P3.3 Sound and waves · KS3 Science

Waves

Waves transfer energy (and information) without transferring matter.
In a transverse wave, the oscillations are at right angles to the direction the energy travels: light, water waves and all electromagnetic waves.
In a longitudinal wave, the oscillations are parallel to the direction the energy travels, making compressions and rarefactions: sound is longitudinal.
Amplitude is the maximum displacement from the rest position. Wavelength is the distance from one point on a wave to the same point on the next (crest to crest). Frequency is the number of waves each second, in hertz (Hz). The period is the time for one wave: period = 1 ÷ frequency.

Wave speed = frequency × wavelength (v = f × λ), in m/s.
Sound travels at about 340 m/s in air, faster in liquids and faster still in solids. It can't travel through a vacuum, because there are no particles to vibrate.
Light travels at 300 000 000 m/s (3 × 10⁸ m/s).

Sound is made by vibrations. A higher frequency gives a higher pitch; a bigger amplitude gives a louder sound.
Humans can hear from about 20 Hz to 20 000 Hz. Ultrasound has a frequency above 20 000 Hz.
In the ear, the eardrum vibrates, the tiny bones pass the vibrations to the cochlea, and nerves carry signals to the brain.
An oscilloscope shows sound waves as traces on a screen.

Waves can be reflected (an echo is a reflected sound), refracted, absorbed or transmitted.
Refraction happens when a wave changes speed as it enters a different material: water waves slow down and their wavelength gets shorter in shallow water.
Ultrasound is used for scans of unborn babies and for sonar. Distance = speed × time ÷ 2, because the pulse travels there and back.
Seismic waves from earthquakes tell scientists about the inside of the Earth.
Derived units are made from other units: m/s comes from metres and seconds.

Wave: A way of transferring energy without transferring matter.
Transverse wave: A wave whose oscillations are at right angles to the direction of energy transfer.
Longitudinal wave: A wave whose oscillations are parallel to the direction of energy transfer.
Amplitude: The maximum displacement of a wave from its rest position.
Wavelength: The distance from one point on a wave to the same point on the next wave.
Frequency: The number of waves each second, measured in hertz (Hz).
Period: The time taken for one complete wave.
Wave speed: How fast a wave travels: frequency × wavelength.
Compression: A region in a longitudinal wave where particles are close together.
Rarefaction: A region in a longitudinal wave where particles are spread out.
Echo: A reflected sound.
Ultrasound: Sound with a frequency above 20 000 Hz, too high for humans to hear.
Pitch: How high or low a sound is.
Vacuum: A space with no particles in it.