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 h

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.

Answer 2

The reference pressure in Leq (sound pressure level) calculations is 20 micro-Pascal (rms). To measure absolute Leq levels, you need to calibrate your microphone using a calibrator. Most calibrators fit 1/2" or 1/4" microphone capsules, so I have my doubts about calibrating the microphone on an Android phone. Alternatively you may be able to use the microphone sensitivity (Pa/mV) and then calibrate the voltage level going into the ADC. Even less reliable results could be had from comparing the Android values with the measured sound level of a diffuse stationary sound field using a sound level meter. Note that in Leq calculations you normally use the RMS values. A single sample's value doesn't mean much.

Answer 3

The problem is likely the definition of the "reference" sound pressure at the mic. I have no idea what it would be or if it's available.

The only audio application I've ever used, defined 0db as "full volume", when the samples were at + or - max value (in unsigned 16 bits, that'd be 0 and 65535). To get this into db I'd probably do something like this:

// assume input_sample is in the range 0 to 65535
sample = (input_sample * 10.0) - 327675.0
db = log10(sample / 327675.0)

I don't know if that's right, but it feels right to the mathematically challenged me. As the input_sample approaches the "middle", it'll look more and more like negative infinity.

Now that I think about it, though, if you want a SPL or something that might require different trickery like doing RMS evaluation between the zero crossings, again something that I could only guess at because I have no idea how it really works.

Answer 4

The units are whatever units are used for the reference reading. In the formula, the reading is divided by the reference reading, so the units cancel out and no longer matter.

In other words, decibels is a way of comparing two things, it is not an absolute measurement. When you see it used as if it is absolute, then the comparison is with the quietest sound the average human can hear.

In our case, it is a comparison to the highest reading the device handles (thus, every other reading is negative, or less than the maximum).

Answer 5

I held my sound level meter right next to the mic on my google ion and went 'Woooooo!' and noted that clipping occurred about 105 db spl. Hope this helps.