Sound waves

SOUND WAVES:
Acoustics:
The study of sound is called acoustics.
Sound:
A sound is a form of energy. It is produced by the vibration of bodies. It travels in the form of waves.
Necessary Conditions For Production Of Sound:
There are two necessary conditions for the production of sound.
1. presence of a vibrating body.
2. presence of a material medium.
How Sound Is Produced:
Sound is produced by the vibration of a body. This vibration also vibrates the air when this vibrating air
strikes the ear, the eardrum also vibrates and the sensation of sound is produced.
Example:
i.
ii.
when we play the guitar, its strings vibrate and produce Sound.
we produce sound when the vocal chord vibrates.
Activity:
when we strike a rubber hammer to a tuning fork, it vibrates and produces sound. We can hear sound
by bringing it near the ear. We can feel vibrations by touching it.
If we tough the vibrating tuning Fork to a freely suspended light plastic bob, it also vibrates.
If we touch the vibrating tuning fork to the water surface, waves are produced in water. this proves that
sound waves are produced by the vibration of bodies.
Sound Required Material Medium For Its Preparations.
OR
Sound Medium Can Not Travel Without Medium.
OR
Sound Waves Are Mechanical Waves.
Sound is a mechanical wave. It always requires a material medium for its production and
propagation without a medium sound can neither produced nor propagated. This can be proved by
an activity.
Activity:
suspend an electric bell in a jar. Make it airtight. By setting on the power supply, the bell will start
ringing. Its sound can be easily heard. Now exhaust the air from jar. There is no medium in the jar
ring the bell again, no sound or feeble sound is heard. this proves that a material medium is needed
for the propagation of sound, or without a medium, no sound can propagate.
Nature Of Sound Waves:
Sound waves are longitudinal in nature this can be proved by an activity.
Activity:
vibrate a tuning fork by hitting to a rubber pad. its right prong moves from mean position to B, it
exerts some pressure on the right layer of air by pushing molecules this produces a Compression.
This Compression transfers to the next layer and becomes normal itself when the prong moves to A,
it produces a layer of less pressure towards right Called rarefaction. This rarefaction transfers to the
next layer.
As the prong moves to and fro, Compression and rarefaction are created in the air. During the
propagation of sound waves, particles of air vibrate in the direction of motion of waves. So the
sound waves are the longitudinal waves.
Characteristics Of Sound:
sounds of different objects can be distinguished on the basis of their characteristics which are
Loudness, Pitch, Quality and Intensity.
Loudness:
It is the characteristic by which we can distinguish between loud and faint Sound. Loudness depends
upon the following factors.
1. Amplitude Of Vibrating Body: Loudness is directly proportional to the amplitude of vibration. If
we beat a drum forcefully, the amplitude of vibration will be more and loud sound will be produced.
2. Area Of Vibrating Body:
Loudness is directly proportional to the area of vibration. The sound of vibrating tuning fork is faint.
If it is touched to the surface of table, loud sound is produced because the area of vibration of table
is more than that of tuning fork.
3. Distance From Vibrating Body:
Loudness is inversly proportional to the distance from the vibrating body. If the vibrating body is far
away from listener, faint sound will be heard and vice versa.
4. Physical Condition Of Ear:
Loudness also depends upon the physical condition of the ear. If the ears are Sensitive, loud sound
will be heard. If they are deffective then faint sound will be heard.
Pitch:
It is the characteristic by which we can distinguish between grave and shrill sound.
Factor Which Effects The Pitch:
The Pitch depends upon the frequency of vibration of the body. If the frequency is more then the
sound will be shrill. The pitch of such sound is more. If the frequency is less, the sound will be grave.
Such sounds have less pitch.
The sound of ladies and children are shriller than that of men, So the Pitch of children and ladies
sound is more than that of men.
Quality:
It is the characteristics by which we can distinguish between two sounds of same loudness and
pitch.
This is due to the difference in the waveforms of the two Sounds, that is why the sounds of Piano
and flute having same loudness Pitch can be distinguished.
Intensity:
Sound energy flowing per second through a unit area held perpendicular to the direction of sound
wave is called intensity of sound.
