Unit-6 | SOUND | Class-09

Overview: 

Sound is a form of energy that gives us the sensation of hearing. Wave is a periodic disturbance produced in a medium. Sound travels in the form of longitudinal waves. SONAR is used to measure the depth of the ocean. Here we will discuss, wave, its types, sound, reflection of sound, sound pollution and its control.



Wave Motion: 

Wave is a periodic disturbance produced in a medium. Wave motion is the transfer of energy from one place to another caused by the vibratory disturbance travelling through a medium. A wave motion travels with same speed in all directions. It is a periodic disturbance produced by a vibrating body.


Types of Wave:

On the basis of propagation of waves through material medium, waves are classified into two types:
  • Mechanical Wave: 
        The wave which requires material medium for its propagation is called mechanical wave. Examples of mechanical wave are sound waves, water waves, etc.

  • Electromagnetic Wave:
        The wave which does not require material medium for its propagation is called electromagnetic wave. Examples of electromagnetic wave are X-ray waves, light waves, etc.

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Classification of Wave:

On the basis of direction of vibrating particles, waves are classified into two types:
  • Transverse Wave:
        Transverse Wave is such type of wave in which the energy is transferred perpendicular to the direction of propagation of the motion of the wave. In this type of wave, crests and troughs are seen. Examples of transverse waves are: light wave, water wave, etc. 

  • Longitudinal Wave:
        Longitudinal wave is such type of wave in which the energy is transferred along the direction of propagation of the wave. In this type of wave, compression and rarefaction are seen. Examples of longitudinal waves are: sound wave, ultrasound waves, etc. 

Some Important Terms Related to Wave:

Amplitude: The maximum distance travelled by a vibrating particle from its mean position is called amplitude. Its SI unit is metre (m).

Wavelength: The distance between any two consecutive crests or troughs or compression or rarefaction is called wavelength. Its SI unit is metre (m).

Frequency: The number of happenings of complete cycles made in one second is called frequency. Its SI unit is Hertz (Hz).

Sound:

Sound is a form of energy that gives us the sensation of hearing. All the objects that can produce sound are called the sources of sound. Sound is produced due to the vibration of the particles of a body. It is a longitudinal wave. Some sources of sound are loudspeakers, headphones, etc.

Spectrum of Sound Wave:

The orderly classification of the frequencies of the sound waves are called spectrum of sound wave.The frequency of sound wave of adult men is about 6.5 KHz and that of adult women is about 8.5 KHz. 
Here is the classification of sound wave on the basis of frequeny:
  • Infrasound:
The sound having a frequency less than 20 Hz is called infrasound. The sounds of earthquake and volcanic eruptions are some examples of infrasound. 
  • Audible sound:
The sound having a frequency between 20 Hz and 20 KHz is called audible sound. It can be heard by human ears. Some examples of audible sound are the sounds produced during talking, turning radio, playing guitar, etc.
  • Ultrasound:
The sound having a frequency more than 20 KHz is called ultrasound. Some animals like whale, bat, mouse, insects and dolphins can produce, detect and use ultrasound. 

velocity of sound (v) = frequency (f) x wavelength (λ)

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Factors Affecting the Speed of Sound in Gases:

i. Density (d)
Velocity of sound ∝ 1/ √density

ii. Temperature (T)
Velocity of sound ∝ √T

iii. Humidity
Density of dry air > Density of moist air

iv. Direction of motion of air 
The velocity of sound is higher in the direction of motion of air than in the opposite direction.

Refraction of sound:

Like light waves, refraction of sound can be defined as the bending away of sound waves when it passes from one medium to another. 

Reflection of sound:

The bouncing back of sound when it strikes on hard surface is called reflection of sound.

Echo:

Echo is the reflection of sound where the sound waves suffer repetition. To hear an echo, the distance between the source and the reflecting surface should be at least 17.2 metres. Additionally, the size of the reflector must be large.

Reverberation:

Reverberation is the reflection of sound where the sound waves suffer prolongation. To hear reverberation, the distance between the source and the reflector mist be less than 17.2 metres.


SONAR:

SONAR is the application of echolocation. It stands for Sound Navigation and Ranging. It is a process of finding the depth or position with respect to the surrounding objects. Fathometer is the device used to find the depth. 
To find the depth of sea (s) = speed of sound in medium (v) x time taken (t) / 2
i.e. s = v x t /2

Musical Sound: The sound that is pleasing and is produced by regular and periodic vibration is called musical sound. Example: violin, flute, etc.

Noise: The sound that is unpleasing and is produced by irregular and non-periodic disturbances is called noise. Example: horns of vehicles, sound of crowd, etc.

Pitch: The shrillness of a sound is called pitch. Because the frequency of the voice of woman is high, a woman's voice has a higher pitch than that of man's voice. 

Intensity of sound (I): Intensity of sound is the measure of vibrational energy present in per unit area per unit time.
i.e. I = Sound energy / ( Time x Area)

Noise pollution: The presence of unpleasant and harsh sound in the surrounding is called noise pollution.
Noise pollution causes deafness, loss of concentration, sleep disturbance and many more undesirable happenings.

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Measures to reduce noise pollution:

Noise producing sources like loudspeakers should not be used in the programs, functions, etc.
Priority should be given to musical sound rather than harsh sound.
Explosion of firecrackers should be avoided during any occasions.

Numerical Problems:

The speed of sound in a medium is 1200 m/s. If the wavelength of sound is 15.5 m, calculate the frequency of sound. 

Solution,
v = 1200 m/s
wavelength (λ) = 15.5 m
f = ?
We know, 
v = f x Î»
or, 1200 = f x 15.5
So, f = 77.41 Hz

A ship transmits sound to the sea of depth 2250 m. If the echo is received after 3 seconds, calculate the speed of sound in water.

Solution:
s = 2250 m
t = 3 s
v = ?
We have, 
s = v x t /2
or, 2250 = v x 3/2
So, v = 1500 m/s



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