Physical mechanism of sound: Air is compressed by a pulse, e.g. from a speaker.
This creates areas of compression within an air column.

Repeated pulses create a wave.

Sound is longitudinal wave

Air is carrier of sound wave.

Without air, there is no sound.

Wavelength of Sound Waves

Consider sound waves with speed

v = 344 m/s

• a) 20 Hz

• b) 20,000 Hz

a)

b)

Audible sound frequencies

Range from 20 Hz to 20,000 Hz (=20 kHz)

f < 20 Hz : infra sonic

f > 20 kHz : ultra sonic

Ultra sound: Many applications

• medical
• navigation
• testing of materials