It is physical quantity and can be measured accurately. Its unit is W/m².
Human Ear Range Of Intensity:
Human ear responds to the intensity of 10⁻¹²W/m² which is the faintest audible sound to the range
of more than 10° or 1 w/m² which is the loud enough to be painful.
Relation between I and L :
Loudness (L) of a sound is directly proportional to the logarithm of intensity (I)
i.e L ∝ log I
or L = K log I
where K is the constant of proportionality.
Sound Intensity Level:
Loudness of sound is directly proportional to the logarithm of intensity.
L ∝ Log I
L = k log I …… (1)
where K is the constant of proportionality.
Let Lo is the loudness of faintest audible sound of intensity Io.
then Lo = Ko log Io •••• (2)
Subtracting (1) and (2) we get
L-Lo = k (log I – log Io)
L-Lo = k log I/Io
the difference between the loudness (L) of unknown sound and the loudness of Faintest audible
sound is Called sound intensity level.
Dependence Of K:
The value of k depends upon the unit of I and Io and the intensity unit of level.
BEL:
If the value of intensity I is 10 times greater than value of intensity Io i.e I = 10 Io
then the unit of intensity level is taken as bel, then the value of K becomes I.
Decibel Scale:
The unit bel is a larger unit of sound level.
Usually a smaller unit decibel (dB) is used.
10dB = 1 bel
If the sound level is measured in dB then.
Sound level = L-Lo = 10 log I/Io.
Reflection Of Sound Or Echo:
when sound is incident on the surface of a medium, it bounces back into the first medium this is
called reflection of sound or echo.
Example:
when we clap near a building at a perticular distance, we hear a same sound little after clapping.
This is called echo.
Activity:
Take two identical plastic pipes of suitable length. Arrange them on the table near the wall. Place a
clock near the open end of one pipe and hear the sound of clock from the other pipe. Adjust the
second pipe such that clear sound of clock is heard.
Draw a normal on the wall between the two pipes. Now measure the angle of incidence and
reflection. The two angles will be equal to each other. This activity proves that the sound waves
obey the law of reflection.
If any one of the pipe is lifted vertically to some height, then no sound will be heard, this is because
at this position angle of incidence is not equal to angle of reflection. the incident wave, reflected
wave and the normal do not lie on the same plane.
Condition To Hear The Echo:
The sensation of sound persists in our brains for 0.1s.To hear an echo the time interval between our
sound and the reflected sound must be at least 0.1s. The speed of sound at the normal temperature
of the air is 340m/s. The distance between the sound and the listener can find out by using formula:
S = v×t
= 340×0.01
= 34m
The obstacle must be at half of this distance i.e 17m from the listener.
Speed Of Sound:
Sound waves can transfer only through some material medium whose particles can vibrate. The
speed of sound is different in different mediums. In general, the Speed of Sound is five times greater
in liquids than in gases. It is 15 times greater in solids than in gases.
Factors Which Effect The Speed Of Sound:
The following factors effect the speed of Sound.
i. Nature of the medium.
ii. Temperature of the medium.
iii. Density of the medium.
iv. Pressure of the medium.
Measuring Speed Of Sound By Echo Method:
Apparatus used in activity stop watch measuring tape 50m long, flat wall.
Activity:
Clap in front of the wall 50m away. you will hear the echo clap 10 times. start the stop watch at the
first clap and stop when you hear the echo of first clap. sound of clap covered 100m distance. Note
the exact distance from the wall and calculate
the speed of sound using formula
V = s/t
Musical Sound:
The sound which produce pleasant effect on our ear is called Musical Sound.
Example: sounds of flute, violin, Piano, Sparrow, etc.
Noise:
The sound which produces an unpleasant effect on our ear is called noise.
Example: the sounds of horns, traffic, alarms, machinery, etc.
Difference Between Musical Sound & Noise :
Musical sound:
i. It produces a pleasant effect.
ii. It is not a kind of Pollution.
iii. The amplitude of the wave changes regularly.
Noise:
i. It produces an unpleasant effect.
ii. It is a kind of Pollution.
iii.
The amplitude of the wave changes irregularly.
