Are audiophile headphones worse than common headphones when you listen to MP3 - Page 2
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A common practice with lower-bitrate lossy audio encoding is lowpass filtering to cut off some of the highest frequencies, so there's less stuff to encode. If the track has some percussive sounds or something else with frequency content out to 16 kHz and higher, then some of this may be removed. Some tracks won't have any frequency content that high, so this quick-and-dirty test won't work on them.
You can test with a CD or lossless file you own. Transcode it to 128 kbps mp3 or AAC or whatever on default encoder settings, and then transcode it again back to a higher bitrate. The high-frequency stuff will still be gone obviously. It's not necessarily this loss in information above a certain frequency that makes the transcoded file sound worse (there's many other changes which may be audible), but it's an easy symptom to check.
Some software like Audacity (which is free) can be used to look at the frequency content. It's under Analyze > Plot Spectrum, for example. I'm sure there's lot more out there. If you just want spectral analysis, Spek is pretty lightweight.
Edited by mikeaj - 1/17/12 at 12:43am