6. Sound

GURUKUL CLASSES ® Sound Dedication to Destination 16 9 th FOUNDATION PHYSICS INTRODUCTION In our day to day life we hear many sounds. Each sound is different from the other in its own way. Some are faint, some are loud, some are mild some are shrill, some are very weak. Each sound has its own speciality. We quickly identify the sound after a long time, even when that sound is heard only once before. Sound is a from of energy so it needs a source to be produced. PRODUCTION OF SOUND A common question which arises regarding sound is that how is it produced? What factors are responsible for production of sound? Let us consider the following examples: 1. Take a scale and hold it firmly on the bench. Flick the other end it. We see that sound is produced. Look at the end of the ruler. We see that the free end vibrates& produces some sound. We see that eventually the scale comes to rest. At that time it does not produce any sound. 2. Tie one end of the rubber string to a rigid support. Pull the other end of the rubber string. 3. Place your fingers against your throat & speak. What do you observe? In all the above cases we see that sound can be produced by plucking stretched rubber string, flicking a thin ruler etc. In all the above described activities, vibrations produced. "Thus, all vibrating bodies produce sound". Also different types of vibrations produce different sound. Some vibration are visible to our naked eyes. Others can just be felt. PROPAGATION OF SOUND We have learnt that vibrations produce sound. Thus we need a material (body) for producing vibrations. It can also be said that a medium is required for sound to travel from one point to another. Sound can travel through solids liquids & gases (or air). Sound can not travel through vacuum. "The matter/substance through which sound is transmitted is called the MEDIUM". Take the glass jar connected to a vaccum pump. Place an electric bell inside it. Switch on the bell. We will hear the sound. Now slowly suck out the air with the help of vaccum pump we will notice that sound fades slowly and eventually we can not hear any sound. This shows that sound travels through air, not through vacuum. Now we know sound needs a medium to travel. But the medium must be a material medium, elastic & continu- ous. It may be solid, liquid or gas. Now let us see how sound travels through any medium. Let us consider the following example. When any source produces sound the air closer to the source is pushed in the direction of sound. As a result of it, the molecules in this layer of air get disturbed and start vibrating about their mean positions. These vibrating molecules then disturb the adjacent molecules & the process continues until the molecules nearest to the listeners's ear start vibrating. These vibrating molecules then cause vibrations in the ear drum of the listeners ear. The vibrating diaphragm of the ear sends signal to the brain. Thus it is the disturbance, i.e. the vibratory motion of the molecules in the air, which travels from the source of sound to the listeners's ear & not the air. The movement of a disturbance produced in one part of a medium to another is called wave motion. Therefore, sound is a wave motion. It means sound propagates as a wave. Mechanism A vibrating string produces sound. During the vibration, the string moves from an extreme position to another around its mean position (Rest). Now let us have a closer look to the mechanism. 3. SOUND SYNOPSIS
GURUKUL CLASSES ® Sound Dedication to Destination 17 9 th FOUNDATION PHYSICS When the string moves from position (3) to (1)the air in front of the string gets compressed & a pulse of compression starts moving forward. 1 2 3 While returning from (1) to (3), the air behind it expands & a pulse of rarefraction strats moving forward. If the string continues to vibrate a series of compressions & rarefractions are created during the propagation of sound & longitudiual waves are produced. (a) undisturbed air (b) Compression (c) Compression Rarefraction (d) Compression Rarefraction Compression Direction of propagation of sound WAVES It is a common observation that ripples are formed when small object is dropped into a pond. The disturbance created by the stone in the water produces ripples which move toward the shore of the pond. If we observe the motion of a leaf floating near the disturbance for a short while, we would see that the leaf moves up & down about its original position, but does not move away with the disturbance. It shows that the disturbance moves from one place to another but the water is not carried with it the water particles simply move up & down about their mean positions. "A periodic disturbance produced in a material medium due to the vibratory motion of the particles of the medium is called a wave" The movement of disturbance produced in one part of a medium to another invloving the transfer of energy but not the transfer of matter is called wave motion. Characteristics of wave motion 1. Particles of medium vibrate about their mean positions. They don't move 2. It travels with same speed in all directions. 3. No transfer of matter takes place only energy is transferred. Mechanical waves: The waves which need a material medium for their propagation are called mechanical waves. Non-mechanical waves: The waves which does not need a material medium for their propagation. All waves: Mechanical & non-mechanical are classified into 2 types. a. Longitudinal wave b. Transverse wave
GURUKUL CLASSES ® Sound Dedication to Destination 18 9 th FOUNDATION PHYSICS Longitudinal A wave is which the particles of the medium oscillate (vibrate) to & fro about their mean position in the direction of propagation of wave is called a longitudinal wave. Particles of medium direction of propagation Sound waves are longitudinal waves as the air molecules oscillate about their mean position in the direc- tion of propagation of sound wave. The portions of the air in which the molecules are pulled apart & have a fewer amount of molecules per unit volume than the normal wave are known as rarefraction (R) . The portions of the air in which the molecules are pushed closer & have large no. of molecules per unit volume than the normal wave are known as compressions (C) . Compression Rarefraction Compression Rarefraction Rarefraction A compression has higher density of particles than normal wave where as rarefraction has a lower density of particles than normal wave. Longitudinal waves can be described graphically as: distance C C C C R R R density Normal density Transverse wave A wave in which the particles of the medium oscillate about their mean position perpendicular to the direction of the propagation of the wave is called a transverse wave. Transverse waves can not travel through gases. For example: Ripples on water surface. T The point of maximum positive displacement on a transverse wave is called a crest. It is the highest point on the hump in the transverse wave. The point of maximum negative displacement on a transverse wave is called a trough. It is the lowest point on the dipression in a transverse wave. (A) Phase: All points on a wave which are in the same state of vibration are said to be in the same phase. D A B C E F G H I J K (1) A,E,I (2) B,F,J (3) C,G,K (4) D,H
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