Bad Effects Of Noise:
i. It causes hearing loss.
ii. It causes hypertension.
iii. It causes sleep disturbance.
iv. It causes high-stress levels.
v.
It interferes with the warning signals and can cause an accident.
Methods To Reduce Noise :
i.
ii.
iii.
Replace the noisy machinery with the low sound producing machinery.
use sound reducing barriers.
use hearing protection devices.
Factors Affecting The Safe Level Of Noise :
The safe level of noise is from 85 dB to 90 dB over 8 hours of the workday.
The following factors affect the safe level of noise.
i. Crossing the safe limit.
ii. Increasing the exposure time of noise.
Action Plan To Help The Affectees Due To Noise :
i. Describe the problem
ii. what are the sources of the problem?
iii. who is being affected?
iv. Suggestions for the solution.
Acoustic Protection :
The techniques or methods used to absorb undesirable sounds by soft and Porous Surfaces are called
Acoustic Protection.
Methods Of Acoustic Protection:
The reflection of sound depends upon the nature of the surface. Reflection is prominent if the surface is
hard and Smooth reflection is less if the surface is soft and irregular.
(i)Soft and porous materials such as curtains and Carpets can be used at the noisy places to reduce noise
Pollution.
(ii) we use reflective surfaces behind the stage to direct the sound to the audience.
(iii) Generally the ceiling of lecture or theatre halls is made Curved so that sound after reflection may be
real in all the Corners.
Reverberation:
when sound is too reflective from the walls, ceiling, and floor, the sound becomes garbled, this is due to
multiple reflections called Reverberation.
Audible Frequency Range:
The range of frequency that the human ear can hear is called the audible frequency range.
A normal ear can hear the sound of frequency lying between 20Hz to 20000Hz. This is called the audible
frequency range. This range is different for different ages people old people cannot hear sounds above
15000 Hz.
Ultrasound Or Ultrasonics :
The sounds of frequency higher than 20000 Hz which is inaudible to the normal ear are called
ultrasound or ultrasonics.
Uses Of Ultrasound :
(i)ultrasonics have a very small wavelength and is very useful for detecting very small objects.
(ii) They are used to diagnose and treat different ailments, these waves enter the human body through
the transmitter. they are reflected differently by different organs, tissues, and tumors. the reflected
waves are amplified to see the image of the organ on the screen.
(iii) They are used to remove the blood clots formed in the arteries.
(iv) They are used to get a picture of the thyroid for diagnosis purposes.
(v) They are used to find the depth of water. This technique is called Sonar.
(vi) They are used to find the objects underwater.
Q. What is the effect of medium on the speed of sound? In which medium the sound travels faster air,
solid, or liquid?
Ans. The speed of sound is affected by the medium. The speed of sound is proportional to the density of
the medium. The density of solid is much more than liquids or gases, that is why the speed of sound is
more in solids than in liquids and gases.
In general, it is 15 times in solids than in liquid and 5 times in liquids than in gases.
Q. sound is a form of a wave, list at least three reasons to support this idea.
Ans. A sound is a form of a wave because
(i) The sound can reflect from a hard
and smooth surface.
(ii) the sound can refract as it enters from one medium to another.
(iii) The sound can diffract when it crosses a slit.
these phenomena can be processed only in waves, so sound travels in the form of waves.
Q. waves manifest phenomenon of reflection, diffraction and refraction. Does sound also manifest these
characteristics ?
Ans. Since sound is a form of waves so it also manifest phenomenon of reflection, diffraction and
refraction.
Q. what is the difference between loudness and intensity of sound?
Ans.
Loudness:
(i) The characteristics by which we distinguish between loud and faint sound.
(ii) It depends upon the physical condition of the ear.
(iii) It is the magnitude of sensations produced in the ear.
Intensity :
(i) It is the sound energy flowing per second through a unit area held perpendicular to the direction of
sound waves.
(ii) It doesn’t depend on the physical condition of the ear.
(iii) It is the magnitude of energy transferred to the ear
Q. What is the unit of loudness? Why do we use a logarithmic scale to describe the range of the sound
intensities we hear?
Ans. The unit of loudness is the same as that of sound level i.e Bel. It is a larger unit. Smaller unit decibel
(dB) is used.
