6 dB rule

[all the audio processing notes are going to be labeled with Image/Video][To Audio label now, 9/22/09]

The amplitude of a sound wave is measured in decibels (dB), it is denoted in Hertz (Hz) in Frequency. The lowest amplitude we can hear is ‘threshold of hearing’, denoted with 0 dBSPL (sound pressure level). The sounds we can hear is about 20 ~ 20k Hz, and 2-4k Hz are the most sensitive to us.

The microphone converts sound wave to voltage:
dBV = 20 x log (x volt / 1.0 volt)

Dynamic Range = Peak Level – Noise Floor (dB). The dynamic range for human is about 120dB.

6dB describes the relation between dynamic range and computational word width:

Dynamic range = 6n dB, (n is the precision bits)

The addition of one bit of precision will make the dynamic range increase of 6 dB. The more bits used, the higher quality of system. 16-bit, 24-bit, and 32-bit’s dynamic ranges are 96, 144, 192 dB. Since 96dB is less than human’s 120dB, 24-bit and 32-bit are used.