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60 Years of Hearing Conservation and Hearing Protection Research and Development at Wright-Patterson Air Force Base
By Richard L. McKinley

Background of Wright-Patterson Air Force Base
The history of the Wright-Patterson Air Force Base, of the Air Force itself, and even of aviation, is of interest in framing the story that I would like to share. The issuance of a patent to the Wright Brothers in 1906 brought about the creation on August 1, 1907, of the Aeronautical Division of the Army Signal Corps. Congress voted the first appropriation for military aviation in 1911 and expanded the service into an Aviation Section of the Army in 1914. In 1917, Wilbur Wright Field was opened to train pilots and gunners during World War I, followed shortly by the creation of the adjacent Fairfield Air Depot, in what is today Fairborn, Ohio. In 1924, with the closing of the McCook Field test facility, the Dayton community purchased 4500 acres including the leased area on which Wilbur Wright Field was located and named the combined facility Wright Field for the Wright Brothers. Wishing to recognize the contributions of the Patterson family (owners of National Cash Register), an area of Wright Field was renamed Patterson Field on July 6, 1931, in honor of Lt. Frank Stuart Patterson, who was killed in 1918 during a flight test. In 1948, Wright Field and Patterson Field were merged under the name Wright-Patterson Air Force Base.

From US Signal Corps to US Air Force

• Aeronautical Division, U.S. Signal Corps 1907 - 1914
• Aviation Section, U.S. Signal Corps 1914 - 1918
• Division of Military Aeronautics 1918 - 1918
• U.S. Army Air Service 1918 - 1926
• U.S. Army Air Corps 1926 - 1941
• U.S. Army Air Forces 1941 - 1947
• United States Air Force 1947 - Present

Hearing Conservation – Hearing Protection at Wright-Patterson Air Force Base

Dr. Henning von Gierke

In 1947, Dr. Henning von Gierke, together with several colleagues from the Helmholtz Institute in Germany, came to the United States to work for the Army Air Corps in the Bioacoustics Section of the Biophysics Branch of the Aero Medical Laboratory at Wright Field near Dayton, Ohio. Henning’s early years at Wright Field were dedicated to understanding the magnitude and effects of aircraft noise on humans. The 40’s and early 50’s saw the development of the first generation of jet engine aircraft. Henning’s Doctor of Engineering dissertation in 1937 accurately predicted the level and spectrum of noise produced by jet flows from a nozzle and in sheet flow. Today’s predictive computer models are only marginally better than von Gierke’s 1937 method.

In 1948, the newly formed Air Force published the first hearing conservation regulation, AFR 160-3 “Precautionary Measures Against Noise Hazards” that established requirements for prevention of illness and injury from operating and testing Dr. Henning von Gierkejet and rocket engines where hazardous noise levels existed. In this 1948 Medical Services regulation, overall exposures were never to exceed 95 dBA and were recommended to be kept below 85 dBA. Individuals exposed to over 130 dB for longer than 5 minutes per 24 hours were required to have weekly audiograms and if the loss was over 20 dB they were temporarily reassigned until hearing recovered. The regulation also made pre-employment audiograms mandatory.

Soon the research group grew to include pioneering researchers like Dr. Horace (Hop) O. Parrack, one of the charter members of the National Academy of Science Committee on Hearing, Acoustics, and Bioacoustics (CHABA) and Lt Col Elizabeth “Betsy” Guild. Hop Parrack led the way in October 1956 to the first recognized comprehensive hearing conservation program, either within or outside the military. This program required measurements of 8-hour integrated noise exposures as well as octave-band requirements. It included a hearing conservation data card to record the noise exposures, audiograms, and hearing protection data. These cards were stored in a hearing conservation repository.

Elizabeth "Betsy" Guild

While Dr. Parrack was working on hearing conservation, Betsy Guild was flying in any and every aircraft which would give her a seat. Betsy would work on measuring noise and speech communication on almost every flight she took. She was a one-person test team. She was the first woman to fly supersonic, the first woman to fly in a B-52, the first person to measure aerodynamic noise at supersonic speeds, and many others. She was awarded the Air Medal for the courageous pursuit of information to protect others. Her achievements included development of adequate hearing protection and voice communications systems, studies of individual and community response to noise, studies of sonic boom and of rocket noise. Few individuals have personally experienced the first–hand knowledge of so many different noise environments to determine their impact on performance and safety. The Military Audiology Association honors her many contributions to the field of hearing conservation by annually presenting The Elizabeth Guild Award for outstanding service.

