U.S. engineers say the sounds a highway bridge makes when rain falls on it can reveal hidden dangers of potential serious flaws in its structure. Researchers at Brigham Young University in Utah report the method, called impact-echo testing, can diagnose the health of a bridge's road deck based on the acoustic footprint produced by falling water. "There is a difference between water hitting intact structures and water hitting flawed structures," engineering Professor Brian Mazzeo said. "We can detect things you can't see with a visual inspection; things happening within the bridge itself."