"G.R. Patterson III" wrote:

ANR can be designed to work on any frequency. I suggest your company might want to
check out the Bose ANR headphones (nonaviation). They seem to block out all
frequencies. Great hearing protectors in a noisy environment.

We were told by the manufacturer that they wouldn't make them for higher
frequencies, since there was risk of increasing noise levels at certain
frequencies. As the wavelengths get shorter, there are problems with
resonances within the earcups.

As far as headsets for noisy environments, the earcups themselves
attenuate higher frequency noises. The idea of ENR is to provide
attenuation at the low frequencies the passive earcups won't handle.