J. Alton Burks
U. S. Bureau of Mines, Pittsburgh Res. Ctr., P.O. Box 18070, Pittsburgh, PA 15236
With the availability of miniature microphones, an in-the-air (ITE) measurement location has become attractive for use in assessing the noise exposure level of workers. This technique is especially relevant in situations where a worker wears earmuffs to reduce his/her noise exposure since there are questions about the actual level of protection provided by these devices. Presently, the approach to this personal protection problem is either to ignore it, or to attempt to derate the manufacturer's published attenuation data to obtain a more realistic estimate of an earmuff's actual performance. The result in either case is that the actual level of protection is still unknown. However, with a miniature microphone mounted under the cup of the earmuff, near the ear canal entrance, there is an opportunity to extract the necessary information about the worker's noise exposure level if one can only define what an ITE measurement under occluded conditions means. In other words, what is the relationship between the sound-pressure level (SPL) measured under occluded conditions at an ITE location and the equivalent SPL measured in the undisturbed sound field (without the worker present)? This paper provides an analytical description of this problem for a reverberant environment. Also, experimental data obtained on a group of ten male subjects with a microphone positioned in the intertragic notch suggest that an ITE measurement represents a conservative basis for estimating the noise exposure level of a worker wearing earmuffs.