Henning von Gierke’s research on intense aircraft noise exposures defined new data on human tolerance limits, noise-induced hearing loss, auditory pain, and hearing protection, all of which stand today. In 1957, he introduced with others the equal energy hypothesis as the time-intensity trade-off for the Air Force hearing conservation regulation. Many years later he chaired the International Standards Organization (ISO) working group that prepared and obtained consensus for the adoption of ISO 1999, which used the equal energy rule as the basis for determining the relationship between occupational noise exposure and the estimate of the resulting hearing impairment.

Also during this time, 1956 & 1957, the Biological Acoustics group at Wright-Patterson working with RCA designed, developed, and demonstrated the first Active Noise Reduction Headset. The work was led by Willard Meeker, and developed the basic equations for an analog feedback ANR controller that are still used today. During this time, work on improved passive hearing protectors, earmuffs and earplugs, and communication headsets was being led by Dr. Charles Nixon. Also during this time Dr. von Gierke developed a lumped parameter model for passive earmuffs that still provides significant insight into the performance of circumaural hearing protectors. The mid-fifties also saw the completion of a dedicated acoustics research building designed by Air Force experts working with consultants from Bolt, Beranek, and Newman (BBN). The building was originally designed as a national acoustics laboratory with 6”-12” monolithic concrete walls, floors, and ceiling throughout the building. Special rooms were actually rooms-within-rooms and over 20 different heating and cooling systems were used to prevent noise from going thru the ventilation system to unintended locations.

Henning also helped to address the noise problems of residents near air bases. Working with his colleagues he led the ten-year development of a comprehensive procedure for prediction of aircraft noise exposure near airports, estimating community response, and land use planning for the Air Force, published in 1964. This methodology provided the basis for the procedures in use today.

The 1960s saw work on high-level continuous noise effects by Henning, Captain George Mohr, and others from the Army, Navy, and NASA (National Aeronautics and Space Administration). The researchers exposed themselves to levels up to 160 dB and recorded non-auditory responses to the noise. This pioneering research led to the inclusion of a 150 dB whole body non-auditory noise exposure limit in the Air Force noise regulation.

In the early 1970s, Major Dan Johnson led pioneering work in the auditory and non-auditory effects of high level 150-170 dB infrasound. In a series of experiments, he investigated the effects of infrasound on vestibular function, cognitive performance, and respiration. Dan also led an interagency agreement with the US Environmental Protection Agency (EPA) sponsoring a longitudinal study of hearing thresholds in children. Captain Mark Stephenson was the principal research audiologist at that time, in charge of the data collection and analysis for this pioneering study. Mark also continued the legacy of hearing protection work for the bioacoustics group. Henry Summer, of the bioacoustics group at Wright-Patterson, in 1974 developed the ear-insert-receiver, a custom molded earplug with a hole or bore and a snap ring to enable the use of an earphone receiver with the earplug. The advantage was that the communications could be delivered directly to the eardrum or tympanic membrane, while the listener had the double noise protection of the custom earplug and an earmuff. This design was used by the F-15 and F-16 maintainers during the seventies and has more recently been reincarnated in systems like the CEP (communications earplug) and ACCES (attenuating custom communication enhancement system).

Also during this time, von Gierke chaired the EPA task force charged with meeting the Congressional mandate to “identify levels of environmental noise requisite to protect public health and welfare with an adequate margin of safety.” The findings in the task force’s “Levels” and “Criteria” reports have provided the basis for non-occupational noise criteria in the United States for the past 30+ years. It should also be noted that Alice Suter, who was then part of the EPA’s Office of Noise Abatement and Control was detailed to Wright-Patterson AFB, and also contributed substantially to the “Levels” document.

Throughout the seventies, the team of Dan Johnson, Charles Nixon, and Mark Stephenson conducted a series of long duration (24-48 hours) noise exposure studies. The results of these efforts were used to establish recommendations for safe noise exposures that are in use today by the Department of Defense (DoD), NASA, OSHA, and NIOSH. In the later seventies, Dan worked with Henning and United States and international researchers developing the first noise dosimeters and standards for human exposures to impulse noise. Dan and Charles also worked together on a single number noise reduction rating that accounted for the general spectral content of the noise. This C-A metric was found to be more accurate than the NRR (Noise Reduction Rating) and was nearly as easy to use. The user only had to measure the overall C-weighted noise level and the A-weighted noise level and subtract the two. The result was then used to look up one of five single-number attenuations for a hearing protector binned on groups of C-A values.

