AAC vs. MP3, an iTunes Sound Quality Test

NOTE: As of July 2009, the test described in this article has been substantially upgraded. A number of readers critiqued the original test - quite rightly - and I've updated the piece based on that feedback to, hopefully, make it more accurate and reliable. Also note: this article is aimed at laypeople and beginners/intermediates looking for basic information. High-end audiophiles will likely find much to quibble with here, which I concede, but they're not the intended audience for this piece.

It’s widely said that AAC files – the native format of iTunes and the iTunes Store – sounds better and takes up less space than an MP3 of the same song. Anecdotally, I’ve long believed that this is true, but wanted to put the proposition to a more strict test in order to better help you decide which file format to use for songs in your iTunes library and on your iPod.

So, to conduct this audio file format shoot out, I decided to encode the two songs three different ways: as 128 Kbps AAC and MP3 files, as 192 Kbps AAC and MP3, and as 256 Kbps AAC and MP3 (the higher the Kbps rating, the bigger the file, but the better the quality as well – at least in theory). For all files, I used the encoder built into iTunes.

Most audiophiles – the people who have great ears and really value the highest possible sound quality – generally detest MP3 and other digital audio formats because they use compression that takes information away from the digital files. While this is true, most average listeners won’t be able to hear the loss in most cases - as I learned during this test (it's worth noting that I'm not an audiophile, use high-end equipment, or anything else that especially qualifies me for performing this test. I'm just the average listener and consumer of music, which I believe to be a useful perspective).

The Test Subject
For my test, I chose two songs: the quiet, intricate Wild Sage, by The Mountain Goats, and the loud, raucous cover of Leaving on a Jet Plane by Me First and the Gimme Gimmes.

Wild Sage is full of subtle pianos and finger-picked/strummed guitar, with high, breathy singing. I chose it because I'm hoping that those intricate sections will reveal a lot of detail in the various versions of the file.

Leaving on a Jet Plane, on the other hand, is fast, loud, bass heavy, and full of complex drum sections. This song will hopefully show some more dynamic range and reveal other things that the quieter Wild Sage won't.

I used my CD copy of both songs (presumably the highest quality) as a baseline.

Here’s what I found:

256K

Wild Sage: Both versions of the song sound great - I can clearly hear the plucking of the guitar strings, the breathiness of the vocal, no distortions anywhere. However, the MP3 and AAC sound nearly identical. No clear winner here. The MP3 version is 1.2MB smaller, though.

Leaving on a Jet Plane: Same situation here. The song sounds great - clear vocal with subtle intonations and a general sense of brightness - but neither sounds appreciably different.

192K

Wild Sage: No surprise here - this version sounds a bit muddied compared to the 256K version. However, there's still no clear distinction between AAC and MP3. The MP3 is almost 1MB smaller.

Leaving on a Jet Plane: The pattern holds true. Not as good as 256K (of course), but very similar between the two file formats.

128K

Wild Sage: Finally a difference! In this version, the AAC is a bit clearer and brighter than the MP3 which suffers from slight muddiness and a slurring together of some sounds. The filesizes are almost exactly the same, making the AAC the clear winner here.

Leaving on a Jet Plane: And here the AAC is slightly superior in drum clarity. Otherwise, the songs sound fairly similar and are exactly the same size.

Conclusion
Though there are, no doubt, differences in the sound waves of the three files, they sound roughly equivalent. Though there may be a bit more detail in the 256 Kbps MP3, it's difficult for an untrained ear to discern and the files are much bigger than either other version. The only place you're likely to hear a difference is in the low-end 128K encodings, but I wouldn't recommend those anyway.

So, given these test results, it seems that the debate between AAC and MP3 may come down to a matter of taste, opinion, or having better ears than me (learn more about file comparisons. If you find different results from your own testing, I’d love to hear from you.

File Size by Encoding Type/Rate

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