Sound & Doppler Effect — A/L Physics

Sound as a Longitudinal Wave Sound is a mechanical wave that propagates through a medium (solid, liquid, or gas) via compressions and rarefactions of particles. The particles oscillate parallel to the direction of wave travel, creating regions of high pressure (compressions) and low pressure (rarefactions). Unlike transverse waves, sound cannot travel through a vacuum. For example, a tuning fork vibrates, pushing air molecules together and apart, transmitting energy to the ear.

Speed of Sound in a Medium The speed of sound depends on the elasticity and density of the medium, given by v = √(E)/(ρ) , where E is the bulk modulus (for fluids) or Young’s modulus (for solids), and ρ is density. In air at 20°C, v ≈ 343 m/s . Sound travels faster in solids (e.g., steel ~5000 m/s) than in liquids (e.g., water ~1500 m/s) because solids have higher elasticity. Temperature increases speed in gases: v ∝ √T . For example, sound travels faster on a hot day than a cold day.