As you may already know, AAC and mp3 are compression algorithms that manage to significantly reduce the size of music files by discarding part of the signal based on psychoacoustics, that is, based on what our ears are really able to perceive. The basic principle is that soft, quiet sounds are easily masked by louder ones. Just think of the difference in loudness the ring of a phone can have when heard at home at night, or during the day, in a noisy environment. The ticking of a clock would be an extreme example of the same phenomenon. These algorithms scan the wave and decide which features can be “safely” removed without (significantly) degrading the audio quality of the original. In my opinion, they are very clever, and do an outstanding job. Of course, the quality of the output will depend on the criteria through which these decisions are made, so different algorithms will lead to different results. In what follows, I will use null testing to analyze the discarded data when using one format or the other.
It is said that AAC (Advanced Audio Coding, 1997) offers better results than mp3 (1995) at equal bit rates. My impression is that this is true, at least at 128 kbps. Let’s see what the null tests say. All the encoding is done with iTunes built-in encoder.
Note: Please let me know in the comments if you experience problems playing the sound samples.
1. AAC 256k VBR vs mp3 256k VBR
Original CD clip:
AAC 256k VBR version:
mp3 256k VBR version:
Now let us listen to the discarded audio signal.
AAC 256k VBR nulled track:
mp3 256k VBR nulled track:
For this clip. the spectrograms show that AAC discards more data in the lower part of the spectrum and less in the high frequencies. The spectrogram for the mp3 nulled track is much more uniform along the whole range of frequencies.
Listening to these nulled tracks, you may be surprised at how much audible signal is removed from the original. It really sounds like a lot to be sacrificed. Notice, however, how quiet these nulled tracks are, compared to the original. All this audio signal is chosen to be discarded because it is almost perfectly masked by the louder part. If you find this hard to believe, you should try my blind test! You’ll see how difficult it is to tell de difference between the original and the compressed tracks at this bit rate. (Whether you find it hard to believe or not, please, try my blind test anyway!)
2. ACC 128k CBR vs mp3 128k CBR
128k bit rate still offers good quality audio, but differencies can be easily detected by a trained ear. Let’s see what the null tests say in this case. We will use the same original CD audio clip, which I am not including again at this point.
AAC 128k CBR version:
mp3 128k CBR:
Notice that, in the AAC 128k version, the algorithm starts cutting at high frequencies above 16 kHz, but there is a lot a data kept up to 18 kHz. In the mp3 version, the cut at 16 kHz is much clearer, but there are some bits of data that go up to 20 kHz. This clearly shows that the algorithms are indeed making different decisions on what to discard.
The discarded audio signal sounds as follows:
AAC 128k CBR nulled track:
mp3 128k CBR nulled track:
I guess you can clearly hear that the discarded audio is much louder now. I also perceive that the cut in low frequencies is much noticeable in the mp3 version. In my opinion, AAC clearly beats mp3 at this bit rate.
Finally, tastes and subjective perceptions aside, my hope is that you realize how useful null testing is for proving (or disproving) claims in the audio world.