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# SCIENCE-9

## Chapter 7 - Sound

1. Sound: It is a form of energy that gives us the preception of hearing. And the study of sound is called acoustics.

2. Wave: A wave is the disturbance produced in a medium due to the transmission of energy from its equilibrium or mean position that propagates with finite velocity from one region of space to another region. It has repeated and periodic motion. The wave that needs material medium to propagate is called mechanical wave. Sound wave is an example of mechanical wave.
What is the difference between sound and wave?

Types of wave:
1. Transverse Wave
2. Longitudinal Wave
3. Mechanical Wave
4. Electromagnetic Wave

Wave can be trasverse or longitudinal based on the direction of motion of particles.

Transverse wave: Those waves in which particles of medium move up and down perpendicular to the directon of motion are called transverse wave. for eg: tides, light wave etc. It comprises creast and trough.

Longitudinal wave: Those waves in which particles of medium move to and fro about a point parallel to the direction of propagation are called Longitudinal wave. for eg: sound wave. It comprises compression and rarefaction.
Compression: It is the region in the wave where the particles density is more.
Rarefaction: It is the region in the wave where the particles density is less.

The to and fro motion of molecules produces a series of successive compression and rarefaction.

Frequency:: The total numbers of complete cycles of wave produced from source per unit. The frequency is measured in Hertz (Hz.) It is the reciprocal of time (s-1)
Frequency (F)= Number of complete cycles of wave / Time taken

Wavelength: The distance between two successive crests and troughs is called wavelength. It is denoted by 𝛌 (lamda), and measured in metre (m).

Amplitude: The maximum displacement made by the particles of medium of a wave is called Amplitude.
It is denoted by 'a'.

Differences between transverse wave and longitudinal wave

Transverse wave Longitudinal wave
Those wave in which particles of medium move up and down perpendicular to the direction of motion. Those waves in which particles of medium move to and fro about a point parallel to the direction of propagation.
These wave develop crest and trough. These wave produce compression and rarefaction.
For eg: light wave, radio wave For eg: sound wave

Differences between Electromagnetic wave and Mechanical wave

Electromagnetic wave Mechanical wave
These wave transmits by carying electric and magnetic field alternately and continuously. These waves need mechanical medium to travel.
These waves have high energy. These wave have low energy.
They have high speed They have low speed.
For eg: light waves. For eg: sound waves.

Relation of wavelength and frequency :
Since,
v = 𝛌 x f
f = v / 𝛌
f ∝ 1 / 𝛌 for constant velocity
Frequency of a wave is inversely proportional to the wave length of the wave i.e. a wave having greater frequency has shorter wave length and a wave having less frequency has longer wave length.

Speed of sound in different medium:
Sound produces mechanical wave to propagate, it needs a material medium. It doesnot mean that in every medium,
sound travels with equal speed i.e. sound travels with different speed in different media.

Speed of sound in some media are :

Medium Speed (m/s)
carbon dioxide 258
air 332
hydrogen 1270
water 1498
brick 5000
aluminium 5100
steel 5200
granite 6000

In short, speed of sound in gas < speed of sound in liquid < speed of sound in solid

The speed of sound is independent of pressure, its frequency and amplitude.

Pitch of sound:

Pitch is the shrillness or sharpness of sound. This relative qualtiy of sound is pitch. The sound from girls are generally with more pitch than from boys. This is because the frequency of sound from girls is higher than the frequency of sound from boys.
Intensity of sound: Intensity of a sound at a point of wave is defined as the rate of flow of energy away from source per unit area perpendicular to the direction of wave.
Intensity of sound is directly proportional to the square of amplitude of wave produced.
i.e.
I ∝ a2

Unit of intensity of sound:
As we know,
teIntensity = energy flow / area x time
= Joule / m 2.s
watt m -2
W m -2

Threshold of hearing:
The threshold of hearing is the lowest intensity of sound that can be perceived by human ears.
The intensity of sound for threshold of hearing is 10 -12 watt m -2

I 0 = 10 -12 watt m -2

Some sources of sound and their loudness :

1. Reflection of sound:
Sound is a form of energy which reflect and follows laws of reflection. If someone shouts in front of a hill, a large wall or a building, a reflected sound is heard.
There are two types of effects develops due to the reflected sound according to their nature:
• Echo:
Echo is distinctly heard repeated reflected sound.
Conditions of echo are:
1. Distance between the source and the reflecting surface should be more than 17 metre.
2. Time interval between the original sound and the reflected sound is more than 0.1 second.
• Reverbration:
It is the prolongation of sound. It is heard when reflection of sound is caused by a close surface.
Conditions of Reverberation are:
1. Distance between the source and the reflecting surface should be less than 17 metre.
2. Time interval between the original sound and the reflected sound is less than 0.1 second.
2. Refraction of sound:
Sound is the form of energy and it also undergoes refraction during its propagation when density of the medium changes.
Questions:
1. Difference between Echo and Reverberation.
2. Differentiate between Reflection and refraction of sound.

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