Like Bats, People 'Hear' Silent Objects

June 11, 2008 -- The song "The Sound of Silence" might hold more truth than ever imagined, suggests a new study that determined people can hear silent objects based on reflected and ambient sound.

Since blind individuals may be particularly tuned in to such sounds, the research helps explain how they can often find doorways, windows and objects without seeing or touching them. It may also lead to a better understanding of echolocation, the technique used by certain animals, like bats and dolphins, to detect reflected sound.

The study also suggests that true silence, except in artificially created environments like soundproof booths, does not exist.

"In the real world there is always some type of ambient noise," said Lawrence Rosenblum, who conducted the study with co-authors Ryan Robart and Ethan Chamberlain.

"Wherever there is sound, such as ventilation and air currents, there are sound reflections and obstructions," added Rosenblum, a professor of psychology at the University of California at Riverside. "One way to convince folks that there is always some quiet 'ambient' sound in a room is to listen to a seashell. It's the seashell's amplification of ambient sound -- specific frequencies of the sound -- that makes it sound like the ocean."

For the study, Rosenblum and his colleagues used a rolling equipment cart to move blindfolded students around a basic campus classroom consisting of a linoleum floor, sheet rock walls, a tile ceiling, two white boards and three large windows covered with plastic blinds.

For the first experiment, students were allowed to speak and then use the sound of their own voices to determine what part of the room they were in. They were nearly always accurate. In follow-up experiments, the students had to be quiet, but either relied upon sounds made by a clicker, recorded sound bursts or no extra sound at all.

The students again aced the tests, although they were not as accurate during the quietest experiment.

The findings will be presented later this month at Acoustics08, the annual meeting of the Acoustical Society of America, which will be held in Paris.

Rosenblum and his colleagues wrote, "The results add to the evidence that sighted listeners are sensitive to reflected sound to the degree that it can be used for perceiving properties of silent, sound-structuring surfaces in the environment."