That said, I never bother with 320 constant bit rate because it just leads to large files. This happens mostly at the top end (cymbals, distorted guitars etc) and especially when there is significant bottom end at the same time (1) The jangles, splashes and other digital artifacting ruins the pleasure I get out of a good piece of music. The real problem I have is in trying to listen to either heavy metal or classical music at low bitrate through a high end system. If you play the same mp3 through a decent or high end audio system it will sound worse, but again, with pop music it is still acceptable. I can happily listen to pop music on an ipod through earphones at low bitrate (well, I say happily - not really a fan of pop music, but what I mean is it sounds as it should) I find it comes down entirely to the type of music and the speakers you listen to it through: I would have thought there'd been a bunch of grad student research in psycho-acoustics available. I couldn't find any academic papers on the matter (but I didn't look that hard), which surprised me a bit. This Maximum PC article gets a lot of attention on the net, though they're doing 160 vs 192 and 320 vs uncompressed. There's been a few magazine attempts at uncovering codec and sample rate differences. There are other double blind approaches, see this article for details.

This kind of test is a little better than an 'assign the bitrate to a sample' type test because the listener can't guess their way through it with any accuracy and skew the results. So A might be 192 kbps and B might be 320 kbps and the listener has to decide whether X is from the 192 or 320 kbps encoded track. The ABX type test seems to be popular for bitrate comparisons: you listen to sample A and then B and then X and decide which of A or B sample X is from. In this case you could have some one listen to samples of music and have them guess the bitrate of the samples. Empirical tests such as waveform analysis will most definitely show differences in the output, but that doesn't mean the differences are audible.

If you're after audible differences the only test you can use is a double blind test.

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