Title:  “Most animals hear acoustic flow instead of pressure; we should too”

N. Miles – miles@binghamton.edu

Department of Mechanical Engineering
Binghamton University
State University of New York
Binghamton, NY 13902 USA

Popular version of paper 2aAB1

Presented Tuesday morning May 14, 2019.  8:35-8:55 am

177th ASA Meeting, Louisville, KY

The sound we hear consists of tiny, rapid changes in the pressure of air as it fluctuates about the steady atmospheric pressure.  Our ears detect these minute pressure fluctuations because they produce time-varying forces on our eardrums.  Many animals hear sound using pressure-sensitive eardrums such as ours.  However, most animals that hear sound (including countless insects) don’t have eardrums at all. Instead, they listen by detecting the tiny motion of air molecules as they flow back and forth when sound propagates.   

The motion of air molecules in a sound wave is illustrated the attached video, RNMilesrandomgaswave.  The moving dots in this video depict motion of gas molecules due to the back and forth motion of a piston shown at the left.  The sound wave is a propagating fluctuation in the density (and pressure) of the molecules.  Note that a wave propagates to the right while the motion of each molecule (such as the larger moving dot in the center of the image) consists of back and forth motion.   Small animals sense this back and forth motion by sensing the deflection of thin hairs that are driven by viscous forces in the fluctuating acoustic medium. 

It is likely that the early inventors of acoustic sensors fashioned microphones to operate based on sensing pressure because they knew that is how humans hear sound.  As a result, all microphones have possessed a thin pressure-sensing diaphragm (or ribbon) that functions much like our eardrums.    The fact that most animals don’t hear this way suggests that there may be significant benefits to considering alternate designs.  In this study, we explore technologies for achieving precise detection of sound using a mechanical structure that is driven by viscous forces associated with the fluctuating velocity of the medium.  In one example, we have shown this to result in a directional microphone with flat frequency response from 1 Hz to 50 kHz (Zhou, Jian, and Ronald N. Miles. “Sensing fluctuating airflow with spider silk.” Proceedings of the National Academy of Sciences 114.46 (2017): 12120-12125.). 


Nature shows that there are many ways to fashion a thin, lightweight structure that can respond to minute changes in airflow as occur in a sound field.   A first step in designing an acoustic flow sensor is to understand the effects of the viscosity of the air on such a structure as air flows in a sound field; viscosity is known to be essential in the acoustic flow-sensing ears of small animals.  Our mathematical model predicts that the sound-induced motion of a very thin beam can be dominated by viscous forces when its width becomes on the order of five microns.  Such a structure can be readily made using modern microfabrication methods.

In order to create a microphone, once an extremely thin and compliant structure is designed that can respond to acoustic flow-induced viscous forces, one must develop a means of converting its motion into an electronic signal.  We have described one method of accomplishing this using capacitive transduction (Miles, Ronald N. “A Compliant Capacitive Sensor for Acoustics: Avoiding Electrostatic Forces at High Bias Voltages.” IEEE Sensors Journal 18.14 (2018): 5691-5698).


Acknowledgement:  This research is supported by a grant from NIH National Institute on Deafness and other Communication Disorders (1R01DC017720-01).

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