Thursday, June 20, 2013

RTA not working on Samsung Galaxy S4

Literally, the application named RTA does not work correctly on the Galaxy S4.  It is unable to show an interesting graph of pink noise response from my living room audio system (pictures below, sorry they got turned upside down somehow automatically).  Furthermore, the response shown doesn't vary much with orientation of the phone from the listening position.  Whether I point the front of the phone at the quite directional electrostatic speaker or not makes very little difference.  (That lack of directionality might be good for some things, like measuring actual noise exposure, but bad for measuring specific sounds or system responses.)  In case you are prepared to argue the problem is the ultimate SPL limit, which may well be around 90dBA as it is for other Android phones or so I have read*, I tried measuring at two different system SPL levels, which were around 75dBA and 65dBA, and they showed the same uninteresting picture, simply having an enormous peak around 8kHz and rolling off dramatically above and below that by 10's of dB's.  (Tthe left is 75dB upside down and the right is 65dB turned left.  And, strangely enough, it matters little which way the phone is turned, just as the pictures might suggest, but actually I had the phone laying flat in the same direction for both pictures.)

In contrast, the application named RTA on my iPhone G3 shows a very interesting and plausible frequency response of my living room system, broadly flat through the mid to highs but with a few peaks and dips below 600Hz, and then rolloff at the very lowest and highest frequencies.  I'm sure the rolloffs at the top and bottom are iPhone limitations, I have 16Hz-25kHz nearly flat response, but I think the rest is fairly accurate, or if not, it's simply the result of 4 and a half years of smartphone wear or careless measurement technique resulting in minor reflections.  The picture changes in interesting ways depending on orientation--notably the extreme highs get flatter when the bottom of the phone is pointed right at the speakers.  That's what I mean by an 'interesting' picture, when you make changes, you can see their effect reflected in the graph.

A different Android app running on the Galaxy S4 gives an equally plausible but different picture.  This is from AudioTool, the paid version:

Because AudioTool does show plausible response, I suspect the problem is that the default calibration of the free version of RTA isn't good for the Samsung Galaxy S4, and unfortunately you can't change the calibration on the free version.

(*I don't know why Android phones are limited to 90dB.  Tiny capsule microphones usually have a much higher limit, something like 110-140dB.  The iPhone SPL app can measure up to about 105dB, but that could be because the app is automatically compensating for known microphone compression.)

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