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### Messages - gorecki

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##### SoundMeter / Re: How to calibrate an iPhone ?
« on: November 15, 2011, 11:11:27 AM »
Hi Ben,

Thanks for clarifying.
I knew about decibels being a logarithmic representation of the ratio between a measured pressure (or voltage) value and a reference value. And I also got that adding/subtracting from a dB value (i.e. "trimming") is equivalent to multiplying/dividing (i.e. scaling) the underlying pressure or voltage value.

However, I really did find some Android apps (at least 2) which applied calibration by scaling dB values themselves. When I filled in a calibration value of 2 (the default being 1), the dB readings in those apps exactly doubled. Another Android app I found explicitly mentioned in the UI that calibration was achieved as follows: dB' = (dB x scale) + trim.
I am not saying that these are smart ways of doing it, only that it is being done. It is very likely that these apps are made by amateurs how do not know much about acoustics (while you obviously do).

So just so that I understand correctly, when you say your apps "scale raw input signals based on measured (calibrated) sensitivity values and then calculate levels in dB", does this mean that, before the dB value is calculated, the (voltage) signal is multiplied/divided by a constant device/model-specific scale factor? For example, on an iPhone1 you multiply the signal by a constant X, on an iPhone3G by a constant Y, on an iPhone3GS by a constant Z, etc.? And X, Y and Z were probably determined experimentally by comparing iphones against reference equipment?

Regards,

gorecki.

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##### SoundMeter / Re: How to calibrate an iPhone ?
« on: November 13, 2011, 10:05:52 AM »
FaberAST: Out of curiosity, how does SoundMeter adjust the measured sound level with regards to the calibration?
I've been looking at some SLM apps for iOS and Android and roughly speaking I found 2 methods:
• "trimming": a phone-model-specific constant is added or subtracted from the measured value, e.g. 78dB (measured) -3 = 75dB (displayed); or 55 (measured) + 6 = 61 (displayed)
• "scaling": the dB value is multiplied or divided by a phone-model-specific scale factor, e.g. 70 (measured) * 1.05 = 73.5 dB (displayed)
• (one Android app used a combination of both approaches)

I was wondering which approach is used in SoundMeter (and in your dB app?) and what your thoughts are on this subject.

Regards,

gorecki.

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