Development of a Standardized Acoustic Shock Tube

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

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.

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 dB148-152 dB 0r 150+2 dB166-170 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

Impulse Source

Distance from Impulse Source

rh

h

q

qFree-Field

Pressure Probe

Impulsive Acoustic Test Fixture

r

Proposed EPA label for HPD

ABC Protective Equipment123 Main StreetAnywhere, USA

MODEL ABC1www.ABCProtectors.com

Federal law prohibits removal of this label prior to purchase

LABEL REQUIRED BYU.S. E.P.A. REGULATIONS40 CFR Part 211, Subpart BEPA

NoiseReductionRating

0 10 20 30 40 50

PASSIVE

IMPULSIVE22 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

Pressure Chamber

Flat Gasket and Puncture Knife Pneumatic Solenoid for Puncture Knife

Flat Gasket and Puncture Knife

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

Support for Control Panel and Ram

Control Panel

O-ring Gasket and puncturing knife

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