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Attenuation Characteristics of Earmuffs at Low Audio and Infrasonic Frequencies

Report Number: AMRL TR 67-27
Author: Nixon, Charles W.
Author: Hille, Harald K.
Corporate Author: Aerospace Medical Research Laboratories; University of Dayton Research Institute
Laboratory: Biomedical Laboratory
Date of Publication: 1967-05
Pages: 22
Contract: Laboratory Research - No Contract
Project: 7231
Task: 723103
AD Number: AD0655939

Sound attenuation and transmission loss characteristics of three different earmuffs models were determined from (1) physical measurements of discrete frequency signals in the range from 1 to 500 Hz inside and outside earmuffs being worn by subjects and (2) psychoacoustical or subjective measurements employing the United States of America Institute of Standards 'Real Ear Attenuation at Threshold Method.' Evaluation of the data showed good correlation between the two measurement methods. Greater attenuation was obtained with the subjective method. It was interpreted to be due primarily to the masking effects at threshold of hearing of physiological noise present under the earcups. Findings show that typical present-day earmuffs provide approximately 10 dB of attentuation in the frequency range from 20 to 100 Hz and very little sound protection below 20 Hz. It is recommended that insert earplugs be used for sound protection in intense low audio frequency and infrasonic sound fields. Good earmuffs in combination with insert earplugs should provide more protection than earplugs alone.

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