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About our testing:
Our audio tests use a head and torso simulator, or HATS, and an electroacoustics analysis program called SoundCheck. With their powers combined they are like a potent combination of science and distilled awesome. That's why they're the same tools used by manufacturers for their own testing. For more information, read this article.
Frequency Response (4.72) 
How the test works:
This test measures how much emphasis the headphones will assign to any given frequency. We begin the test by setting up the headphones in HATS's ears. We then play a frequency sweep from 100Hz to 20kHz at a set decibel level. HATS listens, then tells SoundCheck what it heard. Once it analyzes the data, SoundCheck spits out the graph below, which we post for you, our valued reader. The graph depicts how the left and right channels play different frequencies at different volume levels. The dotted black lines represent the upper and lower limits that this score is based on. Ideally the line would move around within these limits to add or subtract a little emphasis. Things that are bad are sudden spikes or when the line goes way outside of the limits. If you want to learn more, click the orange "i" badge above (hint: "i" stands for "information").
What we found:
The Sennheiser CX 95 had a pretty good frequency response. It might over-emphasize the bass a bit, but we didn't notice any muddiness or booming. As you can see, the response diminishes as it reaches the middle frequencies, where it peeks below the bottom limit a bit. The line then jumps back up around 7kHz, which is the frequency partially responsible for making drums sound sharp (specifically it'll emphasize the attack, or initial hit). After the 7kHz spike the graph drops back down again, with a second, smaller peak around 10kHz, which should help accentuate the sizzle on cymbals.
How the Sennheiser CX 95 compares:
The CX 95 landed square in the middle of all our comparison units. It had a virtually identical response to the MM 50 iP, but it dipped a bit lower when it broke the lower limit, which is why it received a lower score. There really isn't a shining star on the below headphones, but that's typical for in-ear headphones. They can have some really low distortion, but they tend to to overly emphasize certain frequencies (or overly dampen some frequencies—we're looking at you, Apple iPod In-ear Headphones' bass response).
Distortion (11.10)
How the test works:
For our distortion test, we play a frequency sweep through the headphones and have HATS listen. SoundCheck plays the part of interpreter and graciously bestows upon us the below graph. What it depicts is the percentage the playback has been distorted. We again measure the left and right channels separately and we don't score the extreme left and right ends. The ideal response is, of course, 0% distortion.
What we found:
The CX 95 headphones have excellent distortion control. There's thankfully not much else to say. The line hovers around the 0% mark the entire time and really doesn't peak above it in any noticeable way. Bravo, Sennheiser.
How the Sennheiser CX 95 compares:
On a purely score-based comparison, the CX 95 came in second to the MM 50 iPs, also from Sennheiser. Realistically, anything that gets above an 8 on this test is golden. We do think it's interesting, however, that there's such a discrepency between the two Sennheiser headphones and the others below.
Tracking (9.52)
How the test works:
We test tracking by again pumping a frequency sweep through the headphones and into HATS's awaiting microphone ears. What we're checking for here is any differences in the relative volume levels between the two channels (left/right). The graph is set up so that when the left side is louder the line bounces up a bit and when the right side is louder the line droops below zero. Like our distortion test, the extreme left and right of the graph aren't scored.
What we found:
This was another good score for the CX 95. At no point is either channel significantly louder than the other. Emphasis tends to meander towards the right channel, but barely strays further than a 2dB offset before wandering back towards neutral. It's nothing someone with sub-superhuman hearing would notice.
How the Sennheiser CX 95 compares:
Just about all the comparison headphones did about as well as the CX 95s, although the Vibe Duos and iPod headphones were notable for being closer to average. You might notice a bit of a difference comparing the Vibe Duos or iPod headphones to any of the other pairs, but the four below that scored above 8 points are pretty solidly good.
Maximum Usable Volume (10.00)
How the test works:
We test maximum usable volume by running a series of distortion tests. After each one we crank up the volume, which will increase the overall distortion. What we're looking for is the point at which the distortion creeps past 3%, which is very noticeable and distracting.
What we found:
The CX 95s were capable of 123.64dB, which is exactly 3.64dB louder than the max level we award points for. Even if you like it loud, the CX 95s are a great choice for you.
Isolation (5.97)
How the test works:
To test isolation, we blast HATS with pink noise while it's wearing the headphones. Pink noise is when each frequency has the same power, meaning lower frequencies will be played back at a higher decibel level than higher frequencies. The result sounds a lot like the inside of an airplane during a flight. We use HATS to measure how sound gets through the headphones, on a frequency-by-frequency basis. SoundCheck then plots us some graphs, which look a bit like the one below and to the right.
What we found:
The CX 95 was a bit disappointing here, although they scored about average. As in-ears, the CX 95s have a lot of expectation to live up to. They really don't block out much bass at all, which is unfortunate for commuters and others who are the prime in-ear demographic. They do block out quite a bit of higher-pitched sounds, but nothing impressive. Of course, if you're the sort of person who doesn't like total immersion, then the CX 95s offer you a rare service.
How the Sennheiser CX 95 compares:
In-ear headphones have a great capacity for blocking out sound. The CX 95s didn't do too well. Even the MM 50 iPs managed to outscore them, and we thought the MM 50 iPs were scraping the bottom of the barrel when it came to in-ear isolation. The main problem with the CX 95s is they don't block out as much high-end noise as other headphones tend to (with the exception of the MM 50 iPs). Overall, this isn't bad, especially if you're walking or jogging somewhere and want to keep a wary ear out for cars, trains, or other dangers.
One interesting thing to note is that the two Sennheiser headphones have, so far, had very similar audio quality results. We'll need to do a few more reviews before we say anything for certain, but we're wondering if there's some correlation between having very low distortion and not isolating well.
Leakage (10.00)
How the test works:
Our leakage test pits headphones against microphone in a contest to see which headphones are the quietest. What we do is outfit HATS with our headphones, then play back our standard pink noise through the headphones at a known volume. The mic is there to pick up any sound that leaks out.
What we found:
This is another non-issue for CX 95 users. These things might let in a lot of external sound, but they really don't like giving up their own. These headphones would be appropriate for any quiet environment or with a room mate that needs to study. We had no complaints here.
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