[Harlan Messinger]

I've got a pair of Sony MDR-NC6 noise-canceling headphones. They do a
great job of removing the low, rumbling sounds, like the office copier
or the continuous roar on a plane. On the other hand, on a plane, they
do virtually nothing about the higher-frequency airplane sounds
(hissing, whining), and they add their own hiss.

I asked someone selling expensive Bose headphones at Denver Airport
about whether their phones cover the higher frequency ranges, but he
turned out not to be informed on such a technical topic.

Is this issue with my Sony phones normal--are only the lower frequencies
targeted by today's noise-canceling products? Or are there products that
I can rely on to cancel a wider range of frequencies and, ideally don't
introduce significant noise of their own?