Development of a Standardized Acoustic Shock Tube Amir Khan, Edward Zechmann and William J. Murphy Hearing Loss Prevention Team Engineering and Physical Hazards Branch National Institute for Occupational Safety and Health May 22, 2009 The findings and conclusions in this presentation have not been formally disseminated by the National Institute for Occupational Safety and Health and should not be construed to represent any agency determination or policy.

Acoustic Shock Tube High amplitude wavefront Rapid expansion of a compressed gas. Pressurizing a chamber sealed by a membrane Punctured or ruptured spontaneously. Impulse peak levels range is 130-180 dB peak sound pressure level.

Proposed EPA Regulation EPA is proposing new labeling requirements for all Hearing Protection Devices. The current labels for the hearing protection devices only require the Noise Reduction Rating (NRR) for continuous noises. The EPA is expected to propose new methods for passive, active noise reduction and impulsive hearing protection devices.

Current EPA Label for HPD

Proposed EPA Labeling Requirements According to the new EPA regulation the NRR for HPD will have to be evaluated at three exposure levels using A-duration peak height: 130-134 dB 0r 132+2 dB 148-152 dB 0r 150+2 dB 166-170 dB 0r 168+2 dB In order for a standardized shock tube design to be acceptable it must be capable of generating impulses within the three exposure range levels. impulses can also be generated using explosive charges, but then the concerns about the safety at the testing lab can become an issue. The acoustic shock tube is a much more safe and less hazardous method than working with explosives.

Methodology Recommended by EPA for Computing NRR for HPD

Proposed EPA label for HPD ABC Protective Equipment 123 Main Street Anywhere, USA MODEL ABC1 www.ABCProtectors.com Federal law prohibits removal of this label prior to purchase LABEL REQUIRED BY U . S E P A REGULATIONS 40 CFR Part 211 , Subpart B EPA Noise Reduction Rating 0 10 20 30 40 50 PASSIVE IMPULSIVE 22 38 33 40 This product is intended to protect against impulse noise exposures (such as firearms, impact tools, and machinery). The NRR’s are comprised of two numbers. When this product is used as directed, the user may expect to receive a level of protection between the lesser and the greater NRR. See supporting information for details.

Why Do We Need To Develop A Standardized Shock Tube Design? To ensure reproducibility of NRR from laboratory to laboratory and from manufacturer to manufacturer. This will be achieved by: Developing guidelines for designing standardized shock tube capable of complying with the proposed new EPA labeling requirements. Standardizing the methodology for computing the NRR for HPD to impulsive noise exposure.

Evaluate the Performances of Three Different Design Shock Tubes. National Institute for Occupational Safety and Health (NIOSH) shock tube located at the Robert Taft Laboratories in Cincinnati, Ohio. US Army Aeromedical Research Laboratories (USAARL) shock tube located at Fort Rucker, Alabama. US Army Research Laboratories (ARL) shock tube located at Aberdeen Proving Grounds, Maryland.

Criteria To Be Used In Rating The Performances Of Shock Tubes Precision and reproducibility of the shock tube in generating the three exposure levels. The repeatability and reproducibility of the NRR measurements for a wide range of HPD using all three shock tubes. The day-to-day variations in the acoustic environment of each of the facilities.

Experimental Protocol For The Evaluation Of The Performances Of The Shock Tubes The protocol will specify: Type and size of the microphones to be used Standard operating procedure to be used in the calibration of the Microphones Types and number of the HPD to be evaluated Locations of the microphones relative to the mannequin Location and orientation of the mannequin relative to the shock tube Study duration Room conditions Data collection and analysis

NIOSH Shock Tube Flat Gasket and Puncture Knife Pressure Chamber Flat Gasket and Puncture Knife Flat Gasket and Puncture Knife Pneumatic Solenoid for Puncture Knife

Air Pressure Meter with Reset Button Air Pressure Release Valve Air Pressure Meter with Reset Button Pressure Reducer and Gauge Pneumatic Solenoid Trigger

USAARL Shock Tube Square Horn 6 inch diameter Shock Wave Guide Hydraulic Ram membrane clamp Hydraulic Ram -- membrane clamp

Mannequin and Square Horn Support for Control Panel and Ram O-ring Gasket and puncturing knife Control Panel Mannequin and Square Horn

ARL Shock Tube

Evaluate The Effects Of The Atmospheric Conditions On The Performance Of The Shock Tubes Relative humidity Room temperature Atmospheric pressure

Selection Of The Shock Tube As A Prototype For The Design Of The Standardized Shock Tube The shock tube with the best compliance to the EPA regulation will be used as a prototype for the development and design of a standardized shock tube. The design of the selected shock tube will be improved to enhance the reproducibility and precision of the NRR for HPD by generating stable and reproducible impulses at the required exposure levels per EPA regulation. The performance of the new improved shock tube will then be optimized.

Optimization Of The Critical Components Of The Selected Shock Tube The explosion mechanism. The size and design of the chamber downstream of the explosion. The size and design of the chamber upstream of the explosion.

Other things to consider Development of a speaker driven shock tube Use of explosives Choice of transducers for measurement PCB Blast Probe 1/8” microphone