CBSE Class 8 Science Sound Chapter Notes

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Sound

Wave Motion
Wave motion is a vibratory disturbance travelling through a medium in which energy is transferred from one point to another without there being a direct contact between the two points.
Any motion which repeats itself after a fixed period (or interval) of time is known as periodic motion. The motion of earth around the sun, the motion of the hands of a clock, etc are examples of periodic motion. The periodic motion in which a body moves back and forth continuously is called vibratory (or oscillatory) motion.
The most familiar types of oscillatory motion are the motion of a swing and the motion of a simple pendulum. All the vibratory motions have a common feature that they are repetitive and periodic about a mean position. We will now discuss some important definition connected with wave motion.

Amplitude: The amplitude of a wave is the maximum displacement of the particle in a medium on either side of the mean position. It is usually represented by the letter A.

Wavelength: Wavelength of a wave is the length which is equal to the distance travelled by the wave during the time, any one particle of the medium completes one vibration about its mean position. It is usually represented by l(lambda). Its SI unit is metre (m).

Note: The distance between two consecutive crests or two consecutive troughs of a transverse wave is called the wavelength.
or
The distance between two consecutive compressions or two consecutive rarefactions of a longitudinal wave is called the wavelength.

Frequency: Frequency gives us an idea about how frequently an event occurs. Thus, the number of complete oscillations per unit time is called the frequency of the wave. It is usually represented by the letter v (nu). Its SI unit is hertz (Hz).

For example, if 10 complete waves (or vibrations) are produced in one second, then the frequency of the wave is 10 Hz. Similarly in tuning forks, it is often marked with numbers like 256, 512, etc. These numbers signify the frequency of vibrations of tuning forks. A tuning fork of frequency 256 means that its prongs make 256 vibrations in one second when hit on a hard surface.

Time Period: The time taken to complete one vibration is called time period. It is represented by the letter T. Its SI unit is second(s).
     or
The time required to produce one complete wave (or cycle) is called time period of the wave.

Relation between Time Period and frequency: Let the time period of a wave is T second.
Now, In T second, number of vibrations produced = 1
\ In 1 second, number of vibrations produced = 1/T
Since the no of vibration produced in 1 second is called its frequency, therefore Frequency =1/ Time period

or v =1/T  Þ T = 1/v

\ time period and frequency are reciprocal of each other.

Sound

Sound is a form of energy which produces the sensation of hearing. Hearing is one of the primary sensations. The physical cause that produces the sensation of hearing is the vibration of the source.

Production of Sound: Sound is produced when an object vibrates (moves back and forth rapidly). In other words, sound is produced by vibrating objects. Thus, whenever we hear a sound, then some material must be vibrating to produce that sound. When an object vibrates to produce sound, a certain amount of energy which travels in the form of sound waves. For examples, the buzzing sound of bees and mosquitoes is produced by the vibration of their wings, the sound in a sitar, veena or guitar is produced by the vibrations of stretched strings. The sound of our voice is produced due to vibrations in the vocal cords. The sound of school bell is produced by the vibrations of an iron or brass plate when it is hit by a hammer. In most of these cases, i.e., sound producing objects vibrate so rapidly that we cannot see its vibrations with our naked eyes.
Propagation of Sound: When an object vibrates, then the air layers around it also start vibrating exactly in the same way and carry sound waves from the sound producing object to our ears. The matter or substance through which sound is transmitted is called a medium. This medium can be solid, liquid or gas.

A vibrating object creating a series of compressions (C) and rarefactions (R) in the medium.

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 called compression (C). When the vibrating object moves backwards it creates a low pressure region called rarefaction (R). As the object moves back and forth rapidly, a series of compressions and rarefactions is created in the air. These makes the sound wave to propagate through the medium.
Note: Sound needs a material medium like solid, liquid or gas to travel but it cannot travel through vacuum (or empty space), because sound needs material medium for its propagation.

Longitudinal waves: A wave in which the particles of the medium vibrate back and forth along the same direction along which the wave is propagated is called longitudinal wave. For example, The waves which travel along a spring when it is pushed and pulled are longitudinal waves.

In a spring, a compression is that part in which the coils are closer together than normal position and rarefaction Is that part in which the coils are further apart than normal position. As it is already discussed that sound propagates in a medium as series of compressions and rarefactions. Thus, comparing propagation of disturbance in a spring with the sound propagation in the medium we conclude that these waves are longitudinal in nature.

Transverse Waves
For transverse waves the displacement of the medium is perpendicular to the direction of propagation of the wave.

Its standard unit is metre per second.
The speed of sound depends on: (a) the nature of material (or medium) through which it travels. The speed of sound remains almost the same for all frequencies in a given medium under the same physical conditions.
Speed of sound in solids > Speed of sound in liquids > Speed of sound in gases.
b) The speed of sound also depends on temperature. For example, the speed of sound in air is 332 ms–1 at 0°C and 344 ms–1 at 22°C.

The Race between Sound and Light: The speed of sound in air is about 344 ms–1 and speed of light in air is 3 × 108 ms–1. So it is clear that speed of light is very greater as compared to speed of sound. For this reason, the sound of a thunder is heard a little later than the flash of lightning seen. We can also observe that in the game of cricket, the ball is seen to hit the bat first the sound of hiting is heard a little later.

Characteristics of Sound Waves
Intensity or Loudness : Intensity of sound is defined as the amount of sound energy passing per unit time per unit area around a point. Intensity is a purely physical quantity and can be measured easily. Its SI unit is watt m–2.
Loudness is a measure of the response of the ear to the sound. Thus the sensation produced in the ear which enables us to distinguish between a loud and a faint sound is called loudness. It is measured in terms of decibels, (dB) a unit named after Graham Bell. Loudness depends upon (i) Intensity of sound (ii) Sensitiveness of the ear.

Pitch or frequency: Pitch is that characteristics of sound which distinguishes a sharp sound from a dull sound. Pitch depends on the frequency of the source. The faster the vibration of the source, the higher the frequency and hence the higher is the pitch.
For example, frequency of ladies voice is usually higher than that of gents. Therefore, ladies voice is sharper than gents voice.

Quality or Timbre: The quality or timber is that characteristic of sound which enables us to distinguish one sound from the other having same pitch and intensity. It depends upon the waveform of the second.
For example, we can recognise our friends from their voices on the basis of the quality of their sound.
Note: The sound which is more pleasant is said to be of a rich quality. 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 is unpleasant to the ear.

Reflection of Sound
Sound gets reflected from the surface of a solid or liquid in the same way as light and follows the same laws of reflection. For reflection of sound, however, does not require a smooth and shining surface like that of a mirror. Sound can be reflected from any hard surface, whether smooth or rough.
Echo: If we stand in one side of a big empty hall and shout or clap, we will hear the same sound again a little while later. This sound which we hear is called an echo. This happens because the sound that we shout or clap is reflected from the walls of the hall and this reflected sound forms the echo. An echo is simply a eflected sound.
To hear a distinct echo, the time interval between the original sound and the reflected sound must be at least 0.1 s. Now, knowing the minimum time interval required for an echo to be heard, and the speed of sound in air, we can calculate the minimum distance from a sound reflecting surface to hear an echo.
We know that,
Speed of sound in air at 22°C= 344 m/s
Time taken = 0.1 s
Speed = Distance travelled / Time taken

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