Waves: Sound Waves

Sound is a type of longitudinal wave, which means the movement of the wave is parallel to the direction of energy transfer.
These waves involve regions of compression and rarefaction. Compression is where particles are closest together and rarefaction is where particles are furthest apart.
The speed of sound varies in different materials. It is fastest in solids, slower in liquids, and slowest in gases.
The frequency of a sound wave determines the pitch of the sound. High frequency leads to a high pitched sound, while low frequency results in a low pitched sound.
The amplitude of a sound wave determines the volume of the sound. High amplitude waves produce loud sounds, while low amplitude waves produce quiet sounds.
Humans hear sounds in the range of 20Hz to 20,000Hz. Sounds below 20Hz are referred to as ‘infrasound’, while those above 20,000Hz are termed ‘ultrasound’.
Sound waves can reflect, refract, and diffract. Reflection of sound waves can lead to echoes, refraction happens when sound waves pass from one medium to another and diffusion occurs when sound spreads out through a gap or around a barrier.
The Doppler Effect describes how the frequency (and therefore the pitch) of a sound changes for an observer moving relative to the source of the sound.
Echolocation, used by animals such as bats and dolphins, and by technologies such as sonar, is an application of sound waves. It involves generating sound waves and then interpreting the echoes of these waves to locate objects.
Ultrasound waves have various applications in medicine and industry, including imaging (in an ultrasound scan) and detecting flaws in materials.
Noise-cancelling headphones use the principle of superposition and create sound waves that are the exact opposite (180 degrees out of phase) to unwanted sound to cancel it out.