Sound Seems Amplified Over Water

by Ron Kurtus

If you are sitting in a boat, a sound coming from the shore will seem louder than the same sound heard by a person on land. Sound seems to be amplified when it travels over water.

The reason is that the water cools the air above its surface, which then slows down the sound waves near the surface. This causes refraction or bending of the sound wave, such that more sound reaches the boat passenger.

Sound waves skimming the surface of the water can add to the amplification effect, if the water is calm.

Questions you may have include:

• Why does sound lose its loudness with distance?
• What happens to the speed of sound with temperature?
• How does the amplitude increase?

This lesson will answer those questions. Useful tool: Units Conversion

Amplitude decreases with distance

As any waveform expands or spreads out from its source, its amplitude decreases. You can see this with water waves in a pond. With sound, the amplitude of the waves relates to the loudness of the sound that we perceive.

Amplitude of waves decreasing as they spread

Speed and temperature

When the air temperature is 24° C (75° F), the speed of sound is 346 meters per second (m/s) or 775 miles per hour (mph). The relationship between the speed of sound and the air temperature is approximated from the equation:

v = 331.4 + 0.6Tc m/s

where Tc is the Celsius temperature.

If the water was at 15.5° C (60° F), the air just above the surface would be close to the same temperature. The air temperature would then increase at further distances from the water until it reached the normal air temperature.

The follow chart is the air temperature and speed of sound at different heights above the water.

miles/hour

91.5 cm (3 ft) or more 24 75 346 775
61 cm (2 ft) 21 70 344 771
30.5 cm (1 ft) 18.3 65 343 767
Near water level 15.5 60 341 764

Speed of sound decreases closer to cooler water

Effect of refraction and reflection

Refraction of the sound waves in cooler air and the reflection off the surface of the water effect the sound that is heard.

Waves bent by refraction

When a wave strikes a material in which it travels slower, its direction is changed slightly if it strikes the material at an angle. This effect is called refraction. You have seen refraction when light passes through a pane of glass at an angle. The image is displaced, because the light rays were refracted.

(See Refraction of Light for more information on this subject.)

The same thing happens when sound enters a material in which its speed is slower than in normal air. The direction of the sound waves change slightly.

Since the temperature of the water in a lake or ocean is usually cooler than the normal air temperature, the air just above the water level is cooled by the water. The temperature varies according to the distance from the surface of the water. This gradient of speeds would result in a lens effect due to refraction of sound. That means sound would tend to focus and thus increase its apparent loudness.

Cool air bends sound and thus increases amplitude

It is a strange effect that follows the principles of sound and wave motion.

Reflection off surface of water

If the water is smooth or calm, the sound waves skim the surface of the water and are reflected toward the observer in the boat, adding to the amplification. However, if the water is choppy, the sound is randomly reflected and make no contribution to the amplitude of the sound.

Summary

A sound coming from the shore will sound louder to a person sitting in a boat in the water than the same sound heard by a person on land.

Sound seems to be amplified when it travels over water, because the water cools the air above its surface. Cool air slows down the sound waves near the surface, causing refraction or bending of the sound wave. Then more sound reaches the boat passenger.

Sound waves skimming the surface of the water can add to the amplification effect, if the water is calm.

Know the principles of science

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Speed of Sound - Hyperphysics site

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