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Audio Measurement Apps


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Mary Gruszka / 12.21.2011 12:00AM

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.

"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 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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Mobile Apps Making Field Measurements Easier

http://www.tvtechnology.com/article/audio-measurement-apps%E2%80%94things-to-be-aware-of/211051

http://www.tvtechnology.com/article/mobile-apps-making-field-measurements-easier/210840

I enjoy audio and acoustical tests and measurements. So, at the 131st AES Convention recently held in New York, it was fun hanging out with some of my audio friends and colleagues and finding out not only what projects they were working on, but what test and measurement apps were on their mobile devices.

One of these friends, Peter Mapp, principal of Peter Mapp + Associates, a specialist acoustic consultancy in Colchester, Essex, U.K., delivered one of the AES "Hot Lunch" presentations, a 45-minute talk on this very topic.

It's truly amazing the extent of available measurement tools that literally fit in the palm of a hand. Are there limitations? Sure. But if understood, these tools are great for many of the field measurements we need to make; for quick checks; and to determine whether other test gear needs to be brought in for a job.

STUDIO SIX DIGITAL LLC

One of the more popular apps, based on an unscientific survey of my audio friends at the AES convention, seems to be Audio Tools from Studio Six Digital LLC. This is a suite of audio and acoustical test and measurement app modules for the Apple iPhone, iPad, and iPod touch.

Andrew Smith, technology director of Studio Six Digital, is no stranger to audio test and measurement. He founded TerraSonde, the maker of the AudioToolbox range of analyzers.

The basic Audio Tools suite ($19.99 as of October 2011) includes a sound pressure level (SPL) meter; an SPL Dashboard for the iPad, which puts all purchased SPL modules on one screen; a real-time analyzer (RTA); a dual-trace audio scope with triggering and single sweep capture; delay finder to find the signal delay between loudspeakers; signal generator; surround sound generator; monitor for the mic or line input; file upload capability; a calculator designed for audio and acoustical work (includes dB, SPL, sine wave, reactance, room modes, Ohm's law, delay and bandwidth); and a recorder.

If the basic suite isn't enough, the user can add modules for additional fees. Most range from $2.99 to $19.99, with specialty modules higher.

The SPL module can be enhanced with SPL Pro, SPL graph, and SPL traffic light. Additional acoustic analysis modules include FFT (fast Fourier transform), ETC (energy time curve), impulse response, and Smaart Tools. The latter two modules are two of the higher priced modules.

Smaart Tools sound system measurement, optimization and control software is licensed from Rational Acoustics, who makes this program for the PC or Mac.

Additional line input modules for Audio Tools include VU/Peak meter, THD+n (total harmonic distortion plus noise), amplitude sweep, and a phase meter.

Additional speaker test modules include a polarity tester for loudspeakers; a distortion driver; an enhanced impedance meter; and the latest standard (version 4) for speech intelligibility index (STI-PA). The latter is the most expensive module at $299.99 due to the various royalty fees, according to Smith.

As Mapp pointed out in his presentation, STI is a sophisticated measurement and for the measurements to be accurate, the user must use an external interface box.

Conveniently, Studio Six Digital offers such an interface box—the new iAudioInterface2, which was shown at the AES convention. This battery-powered digital interface for the iPhone 4, iPod touch 4, and iPad includes a phantom-powered mic input, balanced line input, balanced line and headphone output, and a Toslink optical digital audio output.

FABER ACOUSTICAL

Mapp, in his AES presentation, also showed some of the audio and acoustical test apps for the iPhone, iPad, and iPod touch from Faber Acoustical. These include IOScope ($149.99), SignalScope Pro ($74.99), SignalScope, SignalSuite, SoundMeter, SpeakerDraft, and dB.

SignalScope Pro includes an FFT analyzer, octave-band analyzer, sound level meter, oscilloscope, and stereo signal generator. SignalSuite is a signal generator, and SpeakerDraft helps loudspeaker low-frequency response. The dB module can record an image overlayed with a display of the sound level.

FOR SMARTPHONES, TOO

Do you prefer smartphones running on the Android operating system? There are audio test and measurement apps for you, too.

Kurt Graffy, a principal consultant with Arup Acoustics in San Francisco, showed me the Audio Tool suite by jjbunn (Bofinit Corp.). This set of modules includes a real-time spectrum analyzer; SPL meter; spectrogram; chart recorder; signal generator; loudspeaker polarity checker; data file storage capability; decibel meter; noise criterion curves; chart recorder; and reverberation time measurements.

Studio Six Digital, working with AudioControl, provides SPL Meter app with characteristics found on analog SPL meters including ballistics, ranges, filters and decay rates.

Smith from Studio Six Digital said, "We have a great relationship with AudioControl. They are handling both the Android development (as it grows), and they are warehousing our hardware and acting as our master distributor. We see more synergistic things in the future."

BACK TO THE SHOW

After listening to presentations on grounding and shielding and hums and buzzes, by Bill Whitlock from Jensen Transformers of Chatsworth, Calif., it became clear that we need a gauss meter in our audio tool kits to see if external magnetic fields are causing problems in our systems.

John Siau of Benchmark Media Systems in Syracuse, N.Y., attended one of these sessions and showed us the Metaloid Field Detector from jjbunn (Bofinit Corp.) on his Android-based smartphone. This app uses the smartphone's built-in magnetometer to measure magnetic fields. Siau said the readings on his smartphone track well with his hardware gauss meter.

Siau also showed me the Speedy Spectrum Analyzer and FFT from Electron Chaos, and Perfect Pitch guitar tuner, both for Android-based phones.

This gives only a brief idea of all the capabilities of the apps mentioned and even of all the audio test and measurement apps that are available.

There are some things to watch out for when doing measurements on mobile smart devices, which will be discussed in a future column.

thats the one above

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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Very informative, thanks for the update Wolf, I've been a fan of Studio Six's FFT for a while now being able to see problematic sounds generated by the lighting departments. Finding whistles from lamp heads and hums from generators, cicadas and the like and being able to make notes for post. Confirming this with the amazing sweepable notch filtering on the Fusion is fun ... now if only the Fusion could simply EQ my chosen mix track and leave my ISO tracks un touched!!!

Cheers, Malcolm

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