Of flies and crickets: Directional microphones for hearing aids

English: A gravid female Ormia ochracea resting on a fingernail. (Photo credit: Wikipedia)

The female parasitic fly, Ormia ochracea, can hear the high-pitched chirp of a male cricket (between 4-5 kHz) as far away as the length of a football field. The fly is sensitive to changes in the direction of sound of up to 1-2 degrees, precision similar to sensitivity in humans (Mason et al., 2001). It uses its detection and sound localization skills effectively to find a male cricket and deposits its larvae on the cricket’s back. The larvae penetrate the body of the host and once inside, feed and grow, ultimately leaving the shell of the dead cricket behind when they emerge as pupae in 10 days (Robert et al., 1992). The ability of the female Ormia to hone in with great accuracy on its host, the male cricket, has to do its extraordinary ears (Miles et al., 2009).

http://www.sciencenewsforkids.org/2006/09/how-to-silence-a-cricket-3/

Unlike human ears, that are separated  by our head, the ears of the Ormia are located on its chest. Its eardrums (tympanum) are adjacent to each other and separated by an ‘intertympanal bridge’ (Robert et al., 1992), about which it executes two different types of motion: a rocking motion, where the two eardrums move in different directions about the pivot (i.e., pitching up and down like a seesaw), and a translational motion where the two eardrums move in the same direction (e.g., flapping like a bird) (Miles et al., 2009).

Humans use interaural time differences (ITDs) and interaural level differences (ILDs) to localize sound sources. That is, they use timing differences based on when the sounds arrive at each ear as well as level differences between the two ears to calculate the direction of the sound. However, ITDs are dependent on the distance between the two ears, i.e., the size of the human head. It is remarkable that the fly, which has its eardrums separated by only 0.5 mm (Mason et al., 2001), is equally sensitive to localization cues as humans. It achieves this feat by effectively amplifying the time difference between its ears by several factors using a combination of mechanical and physiological adaptations.  Mechanically, the eardrum closest to the sound vibrates by 10 dB greater amplitude than the eardrum that is farther for sounds that come from an angle of 45-90 degrees. Furthermore, physiologically, these pressure differences are further processed in the fly’s auditory system as timing (latency) differences in the response of groups of auditory neurons (Mason et al., 2001). Together, these mechanical and physiologic mechanisms make the fly extremely sensitive to tiny pressure (i.e., sound level) differences between its two ears to which it would otherwise be insensitive.

Hearing aids use directional microphones to enhance the intelligibility of speech and reduce unwanted background noise, effectively increasing the signal-to-noise ratio (Ricketts and Hornsby, 2006). Conventional microphones use a design where sound differential on two sides of a diaphragm is used to calculate sound pressure level. Directional microphones are sensitive to differences in sound pressure at sound ports (usually two ports). Directional sensitivity is a function of the distance between the two ports where sounds enter the microphone. One of the drivers of sales in the hearing aid industry is the cosmetic appeal of the aid, which typically means a smaller size. It is extremely difficult to design small directional microphones because the reduction in size comes at the cost of sensitivity to sound pressure changes. Microphone noise becomes an issue, making it difficult to extract the signal from the noise (Miles et al., 2009).

Using biological-mimicry based on the hearing mechanism of the parasitic fly Ormia ochracea, scientists are designing micro-scale directional microphones that could be used in a hearing aid.  Using microfabrication technology, researchers have designed a differential microphone diaphragm that is light and pivots about a central hinge, much like the pivoting motion of the eardrums about the intertympanal bridge of the Ormia. These fly-inspired directional microphones are much smaller than current microphones used in hearing aids, but still have a lower noise differential as a result of low microphone noise (Miles et al., 2009). These directional microphones also employ an optical sensor that reduces electronic noise (Miles et al., 2009). The technology is still under development, and much more work needs to be completed before a commercial product is available. Ultimately, the crucial test will be the demonstration of better hearing aid outcomes when listening in fluctuating background noise, one of the big challenges faced by hearing-aid users.

REFERENCES:

1. Mason, A.C., Oshinsky, M.L., and Hoy, R.R. (2001). Hyperacute directional hearing in a microscale auditory system. Nature, 410, 686-690.
2. Miles, R.N., Su, Q., Cui, W., et al. (2009). A low-noise differential microphone inspired by the ears of the parasitoid fly Ormia ochracea. Journal of the Acoustical Society of America, 125, 4, 2013-2026.
3. Ricketts, T. and Hornsby, B.W. (2006). Directional hearing aid benefit in listeners with severe hearing loss. International Journal of Audiology, 45, 190-197.
4. Robert, D., Amoroso, J., and Hoy, R.R. (1992). The evolutionary convergence of hearing in a parasitoid fly and its cricket host. Science, 258, 1135-1137.

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