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The Sennheiser MM50 iP headphones have great audio quality. They have a strong bass response, although low bass sounds might get a little boomy. They have absurdly low distortion, which audiophiles will like. Their tracking was also good, so you'll get a relatively even dose of noise coming out of each ear bud.
The MM50 iPs don't have the best isolation compared to other in-ear headphones, however. This isn't saying they have poor isolation, because they don't: the MM50 iPs can isolation about as well as an above average pair of active noise-cancellers. They should still be fine for a bus or train commute.
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About our testing:
Our audio tests use a head and torso simulator (HATS) and an electroacoustics program developed by Listen Inc., called SoundCheck. The same tools are used by headphone and cell phone manufacturers when they're doing their own performance tests. For more information on our tests, read this article.
Frequency Response (5.04) 
How the test works:
This test measures how loudly the headphones will play back any given sound. We begin the test by outfitting HATS with the headphones. We next play back a frequency sweep (from 100 Hz and 20 kHz) at a known decibel level. HATS listens, and reports back to SoundCheck exactly how loud each frequency was. SoundCheck then graphs this data and we post it on the internet. In this case, you can find the graph right underneath this paragraph. The graph shows the frequencies tested along the bottom, from low to high, and the left side is a measurement of the output decibel level. What you should look for are long, sharp lines: the longer and more vertical they are, the more drastic the change in decibel level was. A really long, more-or-less vertical line means any instruments or sounds that span those frequencies will have very different decibel levels, regardless of how they're supposed to sound. This could lead to an instrument sound like it's fading out and in. For more info, click the orange "i" above.
What we found:
The Sennheiser MM50 iP headphones didn't have a bad frequency response. First of all, many bass enthusiasts will love these headphones: the low end is very loud. Towards the mid-range frequences, the lines both dip ever-so-slightly below our bottom limit. Though they will sound quieter than the bass, they won't sound overly quiet. Towards the high end, there's a bit of a spike around 7 kHz. This is the pitch of spoken sibilance, and the initial impact sound of some drums (known as the attack). After that point, however, the graph drops down fairly quickly. This isn't unusual: typically headphones underemphasize the really high pitches. Here the MM50 iPs go a bit overboard, but not by much. If your song has a lot of really high-pitched sounds, they might have some issues with erratic emphasis.
Overall: great bass and a good overall frequency response, although the high end might sound like it fades in and out randomly.
How the Sennheiser MM50 iP compares:
Well, the MM50 iPs get beat out by the V-MODA Vibe Duos for the boomiest bass, but otherwise perform well. The high end isn't any more or less erratic than any of the headphones pictured here. They performed about as well as the Apple iPod in-ears, but with much better bass. If you're a purist, and therefore want the least artificial emphasis, then the 6isolators are your best bet. Typical users will be fine with the MM50 iPs, however.
Distortion (12.60)
How the test works:
For our distortion test, we send a series of known sounds through the headphones, and have HATS listen to what comes out. We then have SoundCheck look for the differences between the original playback and what HATS ended up hearing. The percentage of total harmonic distortion is then plotted on the graph below. The y axis represents the percentage of distortion, and the x-axis is the frequency range we tested. The left channel is green, and the right one is red. The best score will keep both channels towards the zero line, because that means they show very, very little distortion.
What we found:
The Sennheiser MM50 iP headphones have hardly any distortion at all. Your playback will sound just like the original source file. If you're an audiophile looking for distortion-free in-ears, the MM50 iPs will be happy to meet your needs.
How the Sennheiser MM50 iP compares:
Well, based on numbers alone it's pretty obvious that the MM50 wins this round. The headphones that came the closest were the Bose QuietComfort 3s, but you can clearly see how there's a bit of distortion in the low end. All the other graphs have noticeable peaks in them and, somewhat less noticeably, tend to hover around the 1% distortion level throughout.
Tracking (8.20)
How the test works:
We test tracking by playing back a frequency sweep and using HATS to record the decibel level of each channel. Typically one channel will be louder than the other, if only by a little. We then produce the graph below, which shows which channel is louder at any given frequency. A few things to note: we don't score based on the extreme low or high end, because the data there isn't 100% accurate. We show the results anyway, however, so you can get a general feel for how the headphones will react.
What we found:
The MM50 iP headphones actually performed pretty well on this test. The line stays within a 2 decibel range of neutral, which is virtually inaudible. Towards the high end there's a dramatic shift, but headphones usually have troubles with frequencies that high. Plus, the shift is rather minor, just eight decibels.
How the Sennheiser MM50 iP compares:
On the tracking graph, it's usually quite apparent why a pair of headphones got the score they did. The one thing to keep in mind: we don't score the extreme high or low end. In this case, a straight line dead across center is ideal. Knowing this criteria, it's easy to see why the 6isolators have the best score. The Denons managed to just barely beat the MM50 iPs because the MM50 iPs tend to wander around a bit more. The other headphones have obvious deviations from the ideal flat line.
Maximum Usable Volume (10.00)
How the test works:
First of all, we define maximum usable volume as the highest decibel level before the playback reaches 3% distortion. To find this level, we run the distortion test at steadily-increasing decibel levels.
What we found:
The Sennheiser MM50 iP headphones were capable of outputting 121.80 decibels, which was enough to reach our perfect score cut-off point. Really, any louder than 120 decibels isn't going to do much by worsen your hearing. This decibel level is about what's output by an airplane engine. There are myriad reasons why you shouldn't insert an airplane engine into your ear, and one of them is a dangerously high decibel level. If you like it loud, however, the MM50 iP headphones are more than happy to oblige.
Isolation (6.60)
How the test works:
For our isolation test, we put the headphones on HATS and blast both with pink noise (it sounds like static, only not random; every octave has equal power). We have HATS listen and pick up any of the pink noise that makes it through. We then use SoundCheck to give us a lovely graph showing exactly how many decibels of each frequency were being blocked out.
What we found:
The Sennheiser MM50 iP headphones did a pretty good job of blocking out ambient noise. Like most in-ear headphones, it performed well in the middle range, which is where most noise-cancelling headphones have a lot of trouble. The MM50 iPs didn't block out a ton of lower frequency noise, however, so low rumblings aren't a particular strong suit. On a commute via bus -- and especially by train, since there's often a lot of metal-on-metal squeals -- the MM50 iPs should hold their own.
How the Sennheiser MM50 iP compares:
Compared to other in-ear headphones, the MM50 iPs were just about average. The 6isolators performed the best, but they also came with triple-flange sleeves: the bane of high-pitched frequencies. Though the MM50 iPs aren't the best isolators out there, they will out-perform the typical in-ear headphones and noise-cancelling on-ear or over-ear.
Leakage (9.47)
How the test works:
We test leakage by setting a really expensive microphone six inches away from a headphones-clad HATS. We then play pink noise back through the headphones. The microphone measures the amount of pink noise that manages to escape into the outside world.
What we found:
The Sennheiser MM50 iP headphones won't leak your playback very far. Realistically you should have nothing to worry about. The only scenario we can fathom where an outsider would overhear the MM50 iPs' playback is if you were listening at max volume and someone was sitting next to you in a library. In this case, turn down your music because it's a quiet room. Seriously, turn it down: I read on the Internet that loud iPods can make you deaf!
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