Sound & Light

Sound

Sound is a longitudinal wave produced by a vibrating source; it travels as compressions and rarefactions and needs a medium (it cannot travel through a vacuum). It travels fastest in solids, slower in liquids, slowest in gases (about $340\ \text{m/s}$ in air). The human range of hearing is about 20 Hz to 20 000 Hz; sound above this is ultrasound (used in cleaning, sonar and medical scanning). A higher frequency gives a higher pitch; a larger amplitude gives a louder sound. An echo is reflected sound.