ALP Stage-2 exam: Physics notes- SOUND (PART 1)

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  • Sound is a mechanical energy which produces sensation of hearing. Sound is produced due to vibration of different objects.
  • Sound wave propagates as compressions & rarefactions in the medium. Sound waves are longitudinal waves.

Production of Sound: Sound is produced by vibrating objects. Vibration means a kind of rapid to and fro motion of an object. The sound of the human voice is produced due to vibrations in the vocal cords.

Propagation of Sound: The matter or substance through which sound is transmitted is called a medium. It can be solid, liquid or gas. Sound moves through a medium from the point of generation to the listener. Sound waves are produced due to variations in pressure & density of the medium.

On the basis of direction of propagation, waves can be divided into 2 types:

LONGITUDINAL WAVES: In these waves the individual particles of the medium move in a direction parallel to the direction of propagation of the disturbance. The particles do not move from one place to another but they simply oscillate back and forth about their position of rest. E.g. Sound waves.

TRANSVERSE WAVES: In these waves, particles do not oscillate along the line of wave propagation but oscillate up and down about their mean position as the wave travels. E.g. Light is a transverse wave.


Compression(C): These are the regions of high pressure and density where the particles are crowded and are represented by the upper portion or peak of the curve called crest.

Rare-factions(R): These are the regions of low pressure and density where the particles are spread out and are represented by the lower portion of the curve called trough or valleys.

Amplitude: The magnitude of the maximum disturbance in the medium on either side of the mean value is called the amplitude of the wave. It is usually represented by the letter A. For sound its unit will be that of density or pressure.

Oscillation: It is the change in density (or pressure) from maximum value to the minimum value and again to the maximum value.

Frequency: The number of oscillations of a wave per unit time is the frequency of the sound wave. It is usually represented by ν (Greek letter, nu). Its SI unit is hertz (symbol, Hz).

  • Larger the amplitude of vibration, louder is the sound. 
  • Higher the frequency of vibration, the higher is the pitch, and shriller is the sound.

Time Period: The time taken by two consecutive compressions or rare-factions to cross a fixed point is called the time period of the wave. It is represented by the symbol T. Its SI unit is second (s).  

Time Period = 1/ Frequency

Wavelength: It is the distance between two consecutive compressions or two consecutive rare-factions. The wavelength is usually represented by λ (Greek letter lambda). Its SI unit is metre (m)

The speed of sound: It is defined as the distance which a point on a wave, such as a compression or a rarefaction, travels per unit time.

Speed = wavelength × frequency

Example: A sound wave has a frequency of 2 kHz and wave length 35 cm. How long will it take to travel 1.5 km?
Frequency, ν = 2 kHz = 2000 Hz
Wavelength, λ = 35 cm = 0.35 m
Speed of the wave = wavelength × frequency
v = λ ν = 0.35 m × 2000 Hz = 700 m/s
The time taken by the wave to travel a distance, d of 1.5 km is
1500/700 = 2.1 s
Thus, sound will take 2.1 s to travel a distance of 1.5 km.

Range of Hearing of sound: The audible range of sound for human beings extends from about 20 Hz to 20000 Hz (one Hz = one cycle/s).

  • Sounds of frequencies below 20 Hz are called infrasonic sound or infra sound. Rhinoceroses communicate using infrasound of frequency as low as 5 Hz. Whales and elephants produce sound in the infrasound range.
  • Frequencies higher than 20 kHz are called ultrasonic sound or ultrasound. Ultrasound is produced by dolphins, bats and porpoises.

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