Frequency Response (8.17) 
Our first audio performance test looks at the frequency response of the media player; how well it takes the many frequencies that music contains and plays them back. To do this, we put a special sound that contains the whole range of frequencies onto the player and hit play, while our testing system listens to the output on the headphone socket of the player. It them produces this graph to the right, which shows the frequency range, from very low (at the left), to very high (at the right. A perfect player would play back all frequencies at an equal volume, providing a smooth, straight line on this graph.
The iPod Shuffle did a very decent job here; the graph is mostly flat, but it takes something of an upturn at the end. However, this is not a major issue; the amount of increase here is relatively minor, as most users are not going to notice a difference of less than a decibel. So, the iPod Shuffle had very decent frequency response
One thing to note here is that we test with the equalizer feature of the player turned off; this works by changing the frequency response to suit the type of music being played.
Distortion (6.30)
Distortion is the curse of the media player; it happens when the sound comes out differently from the way it was recorded. There are a number of things that can cause it, such as cheap amplifier chips inside the device that are being pushed beyond their capabilities , or the player doing a poor job of decompressing the stored music. Again, we test it by playing back a special sound while the testing system monitors the sound, looking for any differences between the sound and the output.
We did notice some minor issues with distortion here; the Shuffle had a number of minor distortion peak, and there was some evidence of a little bit of distortion at higher frequencies. However, these were minor issues, and most users aren't likely to notice them.
Cross Talk (7.53)
You have two ears, so media players have two different channels for the two speakers in the headphones you use. However, some media players seem to get a bit confused about this; sometimes sounds meant for the left side end up coming through a bit on the right, or vice versa. That's what we test here; by feeding a signal into one channel and measuring the other, we examine if there is any cross talk between the two. We measured the cross talk on the iPod shuffle at around -54.1 decibels, which means that it will be almost completely inaudible to most users. So, the iPod Shuffle did not have any major issues here; although it had a bit more cross talk than the iPod Nano, it is still low enough to not be a problem.
Output Power (4.62)
Another major factor for media players is how much power they output. More is better here; having more power to drive a set of headphones means that you can drive bigger and better headphones than the ones that the player came with (which we review below). Many larger and better quality headphones have higher impedence, which means they need more energy to make them move, much like a weightlifter has to work harder to heft the heavy weights. The iPod Shuffle didn't do so well here; we measured the output at 18.5 milliwatts, which is at the lower end of the scale. So, the Shuffle would have difficulty driving a bigger set of headphones. Of course, you probably wouldn't want to drive a big, expensive set of headphones from a small, cheap media player like the Shuffle, but it would still be nice to have the option.
Noise (9.75)
Noise is an inevitable fact of life when it comes to any sort of audio player; the amplifiers inside the device that drive the headphones always introduce some noise. But that didn't seem to be a big problem with the Shuffle; we measured the ratio between the signal it outputted and the noise at a very healthy 89.12 decibels. The bigger this number, the lower the noise relative to the music, and the Shuffle had very low noise.
One thing to note here; the noise we test is the noise that is produced by the media player, which is different than the distortion caused by a poorly compressed file. If you compress your files down to a particularly small size, your music will contain lots of distortion that can sound like noise. However, this is caused by the compression, not by the media player, and the best media player in the world can't do a thing to help if the audio files it is playing back are heavily compressed.
Included Headphones (3.3)
Headphones are often an afterthought for MP3 players; the manufacturers include the cheapest ones they can find to help keep the costs down. But they are a vital part of the package; without them, you can't listen to your tunes. We do extensive test to see how good these are, and we were not particularly impressed with the standard apple ear buds that come with the shuffle. We found that they added significant distortion to the music, particularly at low frequencies. And, while they could handle plenty of volume, we found that they did very little to block outside sounds; you can easily hear what's going on around you. And the people areound you can also hear a lot of what you are doing; we found that a lot of sound escaped from the headphones.
The irony is that iPod Shuffle can produce much better sound than the included headphones, so a better pair of headphones would be a sound investment if you're looking for quality. And a better pair of in-ear headphones will also do a better job of keeping your taste in music private, which could be a definite plus for many people who travel on the same bus route as me in the morning.
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