Contains the full lesson along with a supporting toolkit, including teachers’ notes.
For thousands of years people have noticed that sounds produced at a distance take time to travel. Thunder is usually heard several seconds after the lightning flash that caused it. For every kilometre it has to travel, sound is delayed by three seconds. But what exactly is sound? Is it a thing or substance? What actually travels from the source to the hearer? Does it travel at a fixed speed? Can sound travel through water? Can sound be reflected? This lesson will answer some of these questions and may help you to re-examine your concept of sound. It will also illustrate how sound can be used to construct detailed images of the seabed and help determine its composition.
What exactly is sound?
Waves on water have the following properties: they can travel, they can carry energy from one place to another and they generally have a regular repeating pattern of oscillation and a wavelength. Some other properties are less obvious; waves can add together, they can cancel out one another, they can bend around corners and their speed can change, depending on the medium.
More than 2000 years ago some Greek philosophers speculated that the propagation of sound was similar to the propagation of waves on water. In the 1600s Mersenne was to first to measure the frequency of a note and correctly interpreted how notes whose frequencies were in simple ratios produced harmony.
Robert Boyle showed that sound could not travel through a vacuum; it needed a medium. In 1686 Newton proposed that sound was a pressure disturbance that was propagated through the air or another medium.
Because sound exhibits many of the properties that are associated with water waves, over the last 300 years sound has been described in terms of waves.
Sound waves are not material things; they do not have an independent existence. They are merely disturbances of a medium (air, water etc.). Although a water wave can travel, the water itself does not – it just moves up and down; what travels is the disturbance.
How is sound used for scanning and measuring?
The speed of sound depends on the medium; in air at sea-level it is about 330 m/s, in seawater it is about 1500 m/s, increasing with depth and temperature. Sound emitted by a source, such as a loudspeaker, bounces off surfaces; the reflected sounds or echoes can be detected by one or more microphones.