Part 2: A Noise Cancelling Headphone Review – How Well They Cancelled Noise.

While evaluating sound quality is largely a subjective task with measurements playing a supporting roll, determining how much isolation headphones provide can be done almost exclusively through measurement.  Our web based graphing system can only display four products at a time so I'll show the results in two graphs.  Fortunately that will also allow me to show some measurements of noise cancelling cans that weren't officially in this review as well. We'll start with the low performers. [caption id="" align="alignleft" width="298" caption="Some lower isolation noise cancelling headphones compared to the best which was the Bose QuietComfort 15."][/caption]At the bottom of the graph for reference is the Bose QuietComfort 15, which was most sucessful of all the noise cancelling headphones we tested at reducing low frequency outside noise. Among the Sennheisers higher in the graph (models we did not review for this article) are the Monster Beats by Dr. Dre, which delivered the poorest isolation performance of the bunch we tested. It also turned out to be one of our favorites ... we'll talk about that later.

[caption id="" align="alignright" width="298" caption="Most of the noise cancellers had similar isolation performance, with the exception of the Bose, which was astonishingly good."][/caption]You can see here that most of the other noise cancellers we tested had similar amounts of isolation, except the Bose which beat the other cans by 5-10dB below 1000Hz. You can also see that at the very lowest frequencies some of the noise cancellers actually showed some gain --- in other words, at the very lowest frequencies, some noise cancelling headphones made the ouside noise louder, not quieter. I double checked that our testing system was calibrated properly and found that it was, and repeated some of the tests to be sure this information was correct, and it seems to be. I even performed some alternative tests that would double check this, which include just playing some very low frequency tones in the test chamber while standing there listening to the headphones. Apparently some of these headphones do have a bit of gain in outside noise at very low frequencies, and under some special conditions --- it seems that the direction the low frequency noise is coming from may have something to do with it --- I did indeed find the noise cancellers made outside tones louder.

Time to talk about the sound of each headphone. It's worth mentioning at this point that we have some evaluation systems in place here at HeadRoom and listening tests for these cans were done by six people here in the office with their impressions recorded on a form we use. So the comments here include input from more than just myself. All headphones were evaluated with brown noise being played in the room at levels approximating what you'd be exposed to in an airplane cabin. We found that the quality of the audio did not change in any significant way in the presence of the noise, so critical listening tests were done in a silent room. On to the first of our headphones, the Sennheiser PXC 450 ----------> First article in this series is here.