Sound : class 9th notes

Sound

Sound : A class 9th notes

Wave :- A disturbance that travels through a medium from one location to another without any net transportation of the medium. 

Medium :- a medium is a substance or material which carries the wave. 

• A wave medium is the substance which carries a wave from one location to another.
• A wave  transports energy not the matter 

A disturbance moves through a medium from one particle to its adjacent particle, energy is being transported from one end of the medium to the other.

Classification of wave 

(a) Depending on the number of dimensions in which a wave can propagate. 

• one-dimensional waves:- the waves in which disturbance is propagated along one dimension only. 

Example :- string waves

• (two-dimensional waves :- the wave in which disturbance is propagated along two dimension. 

Example :- water waves (ripples) 

• three-dimensional waves :- the wave in which disturbance is propagated along three dimensions.

Example :- sound waves

(b) Depending on the basis of whether waves requires a medium or not for its propagation:- 

• non mechanical wave :- A non-mechanical wave is a wave which is capable of transmitting energy through a vacuum and not require any medium for its propagation.

Example :- light waves, electromagnetic waves.

• mechanical wave :- mechanical wave is a wave which is not capable of transmitting its energy through vacuum. 

mechanical waves require a medium in order to transport their energy from one place to another.
Example :- sound waves.

(c) Depending on the direction in which the medium particles oscillate with respect to direction of wave propagation.

• transverse wave :- a transverse wave is a wave in which particles of the medium oscillate in the direction perpendicular to the direction in which the wave propagate. 

Example :-  string waves

Transverse wave travel in the form of crest and trough.

(ii) longitudinal wave :- a longitudinal wave is a wave in which particles of the medium move in the direction parallel to the direction in which the wave propagates.

• longitudinal waves travel in the form of a series of compressions and rarefactions.
• wave travelling through a solid medium can be either transverse wave or longitudinal waves.
• wave travelling through the bulk of a liquid (liquid or gas) are always longitudinal waves.
• transverse waves require a relatively rigid medium in order to transmit their energy.
• only longitudinal waves are observed moving through the bulk of liquids.

Transverse wave 

A transverse wave is a wave in which the particles of the medium are displaced in a direction perpendicular to the energy transported.

Crest = D.    

Trough = H. 

Crest :- the point on the medium which exhibits the maximum amount of positive or upward displacement from the rest position.

Trough :- the point on the medium which exhibits the maximum amount of negative or downward displacement from the rest position.

Amplitude :- the maximum amount of displacement of a particle on the medium from its rest position.

• The distance from the rest position to crest.
• The distance from the rest position to trough.

Wavelength (λ) :- The length of one complete wave cycle. 

The distance between two consecutive crest or through.

The distance from a point on the wave to the corresponding point to the next cycle of the wave.

Wave-length = BI = GZ = λ

Longitudinal wave :-

A longitudinal wave is a wave in which the particle of medium are displaced in a direction parallel to the direction of energy transported.

Compression :- a point on a medium through which a longitudinal wave is travelling which has the maximum density. 

Rarefaction :- a point on a medium through which a longitudinal wave is travelling which have the maximum density.

Frequency and time period :- 

Frequency :- the number of complete vibrational cycles of a medium for a given amount of time. 

Or

The number of complete vibrational cycles is produced in one second is called frequency.

Let in time "t" , "n", waves are produced

Hence, in time 1 sec = n/t waves are produced

f = n/t

Units :- 

Cycle/second

Waves/second

Vibration/second

Time period :- the time taken by a wave to complete one oscillation. 

It is denoted by "T" 

If n = 1 

t = T 

Then, f = 1/T

SI unit = second-¹ = Hertz (Hz)

Relation between frequency and time period :- 

f = 1/T

Relation among velocity, frequency and wavelength of the wave :- 

T = time-period

λ = wavelength

V = velocity

Picture 🖼️  = frequency 

Velocity = distance/ time 

V = λ / T

V = 1/T × λ 

V = f λ

Characteristics of wave motion :- 

→ Wave motion is a short of distrubance which travel through a medium 

→ A material medium is essential for the propagation of mechanical waves. The medium must possess three properties amongst the particles of the medium.

• Elasticity

• inertia

• minimum friction

Elasticity :- the medium must have elasticity show that particles return to their mean position after displacement from mean position.

Inertia :- the medium must have inertia so that its particle have the capacity to store energy.

Frictionless :- the medium must have frictionless so there is no loss of energy in propagation of sound through medium.

→when a wave motion passed through a medium, particles of the medium vibrates simple harmonically about their mean position. They don't leave their position and move with the disturbance. 

→There is a continuous phase difference amongst successive particle of the medium. 

Or

→Particle 2 starts vibration slightly later than particle 1 and so on.

→the velocity of the particles during their vibration is different at different positions.

Example:- all the particles cross their mean position with maximum velocity and at extreme positions, their velocity is zero. 

→The velocity of wave motion through a particular medium is constant. It depends only on the nature of the medium. The velocity of wave motion does not depend upon its frequency or wavelength or intensity.

→Energy is propagated along with the wave motion without any net transport of the medium.

Audible or sound waves :- 

The range of frequency is from 20-20000 Hz

Infrasonic wave :- the wave whose frequency is lower than 20 Hz 

Example :- earthquake (seismic wave)

Ultrasonic wave :- the wave whose frequency is higher than 20 kHz 

Example :- certain creature such as dog mosquito and bat.

Supersonic body :- when an object moves with a velocity greater than that of sound is called supersonic body. 

Shockwave :- when a supersonic body travels in air it produces energetic disturbance which moves in the backward direction and diverse in the form of cone such disturbance are called shockwave.

Sonic boom :- Sonic bombe is the sound associated with enormous amounts of sound energy similar to the explosion.

Picture 🖼️ 

Puls :- a disturbance lasting for a short duration called puls. 

Picture 🖼️