Humpty-Dumpty Effect: Acoustically, people resemble large eggs

Using a sound-based scanning technique to determine the shapes of moving creatures and other objects, an international team of scientists has found that the human form bounces sound waves as if each person were a huge, elongated chicken egg.

This new acoustic portrait of people as a hard ellipsoid may aid designers of concert halls and other venues where acoustics are of paramount concern, says Stéphane G. Conti of the Southwest Fisheries Science Center in La Jolla, Calif. Until now, no one had measured the absolute acoustic profile of the human body–that is, how the body scatters sound waves independently of where it happens to be.

This new acoustic description “should have application everywhere a human is embedded in an acoustic soundscape,” comments acoustics specialist Vincent Gibiat of the Université Paul Sabatier in Toulouse, France.

In a hard-walled room, sound waves rebound from an object along paths determined by the object’s shape. So, each object creates a distinctive interference pattern and intensity spectrum. Two years ago, Julien de Rosny and Philippe Roux of the Université Paris VII in France demonstrated that they could infer the three-dimensional shape of an object by recording sound waves as the object moved to various locations in a stark, sound-reflecting tank filled with water. They proposed using the method to automate fish counting in aquaculture tanks.

To arrive at the new acoustic representation of people, Conti, de Rosny, and Roux, now at the University of California, San Diego, and David A. Demer of the Southwest Fisheries Science Center placed a speaker and a microphone in each of two reverberant rooms–a squash court and a fallout shelter. Next, the scientists had a person walk around in one of the rooms, while they recorded the many echoes that resulted from audio pulses emitted by the speaker. The team conducted tests with 27 people from ages 3 to 55.

Analyzing those recordings, the scientists mathematically distinguished the sound waves that had bounced off just the walls of the room from those that had ricocheted off a person. In general, the team found that each person’s bodily influence on the recordings matched that expected from an ellipsoid of the same height and girth as that of the person.

That shape would be proportionately taller and thinner than a typical egg is.

Further measurements showed that clothing absorbs sound, but the body’s acoustic absorption is negligible. Conti described the measurements last Tuesday at a meeting of the Acoustical Society of America in Austin, Texas.

The researchers, whose main interest remains sea life, undertook the human experiments in an attempt to decipher perplexing differences between sound spectra of anchovies and sardines. The challenge seemed to require certain controlled tests, and, Conti explains, “it’s easier to control humans than fish.” However, the new results didn’t indicate why the fish spectra differ.

By building a database of acoustic models of various sea creatures, the team ultimately aims to enable fisheries specialists to automate counts of specific species in the ocean, Demer says. Moreover, he adds, if the work includes models for human divers, it may lead to automated systems that detect saboteurs entering ports.

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