Consider these two audio sine waves:

B has a higher amplitude (2x) than A, it is louder, but B is not twice as loud as A, perceived audio loudness works on a logarithmic scale. The human ear was designed this way, so that the quietest mouse can be heard whilst the loudest jet tolerated (there is many order of magnitudes difference between the two).

Bit depth is the resolution audio samples can be stored with, consider these 3 images as representations of bit depth:

8 bit	16 bit	24 bit

8 bit has the worst detail, it looks coarse, for audio it sounds coarse, but there is not too much difference between 16 bit and 24 bit, they are both reaching the limits of perception. Audio CDs are 16 bit, whereas DVDs are 24 bit, again is it a marketing ploy? yes and no, yes most people cannot hear the difference between the two, no as 16 bit audio CDs have been spoilt by the loudness race: that is CDs produced now are volume compressed, that is the quiet parts are pushed up louder, so that when played on the radio or TV the track sounds louder (a 1980's CD would sound quiet in comparison to one from 2000). The downside is that 16 bit CDs are no longer effectively 16 bit, the full audible range is not being used. 24 bit helps, but in the long run, the same fate (loudness war) might happen to 24 bit DVD-audio discs.

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