Tom Fine wrote:

> Is there any science on the hearing-damage ramifications of these 
> noise-cancelling phones? Do they allow lower-SPL listening or are they 
> putting out more SPL to cancel outside noises?

I've seen no science on the subject and do not use Shure's product, but 
perhaps my experience with other models will be helpful.

The Philips phones I use provide both noise cancellation and signal 
boost. The boost seems to be frequency-dependent, reaching perhaps 2 db 
in the upper midrange. Noise reduction varies with the ambient noise but 
seems to be in the neighborhood of 5 db. The phones begin a few db less 
sensitive than typical over-the-ear models; I have not tried earbuds.

I suggest that on the bottom line these phones allow several db lower 
SPL without sacrificing intelligibility. Since high fidelity and a noisy 
environment are not consistent, the frequency shaping is not a drawback. 
  At the same time, they offer the opportunity for higher SPL due to 
their signal boost, though like most players they run on a single 
1.5-volt cell so cannot peak higher than another headset of the same 

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