Sound PDF
Sound PDF
Sound PDF
SOUND
How sound is produced?
When you place your fingertips against the front of your throat, you can feel the vibration of your
vocal cord when you make a noise. This example shows that sound is caused by vibration. Any
vibrating object can be a source of sound waves.
When a loudspeaker cone vibrates, it moves forward and backward very fast. This squashes and
stretches air in front. As a result a series of compressions (‘squashes’) and rarefactions (‘stretches’)
travel out through the air. These are sound waves. When they reach your ears, they make your ear-
drum vibrate and you hear a sound.
This material is called a medium. Without it, there is nothing to pass on any oscillations. Sound cannot
travel through a vacuum (completely empty space). The experiment below is to prove that sound waves
need a medium to travel through:
Experiment :
Put the electric bell into a jar and remove all the air inside the jar with the help of vacuum pump. Then
connect the bell into the battery and switch on, since the bell ring no sounds hear the outside of the jar.
This means sound required medium to travel through.
Speed of sound
Sound can travel through solids, liquids, and gases. Speed of sound varies with the medium through
which it travels. Sound travels faster in solids (concrete 5000m/s), then in liquids (1400m/s in pure
water) and then in gases (330m/s in air).
Temperature
The Wave Equation V = f λ can also be used to find the speed of sound
The speed of sound in solids like metals are so fast that we can assume/ignore the time it takes
to travel a distance
Compressions: Compression is the region where the particles are close together and has high
pressure.
Rarefactions: Rarefaction is the region where the particles are further apart and has low pressure.
Apparatus needed: stopwatch, starting pistol and measuring tape (trundle wheel).
Procedure:
Observers A and B are positioned at a known distance d apart in an open field. Record the
distance d with the measuring tape. (d must be about 1km)
Observer B, on seeing the flash of starting pistol, starts the stopwatch and then stops it when he
hears the sound. The time interval t is recorded.
Repeat the time taken t and find the average value of time.
Then calculate the speed of sound s in the air by using the following formula.
Echoes
Sound waves can be reflected by large, flat and hard surfaces like walls and cliffs. If the reflected sound
is heard after an interval of time, it is called an ECHO.
Echo sound can be calculated using the formula:
Reverberation occurs when the surface is too close, causing any reflected sound to follow closely
behind the direct sound and prolonging the original sound.
The range of frequencies which a person can hear is known as the range of audibility.
Ultrasound
The sounds above the range of audible frequency (20Hz to 20 kHz) are called ultrasounds.
Ultrasound is the sounds with frequencies above the upper limit of the human range of audibility.
Its small wavelength means less diffraction and the echo formed is more precise in direction.
o Medical applications . Ultrasounds are used to examine the inside of human body.
Loudness
The loudness of a sound wave depends upon the amplitude of the wave.
A sound wave with a larger amplitude contains more energy and is therefore louder.
A factor distinguishing between various sounds.
The larger the amplitude of vibration, the louder the sound
Sound is measured in unit decibels (dB).
Pitch
The pitch of a sound wave depends on the frequency of the wave.
Sound of higher frequency (shorter wavelength) has a higher pitch.
a factor distinguishing various sounds