Mary Gruszka /
12.21.2011 12:00 AM
Audio Measurement Apps—Things to Be Aware Of

With the growing number of audio measurement apps for smart devices becoming available, and for fairly reasonable prices, why should you keep your "old" test gear or consider buying new?

Even though apps offer amazing capabilities already, and are getting better with regular updates and improvements, it's still early days yet for apps to replace other test gear.

TESTING THE TEST GEAR

With any test device, it's important to test the test equipment. Compare measurements obtained with apps with those taken from accurate test gear. Make sure you can perform proper calibrations through an app. If measurements are covered by industry standards, check that the apps follow those standards.

An example of the latter was given by Peter Mapp, principal of Peter Mapp + Associates, during his presentation on apps at the 131st AES Convention in October.

Noise Criterion (NC) or Noise Rating (NR) curves are ways of indicating ambient noise levels in a space. The standard way of deriving these numbers is by using octave band noise analysis.

However, Mapp noted that some sound-level meter apps derived NC or NR curves incorrectly using one-third octave band analysis, even though they correctly got the right numbers with octave band analysis.

Studio Six Digital SPL Dashboard app is a series of four sound pressure label (SPL) measurement displays.
"A lot of people fall into that trap," Mapp said. He noted that a fixed offset could be added to calculate the correct NC or NR curves from fractional octave band readings, so check with your app provider. At least one has provided updates to correct this error.

Another issue is the quality of the built-in or headset microphone and internal circuitry of smart devices. As most were designed to perform as phones, such parameters as frequency response, maximum output level, minimum noise level, and compression aren't usually optimized for audio measurements.

Andrew Smith, technology director of Studio Six Digital LLC noted that on Apple devices (iPhone, iPad, iPod Touch), the frequency response rolls off 24 dB/octave starting at about 250 Hz.

Mapp said that some apps may try to compensate for the low-frequency roll-off by adding bass boost.

As for input levels, the built-in or headset mic and input circuitry may overload if the sound pressure levels are too high. Mapp also noted that there could be compression on the input, which could compromise some measurements.

Apps are frequently being updated with improvements, so it's difficult to put a number on these potential limitations. Also, a problem in an earlier version may no longer exist in later ones.

Consultant Ray Rayburn gave the following example, referring to Audio Tools from Studio Six Digital LLC, in a post to the Syn-Aud-Con listserve on Oct. 16, 2011.

"Apple did not give its developers access to the preamp gain in iOS 4," he wrote. "They also applied a sharp high pass filter that rolled off all the lows on the mic input… [A] bit of good news is that in iOS 5 Apple once again allowed developers to control the mic preamp gain."

"The update for Audio Tools released this week took advantage of that," he continued. "As a result, even using the internal mic, you now can have a very useful sound level meter and RTA [real-time analyzer] in your shirt pocket."

As Mapp pointed out, when taking measurements with your smart device, pay particular attention to where the mic is actually pointing. Particularly with the built-in mic, it's easy to inadvertently point the mic towards yourself instead of at the sound you're trying to measure. It's also just as easy to cover up the mic with your hands.

In either case you won't get valid measurements. Measurement app screen displays may not alert you to this problem, even if they change the orientation of the display as you rotate the device.

Some improvement can be gained by using a higher-quality plug-in mic on a smart device.

At AES, MicW showed the i436 mini omnidirectional microphone, which is designed to plug into an iPad, iPhone or iPod Touch. The company said this mic is calibrated to the IEC61672 Class 2 Sound Level Meter standard.

CONNECTING TO THE DOCK CONNECTOR

However, as Mapp pointed out, there's a caveat to using any mic or interface plugged into a smart device's headset/mic jack. That signal still goes through the internal input circuitry with the limitations already mentioned.

The way around this is to use an interface connected to the device's dock connector. This will allow more accurate measurements in general, and is definitely called for when performing speech transmission index (STI) measurements, using such apps as iSTI from Embedded Acoustics or STI-PA from Studio Six Digital.

Studio Six Digital iAudioInterface2
Studio Six Digital introduced the iAudioInterface2 digital interface for the iPhone 4, iPod Touch 4, and iPad at the convention. The unit contains an XLR connector for a microphone input with phantom power, a dual input 1/4-inch TRS (tip-ring-sleeve) balanced mono/unbalanced stereo line input, and a 1/4-inch TRS connector for either a headphone output or balanced line output.

Smith from Studio Six Digital explained why the interface box is available for Apple devices only.

"Apple iOS is a derivative of Mac OSX, which is based on UNIX. So, really, the phone is running a small UNIX, complete with full support for high-resolution audio and audio processing," Smith said. "Also, the tools are very mature and stable, and provide a great base for developing professional audio apps. The earlier models allowed flat 20 Hz to 20 kHz analog audio input on the dock connector; and now they support 16-bit 48k USB audio, which we use with our interface. Android still does not provide any way to get high-quality audio into a device, and it's unclear if or when they will do that."

With an understanding of the caveats mentioned here, audio measurement tools handily available on smart devices can be very useful for certain audio measurements or for quick checks to determine the scope of further measurements, perhaps with more gear.

As both hardware and software improves, current limitations are likely to diminish.

Mary C. Gruszka is a systems design engineer, project manager, consultant and writer based in the New York metro area. She can be reached via TV Technology.


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