The intensity range of human ear is from 10⁻¹²W/m² to more than 1 W/m².This range is so wide that we
use logarithm scale to describe the range of sound intensities we hear.
Q. what is the difference between frequency and pitch ? Describe their relation graphically.
Ans.
Frequency :
The number of waves passing through a point in 1second is called frequency. Its unit is hertz (Hz).
Pitch:
The characteristic by which we distinguish between the grave and shrill sound. Pitch is directly related to
the frequency. If the frequency is higher the Pitch is also higher and Vice Versa.
Q. Effect Of Change In Amplitude On Loudness:
Loudness is directly proportional to the amplitude of vibration. If we pluck the string of the guitar
forcefully, the amplitude will be more and a loud sound will be heard.
Effect Of Change In Frequency On Pitch :
Pitch is proportional to the frequency of vibration. If we move one end of a rope quickly the frequency
will be more and Pitch will also be more.
Q. If the Pitch is increased, then what are the changes in the (i) frequency (ii) wavelength (ii) wave
velocity (iv)amplitude.
Ans.
(i) The frequency will also increase.
(ii) The wavelength will decrease.
(iii) According to equation v = fλ. If the wavelength is kept constant then velocity will increase.
(iv) Amplitude will remain same.
Q. If we clap in front of a building at a particular distance, we rehear our sound after some time. What
can be the reason?
Ans. The effect of sound persist in our brain for 0.1s. We rehear the sound after at least 0.1s due to
reflection called echo.
CONCEPTUAL QUESTIONS
Q. why two tin cans with a string stretched between them could be better way to Communicate than
shouting through the air ?
Ans. When the string between the tins is stretched, it is a solid Contact between the tins. Sound can
propagate 15 times faster in solids than in air, so Communication through string is better.
Q. We recognize persons speaking with the same loudness. How is this possible?
Ans. The quality of sound of each person is different from the other. So we can recognize the persons
speaking with the same loudness on the base of their qualities of sounds.
Q. You can listen to your friend round a Corner but you cannot watch him. why?
Ans. we can listen our friend round a Corner without seeing him because of diffraction of sound waves.
Due to diffraction the sound waves bend round the corner and can reach the place where it cannot
reach directly so we hear the sound of hidden person.
Q. why must the volume of a stereo in a room with wall-to-wall carpet be tuned higher than a room with
wooden floor?
Ans. Carpet is a soft and porous material. It absorbs the sound waves rather than reflecting that is why
the volume is tuned higher the smooth wooden floor is a hard surface. It reflects the sound. We can
hear it easily, so there is no need to tune it higher.
Q. A student says that the two Terms
speed and frequency of wave are the same things. What is your response ?
Ans. The speed and frequency are related to each other by an equation V=fλ. Speed is the distance
covered by a wave in unit time. Its unit is m/s.
Frequency is the number of waves passing through a point in unit time. Its unit is hertz (Hz).
Although the time Facter is common in both the quantities even than these are different quantities.
Q. Two people are listening to the same music at the same distance they disagree on its loudness. How
this could happen?
Ans. Loudness is the magnitude of Sensation produced on the ear by the sound. It depends on the
condition of the ear. If the sensitivity of ears of both the persons is different, the loudness of both will
be different.
Q. Is there any difference between echo and the reflection of sound?
Ans. Reflection Of Sound :
It is just the bouncing of sound waves when they strike a hard and smooth surface of another medium.
Echo :
It is also the bouncing of sound waves from the hard and smooth surface but it satisfies the two
conditions.
i. The distance between the source and the listener must be at least 17m.
ii. The time difference between the original and the reflected sound must be at least 0.1s.
Q. will two separate 50 dB sounds together constitute a 100 dB sound?
Ans. If two separate sounds each have a 50 dB intensity level but are in phase i.e, they interfere
constructively together constitute 100 dB sound.
Q. why ultrasound is useful in the medical field?
Ans. Ultrasonics are high energy and short wavelength waves. They enter the human body by the
transmitter. They are reflected differently from different organs and form images on the screen. These
images help to detect the defects in the organs.
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