SOUND Sound is a form of energy that travels by longitudinal mechanical waves.
Wave Nature of Sound
Sound undergoes four out of five of the phenomena of waves: reflection, refraction, diffraction and interference, but cannot be polarised. It also needs a medium to travel through.
Reflection: An echo is when a sound wave is reflected off a surface.
Singers at an opera Warm day
Refraction: Sound travels at different speeds in different media, and also in different temperatures of air. Sound travels faster in warm air. This causes refraction of sound in a similar manner to how light refracting can cause mirages. Because of refraction of sound it is possible to hear noises from greater distances at night.
Cold air—sound travels slowly Cold night Warm air—sound travels fast
Warm air—sound travels fast (a)
Cold air—sound travels slowly (b)
Fig 11.1: Refraction of sound in air: (a) sound is refracted upwards and (b) sound is refracted downwards Diffraction:
Sound can be diffracted as it goes through a doorway because its wavelength is in the order of 1 m.
Interference: The fact that sound undergoes interference can be used to prove it is a wave motion.
