Kids ask: Why does sound travel faster in water?

Ms. Jamie Stuart and Ms. Mary Dunn's second-grade students at Diamond Lake Elementary School in Mundelein wanted to know: "Why does sound travel faster in water?"

Sound is the science of acoustics. So many things can change the way things sound. When creating a performance space, acoustical engineers and architects must select materials that showcase sound and create the best possible listening experience for performers and audiences. It's their business to know how sound moves through various elements.

"In air, sound is transmitted by air molecules," said Dirk Noy, Switzerland-based partner of the Walters-Storyk Design Group, an international architectural and acoustical design firm that creates recording studios, broadcast and education facilities and performance venues like Le Poisson Rouge and Jazz at Lincoln Center, both in New York City.

"Sound is motion, the motion of molecules vibrating. Every sound starts with something trembling or vibrating in some way," Noy said.

Most sounds we hear travel through the air, which is a gas. Sounds can also travel through liquids like water, and sound can travel through metals. Since molecules of air, water and metal have different properties, sound travels through gas, liquid and metal at different speeds.

Let's look at air. Sound travels through air at an approximate speed of 1,125 feet per second, and that speed can change with the temperature. When something trembles or vibrates, a chain reaction of molecules bump one another in a wave pattern. Because of the properties of a gas, the molecules that cause the chain reaction are not connected. They continue to bump one another until the heat of the molecules causes them to slow and then stop.

Noy used this example: "Imagine that molecules traveling through air are like free-floating traffic in a quiet neighborhood. It takes a long time for one vehicle to meet another vehicle. Sound is transmitted relatively slowly."

Liquid molecules have different properties.

"In water sound is transmitted by water molecules," Noy said. "These are densely packed. Imagine this like a traffic jam on a busy highway. The vehicles are very close together, so if one vehicle bumps into another, the next vehicle is hit and so on. The sound is transmitted much more quickly."

Sound in water is 4 to 5 times faster than sound in air, rushing at 4,920 feet per second.

Sound zooms through metals at 10 to 15 times faster than air. Sounds heard through steel are clocked at the amazing speed of 19,000 feet per second.

Check these out

Cook Memorial Library in Libertyville suggests these titles on sound:

• "Sound: A Question and Answer Book," by Fiona Bayrock

• "Sound: An Investigation," by Jack Challoner

• "Sound," by Sally M. Walker

• "Exploring Sound," by Carol Ballard

• "Earsplitters! The World's Loudest Noises," by Steve Parker