Max Amplitude from PCM Buffers - Audio Android

Question

I am trying to find maximum amplitude value from PCM Buffer.

My questions are-

1) I found that to find this value in DB, formula is : amplDB=20log(abs(ampl)/32767). Now given that ampl is in range of -32768 to 32767, the value of log((abs)ampl/32767) would be always negative. So is this formula the correct one? Should I just negate the value of amplDB?

2) My values are coming very high. For normal song also the Maximum amplitude value is 32767, which doesn't seem correct. What are the usual amplitude values for a song?

3) I found another formula amplDb=ampl/2700. What is this 2700 for?

4) Is there any other way I can calculate the amplitude value?

Thanks

Answer 1

The formula you are using is correct. Keep in mind that dB is a perceptual measurement that compares an intensity with a reference level you set. Therefore, it makes sense that it is always negative since your reference level being used at the formula is the maximum PCM level. In other words, your dB will always be lower (negative), than your maximum level (0 dB).

Regarding the values you're obtaining, it is quite normal to obtain the maximum amplitude. If it is a commercial song, a common mastering practice is to boost the signal as much as possible. If it is a recording you made, it could have to do with the microphones sensitivity and the sounds you're recording.

Finally, just to be clear, this has nothing to do with the sound pressure levels at which the sound will happen upon playback, since you're only looking at the differences in amplitude of a recorded sound.