Date of Graduation

2007

Document Type

Thesis

Degree Type

MS

Committee Chair

Steve Guffey

Abstract

The purpose of this study is to determine whether the Verifit, an audio‐measurement system developed by AudioScan Inc. (Dorchester, Ontario, Canada), can be used to perform field hearing protection testing that will match results from the measurements done following the Microphone‐in‐Real‐Ear (MIRE) standard ANSI S12.42‐1995 (R2004), which requires hearing protector testing to be performed in a reverberatory chamber. Although the standards have their differences, both specify the hearing protection testing must be performed in a diffuse sound field, which can be achieved only by a reverberatory chamber. This is an onerous requirement that sharply limits who can perform such tests because of the costs of building a reverberatory chamber can be as much as $40,000. A reverberatory chamber is used to ensure that sound levels are equal in all directions so that the sound levels will not vary due to the orientation of the sound to the subject’s body or the ear protector and will not vary due to modest changes in the posture or location of the subject. In this study, the difference between using a MIRE‐compliant reverberatory chamber and using an audiometric testing instrument called the VERIFIT in a non‐reverberatory space were evaluated. The Verifit is an audiometric instrument intended as an aid to fitting hearing aids. It generates sounds and measures noise levels with its two microphones, a reference and probe tube microphone. The “reference” microphone is suspended just below the ear during a test. The other microphone has a 35 mm flexible probe that is inserted into the ear canal just proximal to the hearing aid. The Verifit controls the noise level at the reference microphone to 85 dB at frequencies from 250 to 6000 dB. Hence, the noise level ideally would not vary at the ear even if the subject moves or makes moderate changes to his orientation to the source. According to Audioscan, a diffuse sound field is not needed when the instrument is used. The Verifit offers many conveniences for field testing, including portability, easy set‐up, and, most importantly, the claimed ability to produce accurate results in a typical office space environment when testing hearing aids. A comparison between the mean Verifit, MIRE, and REAT for all the hearing protectors’ types, methods and runs were evaluated. The results for all hearing protector types demonstrate that there was a significant difference between the MIRE and Verifit at the lower frequencies, 125 Hz to 500 Hz. The gap narrowed for the earmuffs at the 1K Hz to the 4K Hz range. The mean difference between the MIRE and REAT has been determined for the muffs where the MIRE

gives a higher attenuation for 500 Hz thru 3K15 Hz and has a median difference of 0.6 dB. The plug results varied across all frequencies for the Verifit and REAT, having a high and low median difference ranges between 11 dB and 22 dB. In contrast, for the MIRE and REAT the plug was at least a protective as either muff at all frequencies and over 10dB better at frequencies above the 3K Hz and was nearly 20 dB better at 125 Hz. Possible explanations for these differences in low frequency attenuation are that there was some degree of air leakage around the probe tube microphone mounting system as well as potential experimenter error. A few of the findings do demonstrate the limitations the Verifit does have in measuring hearing protector attenuation.

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