so the end of the mic is out in free space, and I wedge the cane between the seats. The cable is long enough so the iPhone rests in the cup holder. (When AudioTools is running, the phone does not go to sleep, so I make sure the screen is covered so the light doesn't distract me or anyone else.)
My first outing with the new rig was to see X-Men: Days of Future Past. I ran AudioTools with the iTestMic and my Larsen Davis meter together so I could compare the results, but unfortunately, the Larsen Davis was mistakenly set to A weighting, while AudioTools was set to C weighting, which I recently decided to use because it's a better indication of overall level, including low frequencies. The downside is that OSHA and other standards use A weighting, making it more difficult to determine if an event is within OSHA guidelines.
Here are the results I got with AudioTools:
Leq: 90.2 dBC
Lmax: 115 dBC
L10: 99 dBC
L50: 91 dBC
L90: 71 dBC
The Larsen Davis numbers were all right around 10 dB less, which makes sense when comparing A and C weighting. I was a bit surprised that this movie didn't yield higher numbers—there were some loud scenes, of course, but not as many as I expected.
Last night, I saw How to Train Your Dragon 2 and got the following results from AudioTools:
Leq: 83.5 dBC
Lmax: 102 dBC
L10: 91 dBC
L50: 83 dBC
L90: 73 dBC
I didn't use the Larson Davis meter this time (the battery died). As you can see, the movie wasn't all that loud—I didn't feel like I needed my earplugs, though I did put my fingers in my ears a couple of times when the Alpha dragons roared. (BTW, Dragon 2 is a wonderful movie and a great sequel to the first one.)
I discovered one bug that Studio Six Digital is working to fix. The current version (7.1) doesn't display L10, L50, and L90 (collectively called Ln) after stopping a measurement. You can cause them to appear by zooming into the graph, at which point they appear with the values for the visible portion. Then you can zoom out to see the whole graph, and the Ln values remain on the screen, adjusted for the entire measurement. I'm told this bug will be fixed in version 7.2.
The package I put together is not cheap at $230, but that's way less than a meter like the Larsen Davis, which is several thousand dollars. Plus, AudioTools can do much more that the Larsen Davis, and it's much easier to use. Finally, Studio Six Digital has been very responsive to my questions and problems, which speaks well of its customer service.
If you are looking for a good audio testing and measurement app for your iDevice, I can definitely recommend AudioTools and the iTestMic.
Like AVS Forum on Facebook
Follow AVS Forum on Twitter
+1 AVS Forum on Google+