Question
If noise reduction reduces "good" sounds, too, then how do noise-reducing stereo headsets deliver such good sound? Could their design be used for hearing aids?
Answer
The question appears to be directed towards active-noise reduction (ANR) headsets. Such devices reduce sound by sensing the ambient noise penetrating the earmuff cup, inverting its phase, and reintroducing it via a small earphone to cancel the sound that is present. This only works well in the low frequencies, below about 500 Hz. In the popular consumer headsets, such as the Bose QuietComfort 2, the ANR circuitry reduces the outside sounds, while at the same time the "good sound" you refer to comes from a CD or other audio source plugged into an earphone jack. The ANR system is designed to allow this alternative signal, injected into a different part of the circuit, to pass unaffected. Thus, the normal sound blocking capabilities of the earmuff cup are augmented by the ANR in the low frequencies to provide a quiet background in which to hear the desired audio source (music, speech, or sound track).
Hearing aids do provide compensating circuitry to alter characteristics as they operate in different sound environments, but do not currently provide noise cancellation via ANR concepts. This would require additional transducers and power consumption.
By the way, an alternative means to be able to listen to good sound in an adequately quiet background is to use the popular insert earphones available today, such as the Etymotic ER-6. Using eartips that provide a good seal to the ear, they block sound (even better than the lightweight ANR earmuffs) and as with the QuietComfort2, and include the capability to listen to a high-quality sound source. Your choice will be determined primarily by whether you prefer wearing an earplug or an earmuff, as well as by the ergonomics of using a small earphone vs. a somewhat larger earmuff. Cost is also an issue and depending on the earphone or ANR earmuff that is selected, the price can vary from under $100 to a few hundred dollars.
After receiving his M.S. in Acoustical Engineering from North Carolina State University, Elliott joined the Aearo Company in 1976. As E A R/Aearo's Senior Scientist, Auditory Research, he conducts hearing protector research and development. He has written over 60 articles on hearing protection/conservation, and was the principal editor for the 4th and 5th editions of the AIHA Noise & Hearing Conservation Manual. His principal research has involved evaluation of techniques for measuring and specifying hearing protector attenuation and the limits to their performance, as well means for improving the effectiveness of hearing conservation programs. Elliott chairs ANSI working group S12/WG11 on real-world hearing protector performance, served on a National Academy of Science committee evaluating hearing loss in the military, and is also involved with numerous other standards committees. In 1993 he was the recipient of the National Hearing Conservation Association's Outstanding Hearing Conservationist Award, and in both 1993 and 1998 received their Outstanding Lecture Award. Elliott is Past President of the National Hearing Conservation Association (NHCA), Past-Chair of the American Industrial Hygiene Association's (AIHA) Committee on Noise and a Fellow of the Association, a Board Member of the of the Council for Accreditation in Occupational Hearing Conservation (CAOHC), a Fellow of the Acoustical Society of America (ASA), and a member of the American Speech-Language-Hearing Association (ASHA).