Looking Back to the Future of Hearing Protection:
Vacuum Ear Plug
Martin Lenhardt - lenhardt@vcu.edu
Biosecurity Technologies Inc
Popular version of paper 4pPP1
Presented Thursday afternoon, April 22, 2010
159th ASA Meeting,
In five days God created the universe, the earth and heavens and populated the sea with fish and the land with vegetation. It then took 375 million years in our time for fish to evolve into four legged land animals, the tetrapods. It is a fascinating story of natural bioengineering with the ear as one of its stars. There were two major transitions, from hearing underwater to air and the moving of reptile jaw bones into the middle ear to improve aerial hearing sensitivity. The best transitional form known from fish to tetrapod is the Tiktaalik (large shallow water fish in Inuktikuk), an animal that looks strikingly like a fish except for its flattened mobile head with a neck and wrist like anterior (pectoral) fins that could support its body on the substrate. In adapting from water to land the hypomandibular bone became smaller and less of a brace linking the jaw to the quadrate bone of the skull (now mobile) which poised it for a new auditory function. That new function allowed substrate vibrations to reach the skull and inner ear efficiently; eventually becoming a hearing only bone called the stapes. The hearing role of the stapes (although not the mechanics) was much the same in early tetrapods as it is today in mammals including people.
The evolution of hearing tale typically jumps from stapes enhanced substrate hearing to how early tetrapods were able to hear better in the thin medium of air by further modifications of the middle ear by relocating the jaw hinge bones, that is the second marvelous story of auditory evolution. But lets stay with the early tetrapods a bit longer; there is an important and often overlooked hearing story with great relevance for today. What exactly did these ancient animals hear once on land? The substrate vibration mechanism would be as effective on land as when the limbs (bone and soft tissue) are coupled to the bottom when submerged. Loose skin would cover the otic notches in the skull that housed the stapes and would not be an eardrum as we know it. In more primitive forms the stapes was large and its mass added to the animals hearing sensitivity by substrate -bone conduction. That is to say, vibrations coupled to the head via its limbs vibrated the skull, the stapes motion lag behind due to inertial and acted as a battering ram into the inner ear. The loose skin over the otic notch in the skull acted as a damper. Thus for these animals their ears worked in the reverse as ours today. The thin taunt skin of our eardrum is the input to the inner not the exit.
If environmental sound hits the body of any animal, it will penetrate the body once there is sufficient energy to overcome the skins acoustic impedance. Body vibrations are conducted to the inner ear through bones and soft tissue. For low frequencies (<2 kHz) the relative motion of the head and the middle ear bone, due to it inertia, is the principal mechanism of inner ear vibration. The impedance difference between sound in air and whole body vibration is about 40-50 dB SPL depending on frequency. This 40-50 dB level is what is thought to be the threshold of whole body vibration or the bone conduction limit. If the middle ears are compromised as in cases of congenital atresia in which the external and middle ears are sealed from the environment with bone; hearing thresholds are about 55 dB SPL. Almost 50 year ago, Zwislocki isolated the external and middle ears and found that sound at approximately 5060 dB sound pressure level would penetrate the body and reach the inner ears; thus, determining the maximum level of ear protection that plugs and muffs, worn in combination, could provide.
Hearing protection is necessary in extremely high noise since yje ear and body allow too much energy to pass into the inner ear destroying sensory cells and creating irreversible hearing loss. It is important to not only protect ones hearing against noise transmitted through the air-conduction pathway, but also through whole body conduction. Much of the energy transmitted through the body is concentrated in the mid-frequency range around 12 kHz, which is likely due to the action of the middle ear bones set into motion by the relative motion of the head. Recall the middle ear bones are the inertial mechanism that enhanced sensitivity in primitive tetrapods and it is still a mechanism of hearing today. Ear plugs do not alter relative motion of the middle ear bones in high noise, once that noise exceeds the bone conduction limit
If that middle ear motion can be damped by applying negative static pressure in the ear canal it could partially mitigate this mid-frequency whole body effect and increase the effectiveness of hearing protection in high noise. Static air pressures of -100, -200, -400 mm H2O (or -1, -2, -4 kPa) were applied in the ear canal via a custom modified ear plug with a probe microphone. The vacuum mechanism consisted of bulb on a polypropylene frame embedded in a closed cell urethane flanged housing. By comparing the output of the microphone in the canal between the plug and the drum with one external to the plug the attenuation can be calculated. This provides the attenuation provided by the vacuum plug but not the effect of damping the middle ear bone. The damping is added to the threshold reduction determined behaviorally in noise (white at 96 dB SPL). Finally the effect of the vacuum was verified behaviorally by comparing the attenuation of the plug with and without negative static pressure in the canal. The plug chosen delivered 30 dB of attenuation with a 5 dB of vacuum effect for a static pressure of -200 mm H2O at 1 kHz. If the static pressure is doubled, the attenuation due to the vacuum effect is about 9 dB. Additional attenuation can be designed into a vacuum plug .
Stiffing the eardrum and reducing the mobility of the middle ear bones by vacuum can notably increase the effectiveness of earplugs, especially in military noise as in carrier operations where the levels can reach nearly 150 dBA over a period of hours. The action of the middle ear bone, evolved so many millions of years ago, can be modified to protect hearing in high noise.
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