1. 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.

2. 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 dB peak sound pressure level.

3. 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.

4. Current EPA Label for HPD

5. 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:
dB 0r dB
dB 0r dB
dB 0r 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.

6. Methodology Recommended by EPA for Computing NRR for HPD

7. Proposed EPA label for HPD
ABC Protective Equipment
123 Main Street
Anywhere, USA
MODEL ABC1
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
PASSIVE
IMPULSIVE
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.

8. 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.

9. 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.

10. 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.

11. 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

12. 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

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

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

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

16. ARL Shock Tube

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

18. 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.

19. 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.

20. 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