## 06 December 2011

### IX Physics Ch. Sound as a wave part-1

Sound is a wave caused by the movement of particles that travels through air or water similar to the ripples on a pond or the ocean waves you might see on a beach.

A wave can be described as a disturbance that travels through a medium from one location to another location or, The repeating and periodic disturbance which moves through a medium from one location to another is called a wave.

There are two types of waves: Mechanical waves and electromagnetic waves

Electromagnetic waves are waves traveling through a vacuum and  do not require a medium in order to transport their energy(propagation) e.g. Light

Mechanical waves are waves which require a medium for propagation or to transport their energy from one point to another. It is of two types.

Longitudinal wave :  A  mechanical waves in which the particles and the energy move in the same direction  parallel to the direction in which the wave is travelling.

A sound wave traveling through air is example of longitudinal waves because particles of the medium through which the sound is transported vibrate parallel to the direction which the sound wave moves.
Compressions are the regions where particles are crowded together. The peak represents the region of maximum compression. Thus, compressions are regions where density as well as pressure is high.

Rarefactions are the regions of low pressure where particles are spread apart.

The distance between two consecutive compressions (C) or two consecutive rarefactions (R) is called the wavelength

The wavelength is usually represented by λ (Greek letter lambda). Its SI unit is metre (m).

Transverse wave: A wave in which particles of the medium move in a direction perpendicular to the direction which the wave is travelling. The particles of medium vibrate up-and-down and the energy moves left-and-right. e.g. Water waves on the ocean surface.
A peak is called the crest and a valley is called the trough of a wave.

Propagation of Sound

The matter or substance through which sound is transmitted is called a medium. It can be solid, liquid or gas.

How does the sound produced by a vibrating object in a medium reach your ear?

When an object vibrates, it sets the particles of the medium to vibrate. It then exerts a force on the adjacent particle and displace from its position of rest. This process continues in the medium till the sound reaches your ear.

Why sound waves are called mechanical waves?

In the propagation of sound the particles of the medium do not move forward themselves, but the disturbance is carried forward through medium. Hence,  sound is a form of a wave. As it needs material medium for propagation it is a mechanical wave.

Air is the most common medium through which sound travels. When a vibrating object moves forward, it pushes and compresses the air in front of it creating a region of high pressure. This region is called a compression

This compression  starts to move away from the vibrating object. When the vibrating object moves backwards, it creates a region of low pressure called rarefaction (R)

Hence , a  series of compressions and rarefactions is created in the air make the sound wave propagates through the medium.

Explain how sound is produced by school bell.

When school bell is hit by a hammer, the bell continues to move forward and backward, that creates a series of compressions and rarefactions. This produces the sound of a bell.

Suppose you and your friend are on the moon. Will you be able to hear any sound produced by your friend?

Sound needs a medium to propagate. Since there is no material medium on the moon due to absence of atmosphere, you cannot hear any sound on the moon.

In which of the three media, air, water or iron, does sound travel the fastest at a particular temperature?

The speed of sound depends on the nature of the medium. Sound travels the fastest in solids. Its speed decreases in liquids and it is the slowest in gases.  Therefore, for a given temperature, sound travels fastest in iron.

How a sound wave propagates?

When sound wave propagates the particles do not move from one place to another but they simply oscillate back and forth about their position of rest. The individual particles of the medium move in a direction parallel to the direction of propagation of the disturbance. A sound wave traveling through air is example of longitudinal waves

We can describe a sound wave by its properties

(a) Frequency (b) amplitude and   (c) speed.

The number of such oscillations per unit time is the frequency of the sound wave. If we can count the number of the compressions or rarefactions that cross us per unit time, we will get the frequency of the sound wave. It is usually represented by n .Its SI unit is hertz (symbol, Hz).

The time taken for one oscillation is called time period. It is represented by the symbol  T. Its SI unit is second (s).

Frequency and time period are related as follows:  n=1/T

Pitch:  How the brain interprets the frequency of an emitted sound is called the pitch. The faster the vibration of the source, the higher is the frequency and the higher is the pitch.

The amplitude of the sound : 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. The amplitude of the sound wave depends upon the force with which an object is made to vibrate

The quality or timber of sound is that characteristic which enables us to distinguish one sound from another having the same pitch and loudness. The sound which is more pleasant is said to be of a rich quality.

Notes and Tone: A sound of single frequency is called a tone.  The sound which is produced due to a mixture of several frequencies is called a note

Noise and Music :  The sound which is  pleasant to listen is called music .  The sound which is unpleasant to listen is called noise .

The speed of sound:  The distance (a compression or a rarefaction) travels per unit time is called the speed of sound.
speed, v = distance / time = wave length/ time =  l/ T = l x (1/t)

or,   v = nl ( as n=1/T)

Speed = wavelength × frequency