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Mar 29, 2012

[Note: I had started to write this review in the winter of 2011, but real life happened and I never completed it. There has been interest in my experiment with the iAudioInterface but keep in mind that this review discusses the first-generation iAudioInterface released in late 2010. Studio Six Digital is set to release their second-generation model which includes significant upgrades including microphone phantom power.]

In the fall of 2007 I was contracted to work on a project that required me to work with a Bruel & Kjaer 2250 sound level meter. It was my first time working with the meter, and upon receiving the instruction manual from the client, I became extremely excited. The meter had everything (on paper at least): time history logging, spectral logging, voice annotation, expandable memory via compact flash or secure digital cards, and (what I considered most important at the time), a flexible touch-screen interface that provides interactivity during measurement operations.

It was everything I could have dreamed of.

Looking at the screenshots in the manual, I suspected that the 2250 was built on a Windows CE platform. A quick look at the Microsoft license hologram in the battery compartment proved my suspicion to be correct. But I was still excited to try out the meter. Well, I used it for a week, and I was none too pleased with it. I believed that we could do better.

Something else happened in mid 2007 - Apple released its first iPhone. I didn’t (and still don’t) own an iPhone, but at that time I had played around with it enough to see that the multitouch, stylus-free touchscreen interface was the future of portable computing. I wanted to use a meter that incorporated these technology. And I wasn’t alone.

The “well-respected sound level meter developer” I was talking about in the above link is a guy named Andrew Smith. He was the brains behind TerraSonde, the manufacturer of the Audio Toolbox Plus. While at TerraSonde I emailed him a number of times with tech support questions and he demonstrated a good understandings of the standards process and a willingness to help customers. I was excited to see what he could come up with in using the iPhone (or iPod touch) as a development platform for acoustical measurement equipment.

Smith formed Studio Six Digital which introduced two products last year that puts them well on their way to my Utopian vision: the AudioTools suite of iOS apps (and it is a suite, more on that later), and the iAudioInterface to provide with iPhone/iPod with a precision microphone analog input.I was able to get my hands on an iAudioInterface and a 3rd generation iPod touch (note that the iAudioInterface will not work with the current 4th generation iPhone/iPod touch or the iPad) and put it through it paces.

The AudioTools app is a $20 basic audio measurement program that can also perform more advanced measurements through additional in-app modules that are available for for $8 to $60 each. The AudioTools app alone gets a recording function, a basic sound meter (basically simulating the Radio Shack analog slm), a basic 1/3 octave band meter and a basic signal generator. Available modules include FFT functions, logging sound meter, SPL “traffic light” (which triggers when a SPL threshold is met), impedance measurement tools, and many more functions. As with the Audio Toolbox hardware, the meter functions are designed to meet ANSI and IEC specifications.

The iAudioInterface consists of a breakout box with a dock connector to mate with the iPhone/iPod, an included Studio Six Digital-branded microphone, line input, headphone output (for monitoring) and a mini-USB port for powering the iAudioInterface and iPhone/iPod using a 5-volt power supply.

For my purposes, I want to use the iAudioInterface/AudioTools combination for unattended noise-monitoring applications. Since these measurements may have to meet legal scrutiny, the first casualty of the iAudioInterface setup is the microphone - by all accounts the mic may be suitable for non-critical (and perhaps even critical) acoustical measurement work, but since it is not spec’d to ANSI/IEC, I really can’t use it. Instead, I substituted a precision measurement microphone (BSWA MP201) and ICP power supply connected via the aforementioned line-input.

Now that I have the iAudioInterface mated with a Type 1 microphone, the obvious question is: how accurate is the data that collect with this combination? To answer this, I decided to deploy the iAudioInterface/AudioTools instrument alongside a calibrated Larson-Davis 820 integrating sound level meter. I mounted the microphones side-by-side on a microphone stand and let them run in my backyard over a March weekend (March 12-13, 2011). Both instruments were calibrated using the same acoustical calibrator and were programed to measure A-weighted sound levels and 1-second Leq’s.

The verdict: the data from the two instruments were the same to well-within 1 dB. The figure posted below is typical - over this 10 minute period, it’s difficult to distinguish between the two data sets (click the plot for a larger version).

ld820 vs iAudiointerface data graph

The iAudioInterface + iPod touch combo sips power - after running the instrument for a weekend, there was barely in reduction in the charge of my 14V/9Ah Li-on battery. At that rate, I figure the combo could go for a couple of weeks, although I haven’t tested. Coupled with a 32 GB iPod touch, the instrument can be used for fairly long-term noise monitoring of A-weighted sound levels (as well as WAV file recording). Unfortunately the AudioTools suite doesn’t include a module for octave-band logging, but hopefully this will change in the future (I imagine that enough feature requests would convince Andrew to add it).

The only downside I’ve come across is that the noise floor of the iAudioInterface line input is fairly high, such that using a mic with 25 mV/Pa sensitivity or lower will result in a noise floor around 33 dBA. Applying gain or using a more sensitive mic helps, at the cost of the microphone power supply using more power.

All in all, the Studio Six Digital iAudioInterface along with an iPod touch, AudioTools software and precision microphone makes an effective long-term noise monitor. I expect that we’ll see features added to the AudioTools suite over time that will make the combination competitive with high-end meters like the B&K 2250 but at a significantly lower cost.

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