Have you ever wondered why sound seems to travel better over water than it does over land? Why is it that the joyful screams of children, and the roar of powerboats and personal watercraft, carry so well?
It’s not your imagination. Sound travels better over water. If you’re sitting in a boat on the lake and someone shouts from shore, you’ll hear them better than someone would if they were on land an equal distance away.
The obvious first explanation is that there are no obstructions on the water to interfere with the transmission of sound. With no buildings or trees in the straight line between the source of the noise and your onshore vantage point, you’re likely to hear the noise more clearly. That’s one simple explanation for why you can hear sounds from the water while you’re on land.
But there’s another more intriguing explanation for this phenomenon of how sound travels, and why it seems to be amplified over water when you’re sitting in a boat offshore.
Everyone knows the expression “cooler by the lake.” You could be sweltering on a downtown street on a hot summer day, then pleasantly cooled if you head perhaps just a few blocks to the shores of a large body of water. In Toronto, especially in early spring, it could be 25C at Yonge and Dundas streets, and 15C at the harbourfront.
Water cools the air around it. And sound travels faster in warm air than it does in cold air. The speed sound travels through the air varies quite considerably between a temperature of 30C and a temperature of -25C. At 30, sound travels at 349 metres a second, while at -25, the speed is slowed to 315.8 metres a second.
Air temperature varies widely right overtop a large body of water, too. Think about sitting in a boat on Lake Ontario on a summer day. If you’re on deck, the air temperature could be 24C. But the water is still cold and two feet above the surface, it might have cooled the air to 21C. Right at the water’s surface, that air could be 18C.
The difference in the speed at which sound travels, then, is 346 metres a second up on deck, and 343 metres a second at the surface of the lake. So when the sound of your friend’s shout from the shoreline approaches the boat, the cooler air near the surface slows down the sound waves, causing those waves to bend downwards, or refract.
That means more sound is funelled downwards, increasing the level of sound that reaches your ears.
MyNewWaterfrontHome.com – July 2010