I figure there are probably some people here who aren't aware of this so I'm posting it to spread awareness, as I didn't come across it until a few days ago...
Far left: mp3 160kbps
middle left: mp3 320kbps
middle right: 24 bit flac
far right: 24 bit flac
Obviously not of the same songs, but I'm not here for the full frequencies, I'm here for the cutoffs. As you would expect there's a hard cut at 16 to 18 kbps in the lower bitrate mp3, 20kbps in the higher bitrate mp3, and no cut on the first flac file. When you look at the second flac, there's an extremely low shelf between 16 and 18 k.
Even if your files are flac, you might not have lossless quality. A lot of torrents and free dl's for flac files have been transcoded up from lossy formats like mp3, and will yield the same poor sound quality as the mp3 with the same huge file size as the flac. For this one the poor quality was audible, which is why I checked the spectrogram in the first place. If you aren't sure whether or not your music is actually in the quality you think it is find a spectrogram analyzer and try it out, you may be surprised at how much of your library is transcoded up from a lower quality copy.
>tfw there are probably people here who think they're lossless patricians and are listening to mp3 320 quality from unnecessarily large files
>tfw I was one of those people until very recently
>tfw this kinda says something about how minute the audible difference between mp3 320 and flac is