Sound is one of the most important ways ships and submarines sense the ocean because light is quickly absorbed and scattered underwater. In seawater, sound usually travels about 1500 m/s, which is more than four times faster than in air at room temperature. It can also travel much farther in water because water particles are close together and pass vibrations efficiently.
This makes sonar useful for navigation, mapping the seafloor, finding objects, and communicating underwater.
Sound speed in the ocean is not constant because seawater changes with temperature, pressure, and salinity. Warmer water generally increases sound speed, higher pressure at greater depth increases sound speed, and saltier water increases sound speed slightly. These changes bend sound waves, a process called refraction, so sonar paths can curve through layered water.
In some ocean regions, sound can become trapped in a deep sound channel and travel for hundreds or even thousands of kilometers.
Key Facts
- Typical sound speed in air at 20°C is about 343 m/s.
- Typical sound speed in seawater is about 1500 m/s.
- Distance = speed × time, so d = vt.
- Sonar distance using an echo is distance = sound speed × echo time / 2.
- Sound speed in seawater increases when temperature, pressure, or salinity increases.
- Changing sound speed with depth causes refraction, which bends sound waves toward slower sound-speed regions.
Vocabulary
- Sonar
- Sonar is a system that uses sound waves to detect, locate, or communicate with objects underwater.
- Sound speed
- Sound speed is the rate at which a sound wave travels through a material.
- Refraction
- Refraction is the bending of a wave when its speed changes as it enters a different region or layer.
- Salinity
- Salinity is the amount of dissolved salt in water, usually measured in parts per thousand.
- Sound channel
- A sound channel is a layer of the ocean where sound waves can be guided over long distances by refraction.
Common Mistakes to Avoid
- Using the speed of sound in air for underwater problems. This is wrong because sound travels much faster in seawater, about 1500 m/s instead of about 343 m/s.
- Forgetting to divide echo time by 2 in sonar distance problems. The measured time is for the sound to travel to the object and back, so the one-way distance is half the total travel distance.
- Thinking deeper water always makes sound slower. This is wrong because increasing pressure with depth tends to increase sound speed, although temperature changes can also affect the result.
- Assuming sound waves always travel in straight lines underwater. This is wrong because layers with different temperature, pressure, and salinity can refract sound and curve its path.
Practice Questions
- 1 A submarine sends a sonar pulse and receives the echo 4.0 s later. If the sound speed in seawater is 1500 m/s, how far away is the object?
- 2 A sound travels 12,000 m through seawater at 1500 m/s. How long does the trip take?
- 3 A sonar wave passes from warm surface water into colder water below, where sound speed is lower. Explain why the wave bends and how this can affect what a ship or submarine detects.