ICSE Class 10 Physics Chapter 07 Sound

7 Sound

Theme

In the previous classes children were made aware about and enabled to understand that a sound wave is characterised by its frequency and amplitude. Parameters that focus on loudness and pitch, and are commonly used to characterise sounds produced by different sources, were also highlighted. The loudness depends on the amplitude, hence when the amplitude of sound is large, sound is loud. Pitch is related to the frequency so when the frequency is high, the pitch is high or the sound is shrill. In this class the theme focuses on showing how sound produced by different musical instruments have different pitch and loudness.

In this chapter you will learn to

Relate pitch and frequency;

Understand pitch and frequency in relation to working of musical instruments (wind, membrane and string);

Explain mono tone;

Relate loudness and amplitude;

State the unit of loudness in decibels.

Learning Objectives

Revising previous concepts learnt by children,

Building on children's previous learning.

Explaining terms related to pitch and frequency.

Demonstrating the relation between pitch and frequency.

Demonstration of pitch and frequency of some common musical instruments.

Demonstrating monotone sound.

Knowing Concepts

Pitch and frequency.

Pitch and frequency in relation to working of musical instruments (wind, membrane and string).

Monotone.

Loudness and amplitude.

Unit of loudness decibels.

Introduction

In class VII, we have read that sound is a form of energy which produces a sensation of hearing in our ears. Sound is produced when a body vibrates. Thus, each source of sound is a vibrating body.

For example, take a rubber band and cut it to get a string. Now stretch the string, keeping its one end in your mouth under the teeth and other end in your hand as shown in Fig. 7.1. When you pluck the string from the middle, you see that the string starts vibrating and a feeble sound is heard.

Sound needs a medium for its propagation. It can not travel in vacuum. This is why two persons can not hear each other on the moon or in space where there is no atmosphere. Sound can travel in solids, in liquids as well as in gases. Its speed is more in solids, less in liquids and still less in gases. The speed of sound in iron is nearly 5000 m s-1, in water it is nearly 1500 m s-1 and in air it is nearly 330 m s-1.

When a body vibrates, the particles of the medium also start vibrating. During vibrations, the kinetic energy of particles changes into potential energy and potential energy into kinetic energy. This is why sound is a form of energy.

Propagation of Sound in Air

When a source of sound vibrates, it creates a periodic disturbance in the medium near it (i.e., the condition of medium changes). The disturbance then travels in the medium in form of waves. This can be understood by the following example.

Example: Take a vertical metal strip with its lower end fixed. Push its upper end to one side and then release it. As it vibrates, i.e. moves alternately to the right and left, producing sound. Fig. 7.2(a) shows the steady (or mean position) of the metal strip and normal condition of air layers near the strip.

As the strip moves to the right from a to b in Fig. 7.2(b), it pushes the particles of air layer in front of it. So, the particles of air in this layer come closer to each other i.e., air in that layer gets compressed (or compression is formed at C). The particles of this layer while moving towards right, pushes and compresses the layer next to it, which then compresses the next layer and so on. Thus, the disturbance moves forward in form of compression.

As the strip starts returning from b to a in Fig. 7.2(c) after pushing the particles near the strip, the compression C moves forward and the particles of air near the strip return back to their normal positions due to the elasticity of the medium.

When the strip moves to the left from a to c in Fig. 7.2(d), it pulls the layer of air near it towards left and thus produces a space of very low pressure on its right side. The air layers on the right side of the strip expand in this region thus forming the rarefied layers. This region of low pressure is called a rarefaction R.

By the time the strip returns from c to its mean position a in Fig. 7.2(e), the rarefaction R moves forward and air layers near the strip return back to their normal position due to the elasticity of the medium.

In this manner, as the strip moves to the right and left repeatedly, the compression and rarefaction regions are produced one after the other, which carry the disturbance along it with a definite speed depending on the nature of the medium.

One complete to and fro motion of the strip forms one compression and one rarefaction which together constitute one wave. This wave in which the particles of the medium vibrate about their mean positions, in the direction of propagation of sound, is called longitudinal wave. Thus, sound travels in air in form of longitudinal waves. These longitudinal waves can be produced in solids, in liquids as well as in gases.

Terms Related to a Wave

Amplitude: The maximum displacement of the particle of medium on either side of its mean position, is called the amplitude of wave. It is denoted by the letter a. Its S.I. unit is metre (m).

