Android PCM Bytes

Question

I am using the AudioRecord class to analize raw pcm bytes as it comes in the mic.

So thats working nicely. Now i need convert the pcm bytes into decibel.

I have a formula that takes sound presure in Pa into db.

db = 20 * log10(Pa/ref Pa)

So the question is the bytes i am getting from audiorecorder from the buffer what is it is it amplitude pascal sound pressure or what.

I tried to putting the value into te formula but it comes back with very hight db so i do not think its right

thanks

Answer 1

Disclaimer: I know little about Android.

Your device is probably recording in mono at 44,100 samples per second (maybe less) using two bytes per sample. So your first step is to combine pairs of bytes in your original data into two-byte integers (I don't know how this is done in Android).

You can then compute the decibel value (relative to the peak) of each sample by first taking the normalized absolute value of the sample and passing it to your Db function:

float Db = 20 * log10(ABS(sampleVal) / 32768)

A value near the peak (e.g. +32767 or -32768) will have a Db value near 0. A value of 3277 (0.1) will have a Db value of -20; a value of 327 (.01) will have a Db value of -40 etc.