2. Production of sound
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.
If we bang a tuning folk on a cork, the prongs
vibrate and we hear a sound. If the vibrating prongs
quickly dipped into water, we see that water
splashes.
3. These examples shows that sound is caused by
vibration. Any vibrating object can be a source of
sound waves. Sound waves are longitudinal in
nature.
Propagation of sound wave
When a loudspeaker cone vibrates, it moves
forward and backward very fast. This squashes
and stretches air infront. 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.
4. Compression
Compression is the region where the particles are
close together and has high pressure.
Rarefaction
Rarefaction is the region where the particles are
further apart and has low pressure.
Audible frequencies
Human ear can only hear sounds with a frequency
of between 20Hz to 20 000Hz (20Hz to 20kHz).
This frequency range is called range of audible
frequencies.
5. Sound waves need a material to travel
through
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:
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.
6. Speed of sound wave
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). High
speeds in solids are due to the strong force of
attraction between molecules. Speed of sounds
depends on temperature.
7. Measuring of speed of sound in air
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 A fires the pistol
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.
s =
d
t
8. Echo sound
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:
s =
2d
t
Worked example:
To estimate the width of a valley, a climber starts a
stopwatch as he shouts. He hears an echo from the
opposite side of the valley after 4.0 s.
s =
2d
t
2d = s x t
2d = 340 x 4
d =
1360
2
d = 680 m
9. Characteristics of sound
The characteristics of sound can be shown by connecting
sound producing source to C.R.O.
10. 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.
11. Pitch
The pitch of a sound wave depends on the frequency of
the wave.
Sound of higher frequency (shorter wavelength) has a
higher pitch.
13. Ultrasound
The sounds above the range of audible frequency (20Hz to
20 kHz) are called ultrasounds.
Uses of ultrasound
• Ultrasounds are used to pre-natal scanning (womb
scanning) and also to examine the inside of human
body.
14. • It is also used cleaning delicate (sensitive) machinery.
The machinery is immersed in tank of liquid, then the
vibration of high power ultrasound are used to dislodge
(remove) the bits of dirt’s and grease.
15. • Ultrasound is also used to quality control (detect flaws in
metals). The echo sounding principle can be used detect
flaws in metals. A pulse of ultrasound is sent through the
metal, the there is a flaw (tiny gap) in the metal, the
reflected pulse are picked up by the detector and shows
on C.R.O screen.
16. Pulses of ultrasound are sent into the metal from a
transmitter. A detector is placed next to the
transmitter on the front surface of the metal.
Diagram below shows the oscilloscope trace of the
ultrasound pulses produced if the metal contains no
cracks.
17. • Ships use echo – sounders to measure the depth of
water beneath them. An echo – sounder sends pulses of
ultra sound downwards towards the seabed, then
measures the time taken for each echo (reflected sound)
to return. The longer the time, the deeper the sea.
18. The diagram below shows cathode-ray
oscilloscope (c.r.o.) trace of the pulses of
ultrasound sent from the ship and the reflected
pulses.
19. Bat uses ultrasound to find insects and other
things in front of it. It send out ultra sound pulses
and use its specially shaped ears to pick up the
reflections.
