Instruction: Given a question and the related document, and generate the answer to the question based on the information provided in the document.

Input:
Question: What am I missing? There seems to be zero noise cancellation?
Document: I have now used these headphones for around four hours.....hopefully I can leave a positive update in the future. As other reviews have said the noise cancellation qualities are fair to good. But the sound quality? The mids to highs are fairly crisp and accurate. But where is the bass? The lower frequencies are just about completely missing. For this kind of price, I'm VERY DISAPPOINTED!
After reading many of the comparisons between the Bose and the Sennheisers, I went with the Sennheisers. Although the trend seems to be that the Bose stand out out in terms of noise-cancellation, the Sennheisers produce better sound, but most importantly, are more solidly made.I am a frequent flyer, and in fact, I am in the air en route from JFK-SFO as I write this.
Noise cancellation of these headphones is the best I have experienced, especially for the high-frequency hiss of a loud air conditioner that I am forced to work near.The sound quality, however, is poor. Especially compared to other Sennheiser headphones I have used. Sound quality overall is muffled, muddy, and thin. Seems to be a hollow quality to vocals and bass is thin.
It seems to work really well, however, there are other issues that bothered me about the headphones. If I held my head straight and perfectly still they worked great. The sound quality was excellent and the noise cancellation was at least as good as the Bose Quiet Comfort II headphones. However if I tilted my head as if to read or turned my head to either side I experienced a low level repetitive thumping in the left ear of the headphones.
I have smallish ears and a medium sized head and these have not been issues for me. The Senns were definitely more comfortable, but I wouldn't hesitate to use the Klipsh on a long trip. The noise cancellation of the Klipsch seems to be better as well, although the funky brown-bronze design is not my favorite.Bottom line:  The Senns offer good sound and are extremely comfortable. Sound volume and definition could be better. In terms of the competition, for short stints I like the AKG's better.
It seems like the extra size is part of the noise cancellation strategy with Sennheiser. It could also be what get's you the depth of sound.
A big seller for me is that they do not need batteries to operate (without the noise cancellation)! They are very comfortable to wear - you can forget that you're even wearing them! The only bad thing is that these do leak sound, but only a little.There is one MAJOR PROBLEM, and it seems to be very common in reviews here - once the headphones are put in by-pass mode with the batteries in, there is a very audible "clicking" sound.
I purchased these when myBoseAE2 audio headphoneskicked the bucket, figuring I'd find out what all the fuss over expensive noise-canceling headphones was all about. After thoroughly researching headphones that were within my budget I settled on the Sennheiser PX 450s.When they arrived I could not have been more disappointed. I was actually a little stunned at just how bad these headphones are and I was in denial for a couple days, testing every alternative to make sure I wasn't missing something. But what you hear is what you get.
I wanted to like these headphones. I truly did. The noise cancelling is an impressive feat, music sounds good through them, they're comfortable, sturdy, the talk-through feature is a clever touch...But I had to return them. Why? There seems to be a severe design flaw with the noise-cancelling tech: feedback.
and they seem very sturdy. Noise cancelling is significantly better than the PXC-250, not only because of the passive cancelling afforded by the large ear cups, but the active cancelling seems to have improved dramatically as well. Of course, one has to know what to expect, and I think a fair number of negative reviews are the result of unrealistic expectations.
Answer: There's a small hidden black switch on top of the right speaker. Flick it into 'normal' position, then keep the On/Off switch pressed till you see a red light. Red light indicates that the headphones are in active noise cancellation mode.