🔊 Sound – Class 9 Physics

Sound

A form of energy produced by vibrating bodies

Vibration

To and fro motion of an object producing sound

Medium

Material through which sound travels (solid

Mechanical wave

A wave that requires a medium for propagation

Longitudinal wave

Wave in which particles vibrate parallel to wave direction

Compression

Region of high pressure and particle density in a sound wave

Rarefaction

Region of low pressure and particle density in a sound wave

Propagation of sound

Due to compressions and rarefactions in a medium

Speed of sound in air

343 m/s at 25°C

Speed of sound in water

~1498 m/s

Speed of sound in solids

~5000 m/s (faster than in air and water)

Speed of sound formula

v = f × λ (speed = frequency × wavelength)

Frequency

Number of vibrations per second (f)

SI unit of frequency

Hertz (Hz)

Time period

Time taken for one complete vibration

Time period formula

T = 1 / f

Amplitude

Maximum displacement of particles from rest

Wavelength (λ)

Distance between two consecutive compressions or rarefactions

SI unit of wavelength

Metre (m)

Audible range

20 Hz to 20

Infrasonic sound

Sound below 20 Hz

Ultrasonic sound

Sound above 20

Examples of ultrasonic use

SONAR

Loudness

Depends on amplitude of vibration

Pitch

Depends on frequency; higher frequency = higher pitch

Quality of sound

Characteristic that distinguishes different sounds of same pitch and loudness

Reflection of sound

Bouncing back of sound from a surface

Echo

Reflected sound heard after 0.1 second

Echo condition

Distance between source and reflector ≥ 17.2 m

Reverberation

Repeated reflection of sound causing