Also in the late seventies, a young biomedical engineer, Rich McKinley joined the bioacoustics group and started a modern effort on developing active noise reduction headsets. The bioacoustics lab working with Bose Corporation demonstrated the practical ANR headset for both military and civilian use in 1978. During this time McKinley also worked on developing noise exposure criteria for women who had become pregnant and had jobs that involved significant noise exposures.The bioacoustics group of von Gierke, Nixon, and Johnson also led the debate on noise exposure criterions and exchange rates technically advocating the 85 dBA, 8 hour, 3 dB/doubling or equal energy criterion. They advised the US EPA on the criterion and worked to get it established in the DOD. While OSHA established 90 dBA and a 5 dB/doubling exchange rate and the US EPA was using 85 dBA and 3 dB/doubling, the Air Force, Army, and Navy, chose a political middle ground at 84 dBA and 4 dB/doubling. The DOD criterion was used until the late 90s by the Army and Air Force when they changed to the almost universal 85 dBA and 3 dB/doubling. Currently the Navy is still using the 4 dB/doubling exchange rate.

Photo of an auditory localization facility (ALF).

The 1980s saw an increased focus on speech communication led by Dr. Tom Moore and Rich McKinley. Together they developed a standardized procedure called the Coordinate Response Measure (CRM) for measuring speech intelligibility across many languages. The test has found many uses in multi-talker speech tests. The bioacoustics group, with Rich McKinley and Mark Ericson leading, also demonstrated in 1987 the first synthetic 3-D audio cue generator. This system digitally processed audio streams such as radio into two signals for the left and right ear. When presented over headphones the listener perceived an apparent sound source location. This was the first 3-D audio display technology demonstration. The lab also developed an auditory localization facility (ALF) to assist in their development of 3-D audio displays. ALF is a 15-ft diameter geodesic sphere with 277 Bose loudspeakers over its surface. The mid-eighties also saw a substantial military construction project that doubled the size of the acoustics laboratory and more than doubled the number of acoustic test rooms.

The 5th generation fighter aircraft, the F-22 and the F-35 Joint Strike Fighter began their development in the 1990s. The bioacoustics group was leading the way with improved hearing protection for the pilots using ANR and incorporating 3-D audio displays into flight test aircraft for demonstration and flight test. In 1999, at the ASA (Acoustical Society of America) meeting in Berlin, Germany, Henning von Gierke was awarded the Gold Medal for his outstanding contributions to science and the society.

The new century saw increased focus on improving hearing protection. The motivation was perceived increases in noise levels from new aircraft and the importance of improving communication capability, enhancing safety, and reducing hearing damage risk. John Hall and Rich McKinley collaborated with Adaptive Technologies, Inc (ATI) to develop a high-performance active noise earplug. There was an extensive collaboration of the Air Force and Navy during this time focused Photo of an auditory localization facility (ALF).on improved hearing protection for aviation personnel. Over $15 million was spent during the 7-year period on technology development and demonstration. The combined programs saw developments in passive custom molded communications earplugs, demonstration of active bone-conduction reduction for single frequencies, passive bone-conduction noise helmets, custom earmuffs, high-output (130 dB) earplug drivers, and basic research in bone-conduction pathways, models, and reduction techniques.

Throughout the 60-year legacy, the Wright-Patterson bioacoustics group, now the Battlespace Acoustics Branch of the Air Force Research Laboratory, has been a national and international leader in developing hearing conservation guidelines and criteria, in the development of hearing protection technologies, and in the development of national (ANSI) and international (ISO) standards in acoustics. The contributions and legacy of Henning von Gierke and the group he started, have been recognized by the research community in the many awards they have received and the fond recollections of those who knew them well and have benefited from their leading-edge contributions.

Rich McKinley is the Senior Technical Advisor at the Air Force Research Laboratory at Wright-Patterson Air Force Base. He has enjoyed working with many of the leaders and scientists mentioned in this review article.

References
Eldred, KM (1999). ”Gold Medal Citation for Henning E. von Gierke,” J. Acoust. Soc. Am. 105(2, Pt. 2), awards insert.

Nixon, CW (1998). ”A glimpse of History: The Origin of Hearing Conservation was in the Military?,” Rept. AFRL-HE-WP-SR-1998-0005, Wright-Patterson AFB, OH.

Photographs of Guild E., available at www.militaryaudiology.org/eguild.html.