The better to hear you with Fly ears offer hope of better hearing aids By Susan E. Barker On a warm southern night, a cricket chirps and attracts not the mate it was seeking but the deadly attention of the parasitoid fly, Ormia ochracea. The fly's reproductive cycle demands that it deposit its larvae on or near a live cricket, and evolution has afforded the night-flying Ormia specialized equipment to home in on the hapless host insect by sound alone. Unlike most flies, which rely on sensor hairs on their legs and can "hear" only low frequencies, Ormia has ears on its chest that have actual ear drums. The ears are connected by a tiny mechanical structure that uses two resonant modes of vibration to provide the insect with directional, high-frequency hearing. Flying through the darkness with uncanny accuracy, Ormia lands on the cricket and deposits her larvae. The larvae burrow into the host and begin devouring it from the inside out. Later, the larvae abandon the cricket's desiccated exoskeleton to pupate. Binghamton University Professor Ronald Miles is hoping a tiny mechanical structure that allows the parasitoid fly Ormia ochracea to locate its victims with unerring and deadly accuracy may soon prove equally beneficial to the millions of Americans who suffer from hearing loss. Miles and associate researchers from Cornell University plan to develop a directional microphone for hearing aids that is smaller and more effective than anything currently available. The team has received a $1.1 million grant to accomplish their goal by mimicking the structure in the fly's auditory system. The structure, known as the intertympanal bridge, makes it possible for the fly to localize sounds even though its ears are very close together. Localizing sounds is a special challenge for small animals like Ormia, which is about the size of the common house fly and native to the southern United States and Central America. To be able to localize sounds, small animals must be able to discern and process minute differences in signals that arrive almost simultaneously in ears that are of necessity very close together. The challenge might be easiest to appreciate by thinking about how humans tell where sounds are coming from. When there is a question about the source of a sound, most people have a tendency to cock or turn their heads in an attempt to exaggerate and weigh the difference in signals reaching one ear over the other. Even aided by this behavioral adaptation, higher vertebrates rely on sophisticated neural processing to determine the origins of sounds. For Ormia, whose ear drums are located less than one hundred microns apart on its chest, turning its head to discern minute differences in signals would be nonproductive. And sophisticated neural processing is out of the question. Instead, such discernment is made possible by the intertympanal bridge, which links the insects' ears and provides a sort of mechanical pre-processing that reduces the processing requirements on the central nervous system. The horizontal bridge is designed so that it can rock back and forth across a pivot like a teeter-totter or, like a hinged teeter-totter, simultaneously rocking back and forth and independently flapping up and down on either side of the pivot in response to vibrations excited from a sound wave on the fly's ear drums. These uncoupled resonant modes make it possible for the combined movements to "add" on one side and "cancel" on the other. This causes the two ears to have significantly different responses to sound, helping the fly to recognize where sounds are coming from. The responses of the uncoupled resonant modes are analagous to an electronic circuit for recording stereophonic sound. This type of circuit allows two nondirectional microphones-in this case the tympanic membranes or ear drums of the fly-to construct a small directional receiver. The intertympanal bridge was recently discovered and studied by Miles, Ronald Hoy of Cornell University's Department of Neurobiology and Behavior and biologist Daniel Robert of the University of Zurich, who collaborated on a three-year project funded by a $340,000 grant from the National Science Foundation to explore Ormia's auditory system. Although scientists had been aware of Ormia's ability to localize sound since the 1960s, Robert didn't discover until 1992 where Ormia's ears are, and the NSF project then answered a basic scientific question about how Ormia's auditory system is capable of directionality. The recently announced $1.1 million NIH grant will allow Miles and his new team to take that basic science and put it to practical use in a way that will improve the quality of life for many people, he said. More than 28 million Americans, or about 10 percent of the population, are afflicted with hearing loss. What's more, as baby-boomers head into their senior years, the Better Hearing Institute in Washington D.C. predicts that the number of people with hearing and speech impairments will increase at a faster rate than the total U.S. population through the year 2050. One of the biggest complaints most people have about hearing aids is that they typically do nothing to help the user distinguish speech, which generally comes from in front of the user, from the ambient noise coming from everywhere around the user, Miles said. The new microphone would be sensitive to speech range frequencies from 400 hertz to 4 kilohertz and allow a 20-decibel attenuation of sounds that come from behind a wearer, which should make a "substantial" improvement in speech intelligibility, Miles said. A 20-decibel difference is approximately the difference between a whisper and a normal speaking voice. The smallest directional microphone now available is about the size of a pencil eraser. While that might seem small, it is too large to allow the use of directional microphones in cosmetically preferable in-the-ear hearing aids, Miles said. The new microphone, which is expected to be about one third that size-or only about 2 millimeters across-ought to fit easily in an in-the-ear package, he added. That should mean that in less than five years, in-the-ear hearing aids with directional capabilities will be commonly available, he said. Until now only behind-the-ear and other externally worn and bulky hearing aids helped the wearer determine the direction of the sound. As a vibrations specialist, Miles said he is intrigued by basic research questions. But, ultimately, as an engineer, he is compelled to look for practical ways to use the findings of basic science. By working on consecutive projects to study Ormia's auditory system and then to put that knowledge to practical use, Miles said he's enjoying the best of both worlds.

What's all the noise about? What are the different degrees of hearing loss? With mild hearing loss, in the 26-45 decibel (dB) range, one-on-one conversations are audible if the listener can see the speaker's face and is standing at close range. With a moderate hearing loss, in the 45-65 dB range, detecting conversational levels of speech through background noise is extremely difficult. Hearing and understanding conversational levels is difficult even in quiet backgrounds. With a severe hearing loss, in the 66-85 dB range, hearing is difficult in all situations. Those with profound hearing loss, beyond 85 dB, may not register even the loudest speech or environmental noises.   What is a decibel? A decibel is a logarithmic expression used to express power ratios and is an important unit of measurement for indicating power level and pulse compression. A decibel is 1/10th of a bel. It is equal to the smallest degree of difference of loudness ordinarily detectable by the human ear.   How loud are these sounds? Movie theater-87dB Living room band practice-116 dB Jackhammer- 130 dB Gunshot-140 dB Jet takeoff-150 dB