Two weeks ago, we challenged our readers to see if they could discern the difference between MP3 recordings at different
sampling data rates. Nearly 700 completed our study. So does a very high data rate result in a noticeable difference? Here our are basic results:
Respondents rated two recordings, one by rock guitarist Carlos Santana, and another by orchestral composer Aaron Copland. Each recording was encoded into an MP3 file at three different data rates: 64, 128, and 256 kbps. For both recordings, there was a significant difference between ratings of the 64 kbps data rate and the 128 kbps data rate, but no difference between ratings of the 128 and 256 kbps data rate. It’s looking like the 256 kbps MP3s offer no advantage over the much smaller 128 kbps MP3s.
If you’re not familiar with data rates, here’s a brief primer. MP3’s are “lossy” methods of digitally encoding audio files. This means that some of the information in the original digital recording is removed in order to make the file smaller. A 64 kbps MP3 contains half as much information (and consumes half the disk space) of a 128 kbps MP3. A 256 kbps MP3 is twice as big again. The MP3 algorithm works by attempting to remove parts of the sound that you’re less likely to detect. But obviously this process can’t work forever. A balance must be struck between a compact file size and audio that sounds good. The real question we’re trying to answer is “how big is big enough?”
Recently Amazon.com launched a digital music service which boasted 256 kbps MP3s, instead of the 128 kbps files more common at other sites. Are they simply wasting our disk space? So far these results suggest they are. But there are a few other possibilities.
First of all, it might not be true that everyone can’t tell the difference between 128 kbps and 256 kpbs data rates. In fact, 33 of our respondents were able to successfully rank both recordings, rating the higher data rates as better quality. Still, that’s less than five percent of our listeners. Do they really hear something the rest of us can’t?
One possibility is that the people who can’t hear the difference don’t have sensitive enough speakers and headphones. We asked respondents to listen to the Copland recordings using their computer speakers, and to the Santana using headphones. Some listeners didn’t have headphones, and just listened to the Santana with their speakers. So if the people who used headphones were better able to discern the differences in the recordings, then we might be able to attribute the difference to equipment. However, we found no such difference. The correlation between ability to discern the difference between the Santana recordings and headphone use was -.01, not significantly different from zero.
There was, however, a small, significant correlation (.09) between listeners who had purchased their own (presumably better) external speakers and ability to discern the difference between the Copland recordings.
We also asked listeners how much musical training they had. Though many respondents reported over 20 years experience, we found no significant correlation between music training and ability to discern the higher data rates.
We asked listeners if background noise or concern about making too much noise had affected their ratings. Again, there was no correlation between background noise and the ability to discern higher data rates. Self-reported hearing problems had no relationship to the results either.
There was, however, one factor which did explain some of the individual differences in the results. We asked listeners the following question: “Are you an audiophile? Please rate your level of interest in high-quality audio.” They gave their responses on a scale 1 (don’t care about audio quality at all) to 9 (extreme audiophile). Here are the results:
Those rating themselves as more extreme audiophiles were more likely to be able to detect the difference between the different data rates of the Santana MP3s. This was not attributable to their having better headphones; they simply appear to have better knowledge or hearing ability than those who aren’t audiophiles. Even so, the correlation between audiophilia and ability to detect the better Santana recordings, though significant, is not very strong: just 0.17.
Part of this may be due to the particular excerpts I chose for the study. As many commenters pointed out two weeks ago in the survey thread, the easiest way to discern artifacts due to MP3 encoding is in the cymbals, and neither the Copland nor the Santana had cymbals. I suspect the acoustic guitar in the Santana has some of the same properties as cymbals, which is what made encoding differences easier to detect there than in the Copland.
In case you didn’t get a chance to participate in the survey, here are all the samples once again. You can guess which is which in the comments.