determining quality of an mp3

is there a definitive way of doing this, using a program like soundforge or ableton?

say i've got an audio file. winamp tells me it's a 320 mp3 but i suspect it's been re-encoded a few times, possibly from 128 or maybe 256. how can i determine which? i heard there's a way of looking at the frequencies of the wav that tells you but it doesn't seem to be widely known?

i know many people will say 'just use your ears' but while it might sound fine on home speakers and headphones, it can be a different story when played out over a big club soundsystem so i like to know in advance.

only download VBR mp3s to be sure

I think the only real way to know is to have a direct comparison with a lossless rip. If you try to null the two, you can see where the frequency losses are.

Also, how are you expecting to have your answer to "how good is this mp3", in a qualitative or quantitative format?

similar thing is ripping the audio from a youtube video file. downloading the video as mp3 using offliberty.com always results in a crummy 128 file (eg 3mb for a 3 min song) but downloading the actual video creates a large flv file (33mb for same 3 min song). the uploader told me to rip it from there as he uploaded the vid using a wac as the sound file but i dunno, something seems v iffy about downloading tracks from youtube to play out. (it's a bootleg the guy made, unavailable anywhere else, not an about-to-be-released track or anything)

xpost quantitative. don't some frequencies get shaved off the peaks of the wav, as bitrate quality decreases? if true, that'd show hard evidence for the quality of the track

puzzles me there isn't a tool for easily doing this, which makes me guess it's not as simple as i imagine.

(by wav i meant waveform)

you might try posting in "I Make Music."

If you're sourcing things from video, which sometimes is the only way, then you may as well go ahead and expect the audio to be of inferior quality.

yeah i pretty much am, certainly not an ideal way of sourcing music. just curious if there's a way of finding out the actual bitrate. the 33mb flv file is what intrigues me, there's no way that one still image is 30mb of the file (esp when equiv flvs ripped from youtube come in at 10mb or less)

for anyone interested here's the track:
https://www.youtube.com/watch?v=SCEw5X002sM
(my mistake, it's a video not a still image. but i don't think it's 30mb worth of video)

a bootleg of chic 'good times' and soul ii soul 'back to life'. discovered through hunting for the superb 'my forbidden lover' / back ii life bootleg at 1hr 11mins on this: https://soundcloud.com/disco-tech-edits/disco-tech-live-warm-up-mix

winamp tells me it's a 320 mp3 but i suspect it's been re-encoded a few times, possibly from 128 or maybe 256. how can i determine which? i heard there's a way of looking at the frequencies of the wav

I've never really been sure about the results when trying to do this myself, but the magic search engine keyword for a re-encoded mp3 is "transcode"

here's some screenshots using Audacity: http://www.torrentech.org/index.php?showtopic=186245
and here's some in Cool Edit Pro / Adobe Audition: http://hasitleaked.com/discussion/topic/how-to-spot-a-transcode/

ah great, thank you