But studies conducted in the 1990s show that industrial sound booths in mobile vans and in fixed clinics routinely fail to comply with OSHA’s allowable limits, especially below 2000 Hz. It might be tempting to think that simply having a sound booth would resolve this situation, and it often does significantly help. In fact, Berger & Killion in 1989 demonstrated that levels as high as those permitted by OSHA masked thresholds between 5 Hz by at least 12 dB. Depending upon frequency, they are 13 to 25 dB higher than what is really necessary in order to accurately test down to 0 dB HL. They are a political compromise…it’s what could pass Congress and be enacted into law in 1983. The OSHA Appendix D MPANL values are simply not based in good science. Anybody who has taken a hearing test in a somewhat noisy environment knows this intuitively. As the noise level increases, the masking effect spreads to higher frequencies. At 500 Hz, supra-aurals provide only about 5 dB of noise reduction and often less. In other words, low-frequency background noise can make a person’s low-frequency hearing appear worse than it really is. ![]() Second, supra-aural earphones mounted in typical rubber cushions provide little noise reduction, especially at 500 Hz which is a threshold susceptible to “masking” by background noise. First, they are specifically for supra-aural earphones, not insert earphones, and these MPANLs are inappropriate for inserts. There are multiple issues with these MPANLs. In other words, OSHA allows no more than 40 dB SPL of background noise at 500 Hz, 47 dB SPL of background noise at 2000 Hz, etc. The technical term for this is “Maximum Permissible Ambient Noise Level,” or MPANL. This table specifies the amount of background noise permissible for audiometric testing. Appendix D in the OSHA Noise Exposure Standard (.95) includes Table D-1, Maximum Allowable Octave-Band Sound Pressure Levels for Audiometric Test Rooms. However, it left an important compliance-related issue unresolved. So after nearly 30 years of sustained effort by hearing conservation professionals, this 2013 letter of interpretation is certainly good news. However, in 2013 OSHA recognized in a letter of clarification that the calibration issues had been resolved and scientific data showed “the threshold differences between supra-aural and insert earphones are unlikely to cause difficulty in correctly identifying an STS…Insert earphones may be used interchangeably with supra-aural earphones for obtaining audiograms.” This double testing and the comparison process were so expensive and difficult that it effectively precluded the switch to inserts. In order to switch to inserts, OSHA required each worker be tested with both supra-aurals AND inserts. OSHA was also concerned with lack of well-established calibration procedures for inserts. In 1993, OSHA issued a letter of interpretation expressing concern that results obtained with insert earphones be comparable to those obtained with supra-aurals, so that comparison of annual with baseline audiograms not be affected by differences between the two earphone types. These reasons will be discussed in a later blog installment, along with why your clinic should switch from supra-aurals to inserts.Īdoption of insert earphones for industrial hearing testing took significantly longer. For a variety of reasons, insert earphones were quickly adopted by Audiologists in clinical practice as a substitute for supra-aural earphones. ![]() ![]() In the early 1980s, another type of earphone was developed– the insert earphone. Though improved over the decades, the audiometric earphone used to conduct the great majority of industrial audiometric testing in the year 2017 is still essentially 1930s-era technology. This technology has been around for many years note the earphones worn by this Air Force bomber pilot during World War II. The term “supra-aural” means that the earphone sits on the ear. Well before OSHA’s 1983 Hearing Conservation Amendment mandated audiometric testing for noise-exposed workers, audiometric testing in industry was primarily conducted with supra-aural earphones.
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