How sound travels under water

Use this animated video to see 3 things needed for sound to be heard. We’re heading under water to see how the sound of a kina¹ travels.

Transcript

Reef animals generate vibrations and these vibrations cause sound to travel trough the sea. Kina is one such reef animal. It likes to eat algae² that grow on the reef’s rocky surface.

Kina make sharp scraping noises when their teeth grate on the reef rocks. This is amplified³ as the vibrations resonate within the Kina shell.

This scraping sound is emitted from Kina in all directions as a series of spherical pressure waves⁴ that travel at speeds around 1500 m/s through the water. As the sound waves spread out they get weaker because the energy in each wave is spread over a greater area.

Lines of maximum pressure⁵ are drawn in the diagram. The distance between these lines is called the wavelength⁶. Higher pitched sounds have waves closer together with smaller wavelengths. Only three of the sound waves are drawn to make things simple. A real scraping sound would produce a hundred or more waves with a range of wavelengths.

There is little reflection⁷ off small crustacean larvae⁸ that are smaller than the sound's wavelength.

However, bigger fish that are larger than the sound’s wavelength reflect the sound in various directions.

Very little sound is transmitted into the air when the waves reach the surface because the difference in speeds of sound between seawater and air is so great. Instead, the sound waves reflect back into the sea.

Of course there are many more sounds sent out from a reef. We have concentrated on one kind of sound from only one Kina to simplify our explanation of how sound travels. Small crustacean larvae use the whole collection of sounds from the reef to guide them towards the reef where they settle and progress to their next stage of growth.