Time period: The time taken by a particle of medium to complete its one vibration is called the time period of the wave. It is denoted by the letter T. Its S.I. unit is second (s).

Frequency: The number of vibrations produced by a particle of the medium in one second is called the frequency of the wave. It is also defined as the number of waves passing through a point in one second. It is denoted by the letter f. Its S.I. unit is second-1 or hertz (symbol Hz).

The frequency of a wave is equal to the frequency of vibrations of its source. It is the characteristic of its source which produces the sound. It does not depend on the amplitude of vibration or on the nature of medium through which the wave propagates.

Relationship between time period (T) and frequency (f): If T is the time period of a wave, then by definition

In time T, the number of waves = 1

In 1 second, number of waves (or frequency) \(f = \frac{1}{T}\)

Thus, frequency = \(\frac{1}{\text{Time period}}\)

or time period = \(\frac{1}{\text{frequency}}\)

Wavelength: The distance travelled by the wave in one time period of vibration of particle of medium is called its wavelength. It is denoted by the letter \(\lambda\) (lambda). Its S.I. unit is metre (m). It depends on the nature of medium through which the wave travels.

In a longitudinal wave, the distance between two consecutive compressions or between two consecutive rarefactions is equal to one wavelength (\(\lambda\)).

Representation of a Wave

A wave while propagating in a medium can be represented by the following two graphs:

Displacement-time graph, and

Displacement-distance graph.

Displacement-time graph: Fig. 7.3 shows the variation of displacement of a particle of the medium with time at a given position, when a wave propagates through the medium. It is called displacement-time graph. The amplitude is represented by the letter a and time period is represented by the letter T in Fig. 7.3.

In Fig. 7.3, the amplitude of wave is 2 cm and its time period is 4 s (i.e. frequency is 0-25 Hz).

Displacement-distance graph: Fig. 7.4 shows the variation of displacement of particles of the medium at different position with distance at the same time. Here amplitude of wave is shown by the letter a and wavelength is shown by the letter \(\lambda\).

For example in Fig. 7.4, amplitude of wave is 2 cm and its wavelength is 2 m.

In a longitudinal wave, in Fig. 7.3 and 7.4, the displacement on + Y axis shows the motion of medium particles in the direction of propagation of wave while the displacement on - Y axis shows the motion of medium particles in direction opposite to the propagation of sound.

If the particles of medium vibrate normal to the direction of propagation of wave, the wave is called transverse wave.

Sound waves produced in strings are transverse waves.

Characteristics of Sound

A sound wave is characterized by its amplitude and frequency. Depending upon the amplitude and frequency of the sound wave, two sounds can be distinguished from one another by the following three different characteristics:

Loudness,

Pitch (or shrillness), and

Quality (or timbre or wave form).

The above characteristics of a given sound can be known from the wave pattern of that sound.

Loudness:

Loudness is the characteristic of sound by virtue of which a loud sound can be distinguished from a faint sound, both having the same frequency and same wave form.

The loudness of a sound depends on the amplitude of vibration of the vibrating body producing sound. Greater the amplitude of vibrations, louder is the sound produced.

Fig. 7.5 shows two waves A and B of same frequency and same wave form, but the amplitude of wave A is 2 m while that of the wave B is 4 m. Thus, the amplitude of wave B is greater than that of A, hence the sound B is louder than sound A.

Examples: (i) If you gently pluck the string of a sitar or guitar, a soft (or faint) sound is heard. But if you pluck the string hard, it gets displaced more from its rest position i.e., its amplitude of vibration increases and so a loud sound is heard.

(ii) If you strike the drum gently, a faint sound is heard. But if you strike it hard, you hear a loud sound.

(iii) If you gently strike a tuning fork on a rubber pad, you will hear the feeble (or soft) sound, but if you strike it hard on the rubber pad, a loud sound is heard.

Similarly, if the key of a piano is hit harder or a pipe is blown harder, we put more energy in the vibrating system due to which the amplitude of vibration is increased and a loud sound is produced.

The dependence of loudness on amplitude of vibrations can be demonstrated by the following activity.

Teacher's Note

Sound waves propagate through compressions and rarefactions in air, similar to how ripples spread in water - the medium itself doesn't travel, but the energy does, which is why we hear sounds from distant sources.

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