A Cochlear Analogue Bio-mimetic Muffler

 

Sripriya Ramamoorthy - ramamoor@ohsu.edu

Alfred L. Nuttall nuttall@ohsu.edu

Oregon Hearing Research Center

Department of Otolaryngology/Head and Neck surgery

Oregon Health & Science University

Portland, OR 97239, USA

 

Karl Grosh - grosh@umich.edu

Department of Mechanical Engineering

University of Michigan

Ann Arbor, MI 48105, USA

 

Popular version of paper 2aNSa8

Presented Tuesday morning, April 20, 2010

159th ASA Meeting, Baltimore, MD

 

 

Noise exposure is a nuisance, but more importantly, noise exposure at high levels affects the cellular structures inside the snail-shell shaped hearing organ cochlea, which can lead to hearing loss. Those in active military service are especially vulnerable to hearing loss, due to exposure to high levels of noise. Such hearing loss is permanent, and thus affects their personal lives following service. The rehabilitative health care costs are enormous. Noise exposure in industrial or construction settings is also significant, and it is regulated by the Occupational Safety and Health Administration (OSHA, US Department of Labor). Mufflers and personal hearing protection devices could help prevent noise-induced hearing loss. We designed a muffler inspired by the mechanics of the cochlea.

 

After sound enters the outer ear, it reaches the ear drum and vibrates three bones in the middle ear. This vibration is passed on to the cochlea. Inside the cochlea, there is a membranous flexible plate of gradually increasing width and decreasing thickness, which separates two fluid-filled ducts. The vibrations of the middle ear bones cause acoustic pressure in the fluid ducts that sets a wave traveling along the flexible plate in the cochlea, much like a ripple moving across the surface of water.

 

The cochlea separates the incoming sound to frequency-specific locations on the flexible plate. This separated information is sent to the brain via auditory nerve fibers. The spatial separation of frequencies is analogous to keys in a piano, except, these frequency-specific locations inside the cochlea are a broad continuum instead of distinct notes. This continuum enables us to have a wide frequency range of hearing and differentiate two closely spaced frequencies, an essential aspect for enjoying music and understanding speech. The frequency specificity is partly due to the gradation of the flexible plate inside the cochlea. Additionally, when there is no hearing-loss, specialized cells in the hearing organ greatly enhance the sharpness of our pitch-discerning ability by up to 1000 times by mechanisms researchers do not fully understand.

 

The waves traveling along the flexible plate in the cochlea slow down significantly at and near their frequency-specific site. The slowing of the traveling wave gives more time for energy to dissipate and less energy is transmitted further. A common example of the effect of energy dissipation is that of a swing which eventually comes to a stop unless we continue to add energy to move it.

 

The gradual rather than sudden change in the plates profile permits the wave to travel to its frequency-specific site and allows for the dissipation mechanism to operate over a wide frequency range. In noise-control parlance, the cochlea is a most efficient broadband muffler that reduces the acoustic fluctuations by about 80 dB, or 10,000 times, over a broad frequency-range from 20 Hz to 20,000 Hz.

 

By mimicking the cochlea, we designed a muffler which has a flexible plate interacting with a fluid-filled duct. We designed this engineered device employing non biological materials using numerical analysis, and validated our findings by physical testing. We identified the similarities and differences in the performance of the engineered device with that of the passive cochlea (with non-functional specialized cells). Our muffler exhibits broadband noise reduction like the cochlea. However, unlike the cochlea, the wave traveling along the flexible plate is partly reflected back in the engineered muffler and another portion is dissipated. We demonstrated more than 20 dB or 10 times noise reduction at low frequencies in air using a device approximately 25-cm long and predicted about 40 dB or 100 times noise reduction in water using a 5-cm long device. Applications for our device include automotive exhaust systems, HVAC systems, and industrial gas lines.