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."
Before this set of experiments, the scientists conducted tests that proved blindfolded listeners could accurately describe the shape of foam core objects blocking speakers emitting sound. They could also determine the width and size of a changeable doorway-like frame when sounds from built-in speakers came out of it.
Although people are not always conscious of reflected and ambient sound, our brains are always processing such information.
"For example, we take into account an individual's distance and surroundings based on reflections as we process their voice," Rosenblum said. "We do the same when we are determining the simple position of a sound source."
Another team led by Osaka University's Takahisa Furukawa will announce at the Paris meeting that detection of background noise appears to be affected by genetic or cultural differences. Furukawa and his colleagues discovered Japanese, American and German people in an office setting heard background noises -- in this case, the sound of a copy machine -- differently, although all listeners were annoyed by the machine.
Japanese participants complained of the "impulsiveness" of the sound, while the American and Germans found it to be too "sharp."
In another related study, published in Physical Review Letters, University of Illinois at Urbana-Champaign researchers found that silent objects really do emit noise at the nano level. Using special equipment, they picked up tiny vibrations of thermal energy that exist naturally in all objects, alive or not. The resulting mini-sound waves were named phonons.
Richard Weaver, a professor of theoretical and applied mechanics at UI, described them as being "like BBs rattling inside a box" that bounce off the object's walls.
In the future, detection of phonons could be used to identify random vibrations from distant earthquakes, Weaver said.