Conference Report: Making Sense of the World of Sound

By Kathy A. Svitil|Monday, December 01, 2003

From November 10–14, a diverse lot of physicists, engineers, oceanographers, physiologists, and other researchers met in Austin, Texas, for the 146th meeting of the Acoustical Society of America. Over 700 papers were presented. The highlights include:

Facing the music: Many professional musicians shell out hundreds of dollars to throw their trumpets, tubas, saxophones, and other instruments into a deep freeze. Fans of the technique, called cryogenic tempering, say it improves the tone of instruments and makes them easier to play by realigning the molecules that make up the lattice of which metal is composed. But scientists at Tufts University say the musicians are probably just blowing their money away. Jesse Jones IV and Chris Rogers had a cryogenics company chill five professional-grade trumpets down to –321 degrees Fahrenheit in a chamber cooled by liquid nitrogen, let them sit for 10 hours, then slowly warmed them up to room temperature over a period of 20–25 hours. Jones and Rogers found no statistical difference in the quality of the sound produced by the treated trumpets compared to that of five untreated instruments; a microscopic analysis of the trumpets did not reveal any structural changes. Six trumpet players, beginners to professionals, were asked to assess the quality of all 10 instruments and guess whether or not they’d been treated. They detected no difference in the tone or playability of the instruments and correctly identified the cooled trumpets only 52.5 percent of the time: “Virtually a coin flip,” Jones says. “I also found that there was a great variation in a single player’s tone when he was well practiced versus somewhat out of practice. This leads me to believe that you’re better off keeping your trumpet at home and practicing rather than sending it away to freeze it.”

Pumping up the ear: The thousands of tiny hair cells in the cochlea of the inner ear are the key to the sensitive hearing of humans and other mammals, but hearing researchers have been at a loss to explain exactly what the hair cells do to boost our acuity. To find out, neuroscientists Domenica Karavitaki, then a graduate student at MIT and now at Harvard Medical School, and David Mountain of Boston University used a high-speed strobe light, which flashes up to 10,000 times per second, to snap freeze-frame images of the outer hair cells as they respond to electrical stimulation at frequencies in the normal hearing range. Karavitaki and Mountain discovered that when the hair cells contract in response to stimulation, they push fluid through a channel in a special sensory organ called the organ of Corti. The resulting pressure pulse, says Mountain, “distends the organ, almost like a water balloon that is squeezed at one end.” When the cells relax, the fluid flows back out of the tunnel. “It may be that cells are sensing those pressure changes and that this is the real cause of amplification of sound,” he says. He acknowledges that the results won’t immediately lead to hearing loss cures or tricks to develop supersensitive hearing, “but it’s hard to fix the system if you don’t know how it is supposed to work.”

Hearing the human body: Scientists are figuring out not only how we hear but also how our bodies influence the sounds around us. Stéphane Conti of the National Oceanographic and Atmospheric Administration’s Southwest Fisheries Science Center in La Jolla, California, and his colleagues measured sound pulses bouncing around inside an empty, highly echoing room (they used a squash court and the fallout shelter located in the basement of their laboratory) and then measured the change in reverberated sound when a person walked into the room. Conti and his colleagues found that the human form produced the same effects as a hard ellipsoid (an egg shape) of the same volume. Sound waves were scattered by the body but absorbed by clothing; the more clothing a subject wore, the higher the absorption. The study could help improve the design of home entertainment centers and concert halls. “These results give absolute values for the scattering and the absorption to expect from the humans in a concert hall, parameters that can be used to model sound propagation,” Conti says. They could also be used to design systems that compensate for the acoustic effect of human bodies and their clothes: “You can imagine a system that adjusts the sound level on different speakers depending on the number of people in the room and their location.”

Chain gang: Road crews search for defects in concrete freeway bridges and decks by rattling chains and measuring how sounds waves travel through the structure. When the steel rebar forming the inner skeleton of a concrete bridge corrodes and expands, it can cause the top layer of concrete to separate from the layers below, creating a gap, or delamination, that can grow into a pothole. Chains pulled over the delamination produce a distinctively hollow sound. The problem is actually hearing and quantifying the difference when working on a noisy, crowded road. R. Daniel Costley and Gary Boudreaux are applying some new acoustical science to the low-tech method. Costley and Boudreaux, of the Oxford Enterprise Center in Oxford, Missouri, devised an automated chain drag system, which consists of a chain slung from the bottom of a pushcart; a microphone; a recorder; and a microprocessor, which analyzes the sound produced by the dragged chain. In a test stretch on a bridge on Minnesota’s Highway 10, the engineers were able to spot dozens of delaminations while filtering out the clamor of cars and trucks passing barely 10 feet away.

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