Testing in Creare’s Sound Booth.

At Creare, we’ve had the opportunity to work on a series of projects to improve hearing protection devices for use in extreme noise environments. Our approach has been to understand the mechanisms of sound conduction through the head and body, then design protective equipment based on our understanding of the conduction paths. We have made unique measurements of bone-conducted sound transmission, and have managed for the first time to characterize the response of the human head to an external sound field. In the process, we have developed a number of innovative measurement techniques and built a sophisticated on-site sound laboratory.

Even the best protection against air-borne sound transmission through the ear canal does not protect against sound conducted through the bones and tissues of the head and the body. In a series of projects for the Air Force and the Navy, we performed in-depth research into the mechanisms and pathways of bone-conducted sound. We investigated the transmission levels of bypass sound to the cochlea and examined the effectiveness of different types of hearing protection in attenuating this sound. In order to make detailed measurements of the vibration of skull bones in an intense sound field, we developed a head simulator.

Skull with Accelerometers, Jaw Added and Organs Simulated by Latex Balloons Filled with Silicone.	Tissue Simulated by Silicone, Muscle Tension by Elastic Tape.	Skin Simulated by Balloons, Remaining Cavities Filled with Silicone Oil to Simulate Cerebrospinal Fluid.

The simulator comprises a human skull, instrumented with bone-anchored accelerometers, and reconstituted with organs and skin made from silicone and latex. We validated the response of the head simulator by comparing its driving point impedance to values reported in the literature for human subjects and cadavers. We then conducted new experiments in our hemi-anechoic chamber to characterize the response of the simulator to extreme noise fields. We also carried out tests with human subjects designed to expose the torso to sound, while isolating the head and ears from the noise.

Military jet aircraft operating at full afterburner can produce massive sound levels, in the vicinity of 150 dBA. The cranial helmet and earplugs currently worn by Navy personnel on aircraft carriers provide enough protection for only three launch or recovery events before crews reach their safe exposure limit for the day. As crews can launch and recover over 30 aircraft in a given shift, hearing damage is a real concern. The magnitude of the hearing protection problem in the military is illustrated by the $400 million paid out each year by the Department of Defense for hearing-related disabilities.

Hearing Protection System. Three helmet sizes, each comprising a hard shell, viscoelastic foam liner, synthetic leather edge seal, earmuffs, face shield and ear plugs.

We have applied our research results in a project for the Navy to design and develop an improved head and hearing protection helmet for aircraft maintenance personnel. Our goal was an advanced helmet that provided improved impact protection and the highest possible level of hearing protection while remaining functional and comfortable. In tests with our prototype helmet, we demonstrated a sevenfold increase in allowable exposure time, roughly the equivalent of 21 aircraft launch or recovery events. The helmet components include a layered composite shell, in which each layer serves to add a desired characteristic to the system, and a composite viscoelastic foam liner and edge seal.

Our work in this area has resulted in significant advances in hearing protection, but Creare’s efforts are not over. Continued research and development of hearing protection and communication technology will be required to ensure sufficient protection for Navy carrier deck crews and other personnel required to work in extreme noise environments. Current projects for Tony Dietz, in addition to hearing protection technology, include the development of an acoustic height sensor to enable Army precision airdrops, and the development of a portable Flight Test Assistant for test pilots.

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