The most popular standard for AAC and MP3 files is 128Kbits/sec encoding, but Apple's iTunes Plus files use a 256Kbits/sec bit-rate, and when you rip your own CDs you can often choose a bit-rate up to 320Kbits/sec. Higher bit-rates result in greater audio fidelity, but they also produce proportionally larger files.
Are high bit-rates worth it? As a simple test, we presented our listeners with two copies of our three music clips. Unknown to them, one copy was encoded as a 128Kbits/sec AAC file, while the other used twice the bit-rate, effectively representing the difference between a standard iTunes download and iTunes Plus (an important distinction, given that Apple charges customers extra to "upgrade" purchased tracks from 128Kbits/sec to 256Kbits/sec).
We invited our judges to listen to the two files through our high-quality Denon headphones, and to compare them as closely as they wished.
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| Bit-rate test: click on image for larger size |
When we asked if they thought one clip sounded in any way better than the other, 11 of them declared that both clips were effectively identical. The remaining nine agreed that there was no major difference, but felt that one clip sounded slightly warmer or cleaner than the other.
These nine were then given another identical pair of files and we asked them to pick which two, out of the four, they felt sounded the best. Four of them picked at least one 128Kbits/sec file, suggesting that they couldn't hear a clear difference.
Only five listeners correctly selected both high-bit-rate tracks, and all of them required extensive close listening to do so. The conclusion is obvious: for everyday use 128Kbits/sec is sufficient. Unless you're a real hi-fi buff, there's no need to pay extra and waste storage space just to get higher bit-rates.