Abstract
During a study of bone conduction in a free sound field, it has been observed that small Helmholtz resonators attached to perforated earplugs can considerably increase the sound attenuation in the vicinity of their resonance frequency. Some improvement is also present at lower frequencies. Unfortunately, above the resonance frequency the sound attenuation is usually decreased because of an anti-resonance effect. Recently, using the results of impedance measurements at the entrance to the ear canal, it became possible to eliminate the negative effect without diminishing the positive one. This development has opened the way for practical applications in the areas of ear protection, communication, and hearing testing. Earplugs and semi-inserts have been designed with an attenuation characteristic equivalent to that of some of the best earmuffs available commercially. The paper discusses some experimental results as well as the underlying theory. (Work supported by a contract between the Office of Naval Research and Syracuse University.)
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