Noise reducing muffs for audiometry.
Franks-JR; Merry-CJ; Engel-DP III
Hearing Instrum 1989 Nov; 40(11):29-36
The effects of noise reducing muffs on the sensitivity of audiometric testing were examined. The purpose of the study was to determine if commercially available noise reducing earphone enclosures allowed adequate attenuation to permit testing at a 0 decibel (dB) hearing threshold level and whether they affected earphone calibration or the accuracy of auditory threshold limits. Four volunteers with normal hearing were fitted with standard TDH-39 earphones with or without type PE51 Audiocap or Audiomate noise reducing earphone enclosures. Audiometric testing was performed in the presence of narrow band noise as specified by OSHA over the frequency range 125 to 8000 hertz (Hz). Real ear attenuation at threshold (REAT) values were measured. The Clark 10A ear muff was tested for comparison purposes. The overall mean REAT for the Audiocap and Audiomate earphone enclosures, averaged over the 125 to 8000Hz range, was 15dB. Attenuation was similar for both enclosures between 500 and 6300Hz. At 250Hz, the REAT for the Audiocap was - 1dB and that of the Audiomate 9dB. At 8000Hz the REAT for the Audiocap was 10dB higher than that of the Audiomate. The Clark ear muff showed REATs ranging from 10dB at 125Hz to around 37dB at 4000 to 8000Hz. The electroacoustic responses of the TDH-39 were consistent with Bruel and Kjaer NBS-9A and flat-plate couplers and varied consistently when the Audiocap and Audiomate enclosures were used. The audiometric thresholds were altered below 500Hz and above 4000Hz. When the electroacoustic response was tested with the Knowles electronics mannequin for acoustic research (KEMAR), only the earphones and the Audiomate gave consistent results. The authors conclude that neither the Audiocap nor Audiomate noise reduction earphone enclosures provide adequate attenuation to determine hearing thresholds at OdB background noise.
Audiometry; Testing-equipment; Hearing-threshold; Noise-levels; Laboratory-testing; Equipment-reliability; Industrial-hygiene