NIST Hearing Aid Measurements

An echo-free chamber at NIST is used to improve measurement of hearing aid performance. ® Robert Rathe

Hearing loss afflicts approximately 30 million Americans, including two recent presidents (Clinton and Reagan), and is certainly among the most frequently occurring health impairments in the United States. Americans of all age groups are affected, including more than 30 percent of people over age 65, 14 percent of people between 45 and 64, nearly 8 million people from 18 to 44, and over a million school-age children.

The National Institute of Standards and Technology (NIST) supports the development and use of better hearing aids by developing measurement methods and by participating in voluntary standards activities that benefit manufacturers, dispensers, and users of hearing aids.

For several decades, work at NIST in hearing aid testing and measurement methods has been sponsored by the Department of Veterans Affairs (VA), the largest consumer of hearing aids in the United States. This work has established dedicated NIST laboratory facilities, including a medium-sized anechoic chamber and computer-controlled test systems. These all support the development of measurement methods and their standardization for hearing aids, and provide research and test data to VA clinicians, researchers, and consultants to enable them to make informed decisions in selection of new and evolving types of hearing aids and new technologies for their patients. These data also have been used by audiologists in active-duty military hospitals of the Army (e.g., Walter Reed), Air Force, and Navy, and sometimes by other federal government officials. The results also have been made available by VA to private clinicians and thus to the public.

The photograph on the right shows apparatus for measuring the directional properties of hearing aids in the large anechoic chamber at NIST. The hearing aid under test is placed on an anthropometric manikin mounted on a computer-controlled turntable. A microphone in an ear simulator in the manikin measures a sound pressure representing the sound pressure that the hearing aid would produce at the eardrum of a person. To determine the directional response patterns of the hearing aid, a sound source is adjusted to a number of elevations for each of a number of rotational positions of the manikin. These patterns are particularly important because suitably directional hearing aids can enhance significantly the ability of the hearing impaired to understand speech in the presence of interfering sounds and reverberation. In these conditions the hearing impaired have even greater difficulty understanding speech than do persons with normal hearing. By consensus of the field, this difficulty is considered one of the most important (many would say the most important) of the unsolved problems involving hearing aids.

Contact: Victor Nedzelnitsky, (301) 975-6638, victor.nedzelnitsky@nist.gov.