How to calculate decibel from maxAmplitude, I wrote an android application to get maxAmplitude at regular interval, I need to show the o/p to the user in decibels.
Decibels are a relative unit, they express the power of your signal relative to some reference power.
If you are working with amplitudes, then the formula is:
power_db = 20 * log10(amp / amp_ref);
(See http://en.wikipedia.org/wiki/Decibel#Field_quantities).
Note also that maximum amplitude is not usually a very good indicator of loudness (or even of power). More typically, you should measure the RMS power of your signal, and convert that to dB instead.
Regular phone microphones aren't calibrated to measure absolute loudness, so it's not possible without also having a sound meter to initially calibrate the phone. As Oli mentions, you may be able to calculate a relative change in loudness, but I expect you want to replicate a real sound level meter.
来源：https://stackoverflow.com/questions/6571894/calculate-decibel-from-amplitude-android-media-recorder