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In-Depth Survey Report: Design and Construction of an Acoustic Shock Tube for Generating High-Level Impulses to Test Hearing Protection Devices
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2012/05/01
[PDF-1.91 MB]
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Description:Background: The noise reduction performances of Hearing Protection Devices (HPDs) have normally been evaluated at or near the threshold of hearing because the performance of the device was assumed to be constant across the typical range of occupational noise exposure levels. With the advent of new technologies in hearing protection and associated electronics, the noise reduction performance for some HPDs was found to increase with the increase in the sound levels (Parmentier et al. 2002; Zera and Mlynski, 2007; Berger and Hamery 2008; Murphy et al., 2012). These devices are typically described as amplitude-sensitive, non-linear, sound-restoration or level-limiting hearing protectors. The electronic level-limiting or sound restoration HPDs detect the external exposure level and then adjust the gain of the amplification circuit to limit the level of sound played back underneath the protector. Passive HPDs with a non-linear valve or orifice rely on increased acoustic impedance with an increased pressure differential across the two sides of the orifice or valve. In order to accurately assess the performance of these types of protectors, the American National Standards Institute revised the ANSI/ASA S12.42 (ANSI, 2010) standard to include a method to measure the impulse peak insertion loss of all hearing protectors over a wide range of impulse levels using a dual-ear acoustic test fixture and a field microphone. The ANSI/ASA S12.42-2010 requires that impulse tests be performed at three ranges of impulse peak sound pressure levels. The low-level impulse range is between 130 and 134 dB peak SPL; the mid-level range is between 148 and 152 dB peak SPL; the high-level range is between 166 and 170 dB peak SPL. For all impulses, the duration of the initial positive pressure phase of the waveform (A-duration) is required to be between 0.5 and 2.0 milliseconds. The standard recommends generating the required impulse sound events using explosive charges or an acoustic shock tube. Description: This report describes the design and construction of the NIOSH acoustic shock tube. The first prototype shock tube was built under a contract to the University of Cincinnati, but suffered from design flaws that prevented it from achieving the specified impulse ranges. A second prototype shock tube was designed and constructed by Cauble Precision Machine Inc. of Lawrenceburg, IN. The second prototype could generate the specified impulse ranges, but because the manual clamping system to hold the membrane and pressurizing the shock tube chamber was unwieldy to use for a sustained period. NIOSH modified the shock tube and improved the pressure control and installed a pneumatic clamping system on the third prototype shock tube. Safety features were integrated into the pneumatic clamping system to prevent inadvertent crushing of hands or fingers between the flanges. An acoustic horn was designed and manufactured to provide impedance matching between the shock tube and the room. The horn provided a small amount of gain necessary to generate impulses at the highest peak pressure levels 166 to 170 dB SPL. In summary, the operational performance and safety of the third shock tube was significantly improved relative to the second prototype. The modifications to the acoustic shock tube have been incorporated into subsequent versions of the shock tube manufactured and sold by B/C Precision Tool (formerly Cauble Precision Machine Inc.). Conclusions: The NIOSH acoustic shock tube has demonstrated that impulses can be successfully produced using a polyester membrane of thicknesses of 0.001 or 0.002 inch thickness. In addition, this system, developed in support of the Environmental Protection Agency hearing protector labeling requirements (40CFR Part 211 Subpart B), fulfills the operational requirements specified in the ANSI S12.42-2010 standard. This report describes the design, construction and basic operation of shock tube.
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DOI:10.26616/NIOSHEPHB35012a
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Pages in Document:1-42
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NIOSHTIC Number:20041073
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NTIS Accession Number:PB2012-112931
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Citation:Cincinnati, OH: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, EPHB 350-12a, 2012 May; :1-42
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Federal Fiscal Year:2012
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Peer Reviewed:False
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Resource Number:EPHB-350-12a;B07182012;
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