CODEWITHSUNDEEP
iosobjective-caudio
william205
william205

Reputation: 688

Objective-C normalise dB level from iPhone's built in mic

I am building a dB meter as part of an app I am creating, I have got it receiving the peak and average power's from the mic on my iPhone (values ranging from -60 to 0.4) and now I need to figure out how to convert these power levels into db levels like the ones in this chart http://www.gcaudio.com/resources/howtos/loudness.html

Does anyone have any idea how I could do this? I can't figure an algorithm out for the life of me and it is kind of crucial as the whole point of the app is to do with real word audio levels, if that makes sense.

Any help will be greatly appreciated.

Upvotes: 0

Views: 1064

Answers (2)

marko
marko

Reputation: 9159

Apple's done a pretty good job at making the frequency response of the microphone flat and consistent between devices, so the only thing you'll need to determine is a calibration point. For this you will require a reference audio source and calibrated Sound Pressure level meter.

It's worth noting that sound pressure measurements are often measured against the A-weighting scale. This is frequency weighted for the human aural system. In order to measure this, you will need to apply the relevant filter curve to results taken form the microphone.

Also be aware of the difference between peak and mean (in this case RMS) measurements.

Upvotes: 2

ezig
ezig

Reputation: 1229

As far as I can tell from looking at the documentation, the "power level" returned from an AVAudioRecorder (assuming that's what you're using – you didn't specify) is already in decibels. See here from the documentation for averagePowerForChannel:

Return Value

The current average power, in decibels, for the sound being recorded. A return value of 0 dB indicates full scale, or maximum power; a return value of -160 dB indicates minimum power (that is, near silence).

Upvotes: 0

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