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MEASURE WITH YOUR iPHONE

 

iphone-meter

Both visitors and manufacturers were brandishing a new audio device at this year's Bristol Show in the UK: an Apple iPhone loaded with audio measuring software. My eyes nearly popped out. That's because at Hi-Fi World we use sophisticated measuring equipment to test audio products and here was a phone masquerading as a substitute. It barely seemed possible. Now you can walk into a demo room at a show and measure loudspeakers on the spot, with a phone no less! Or can you?

 

Well, the App I saw from Studio Six Digital of Boulder, Colorado, U.S.A. certainly looks the part. You can find it by searching for 'AudioTools' in Apple's internet Application (App) store within iTunes, priced at a paltry £11.99 (UK). There are numerous software add-ons for around £5 or so, allowing the system to be extended, but the basic App is so comprehensive and entertaining it's best to consider the add ons later. And as you might expect there are limitations to consider too.

 

 

audiotools
Here's what you get. First there is a Sound Pressure Level meter. Point the phone's tiny mics at a loudspeaker from where you are sitting and it will tell you exactly how loud the sound is in dB. You can  balance left and right loudspeakers with this, set each loudspeaker in a surround-sound system precisely to same volume, or measure the noise of your neighbour's lawnmower on Sunday morning to lodge a complaint. No test signal is generated so you cannot send  a steady measurement signal through the hi-fi, like pink noise. AV receivers usually have a noise generator on-board, but with a hi-fi a test CD could be used, or even a VHF tuner tuned to noise. You can read more on this under loudspeaker Tests.


You also get a more powerful and flexible third-octave 'RTA' spectrum analyser – and here it is time to take a deep breath. This will in theory tell you how loud various frequencies are across the audio band, so you can see excessive bass and put a figure on it, and you can see spiky treble too and identify it. But only in theory. In practice our measurements showed the i-phone's tiny mics, purposed for speech intelligibility (well, they should be) are inaccurate for this sort of audio measurement.

 

A measuring mic costs from £280 (Clio) to £2000 (Bruel & Kjaer) so we can't expect too much from the i-phone and if you look at what it made of Unity Audio's The Rock loudspeaker against our Clio measuring microphone (accurate to less than 1dB) you can see obvious limitations. There's a steep treble spike of +8dB at 20kHz (far right bar) and a dip of -7dB at 4kHz in the i-phone analysis – 15dB variation no less  – where our measuring mic clearly shows the loudspeaker to be flat within 2dB variation. This is a misleading picture then; the RTA can only be used for very approximate views of spectrum content.


iphone-rtathe-rock-pink-noise

Left shows the AudioTools analysis of a Unity Audio 'The Rock' ' loudspeaker, right shows the actual response. Both vertical scales 10dB/division.

 
That's the bad news. The good news is you can get a more accurate external microphone able to feed the i-phone through its digital input (there is no analogue input) from Studio Six Digital called iAudiointerface. But when I looked it had sold out. Perhaps a lot of users had already discovered this limitation!


Even better news is that the RTA, unlike the SPL meter, simultaneously runs a signal generator through the i-phone's headphone output. Connect into the hi-fi with a 3.5mm stereo jack to phono plugs adaptor lead (Maplins) and you can dial up pink noise, white noise, square waves or sine waves from a natty  generator that measured very well: it was frequency accurate and amplitude flat. Funnies were some low frequency noise  below 50Hz on the display that gave a reading of 65dB SPL (unweighted) even in a perfectly quiet room, electronic flicker noise or l.f. bounce in the i-phone's power line being likely causes. Choosing IEC C weighting (see Wikipedia) helped a little and IEC A a lot to reduce influence on the SPL reading. The RTA has many facilities including a range of decay times, peak hold and averaging. There are weightings, overlays, max and mins and more. The scale can be expanded and moved (ranged) with the fingers, in normal i-phone fashion, or locked to prevent this. Overall, this is an impressive piece of software and not a toy; pity about the mics.


That is not all though –  not by any means! There's line measuring input, only functional with the iAudiointerface, and an oscilloscope that works off the mics or from the iAudiointerface. There is a delay/distance finder that generates a short impulse and measures its time from the loudspeaker. I put the i-phone 0.5m in front of a loudspeaker and it told me 7m, but that's because it was an AV receiver analogue signal path via its internal DSPs that were adding a delay. Pressing the Direct button got me an accurate reading. Measurement has its difficulties.


Utilities throws up another signal generator (phew!) a headphone monitor and measuring file export to a computer via the internet, by entering the phone's URL in a web browser. And for engineers and students there are calculators for dB conversion, SPL source addition, wavelength/frequency, capacitive and inductive reactance (but not Impedance?) and even room modes, so you can calculate where your room will honk and then measure the honk with the RTA. Ohms Law values can be calculated, delay times and bandwidth Q. Some of this is quite hardcore audio/electrical engineering.


Enough? Well, yes that is it for your money, all £11.99 of it. If you want more there are oodles of add ones, such as a full FFT analyser of 16,000 lines no less. However, i-phone screen size limits serious work and an i-pad may be better to overcome this limitation. You also need to bear in mind that whilst the software is very good, for meaningful audio measurement the input amps and ADCs are a big (hardware) issue, not to mention ranging and protection circuitry. That's why we'll be sticking with Rohde & Schwarz.


At the price this i-phone app. is amazing in its scope and power. A great tool then for audio terrorists. If the measurements don't stun people then your tech talk will. Believe me, I know!


Noel Keywood


www.studiosixdigital.com


 



 

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