(a) Sound is that form of energy which makes us hear. Sound waves are longitudinal waves in air.
(b) Sound cannot travel through vacuum. This can be shown by the following experiment:
(i) A ringing electric bell is placed inside an air tight glass jar containing air. We can hear the sound of ringing bell clearly. Thus, when air is present as medium in the bell jar, sound can travel through it and reach our ears.
(ii) The bell jar containing ringing bell is placed over the plate of a vacuum pump. Air is gradually removed from the bell jar by switching on the vacuum pump. As more and more air is removed from the bell jar, the sound of ringing bell becomes fainter and fainter. And when all the air is removed from the bell jar, no sound can be heard at all. Thus, when vacuum is created in the bell jar, then the sound of ringing bell placed inside it cannot be heard.
This shows that sound cannot travel through vacuum.
(a) Sound is produced when an object vibrates. For example, the sound of our voice is produced by the vibrations of two vocal cords in our throat caused by air coming from the lungs.
(b) When an object vibrates (and makes sound), then the air layers around it also start vibrating in exactly the same way and carry sound waves from the sound producing object to our ears.
Suppose a tuning fork is vibrating and producing sound waves in air. Since the prongs of the tuning fork are vibrating, the individual layers of air are also vibrating. Sound travels in the form of longitudinal waves in which the back and forth vibrations of the air layers are in the same direction as the movement of sound wave.
(a) If the air is gradually pumped out of the glass vessel, no sound of the electric bell can be heard because vacuum is created in the vessel and there are no air molecules to carry sound vibrations.
(b) Sound cannot be heard on the surface of moon because there is no air on the moon to carry the sound waves.
Astronauts talk to one another on the surface of moon through wireless sets using radio waves. This is because radio waves can travel even through vacuum though sound waves cannot travel through vacuum.
(a) The number of vibrations per second is called frequency.
The minimum distance in which a sound wave repeats itself is called its wavelength.
The distance travelled by a wave in one second is called velocity of wave.
Relation between velocity, frequency and wavelength of a wave:
Velocity of wave= frequency x wavelength
v = f x λ
(b) time period , T= 1/256 s
Velocity , V = 350 m/s
(a) A wave in which the particles of the medium vibrate back and forth in the ‘same direction’, in which the wave is moving, is called a longitudinal wave. These waves can be produced in all the three media: solids, liquids and gases.
A wave in which the particle of the medium vibrate up and down 'at right angles' to the direction in which the wave is moving, is called a transverse wave. It can be produced in solids and liquids but not in gases.
(b) Longitudinal waves:
(i) The waves which travel along a spring when it is pushed and pulled at one end.
(ii) Sound waves in air Transverse waves:
(i) The waves produced by moving one end of a long spring up and down rapidly, while other end is fixed.
(ii) The water waves or ripples formed on the surface of water in a pond.
(a) A compression is that part of a longitudinal wave in which the particles of the medium are closer to one another than they normally are, and there is a momentary reduction in volume of the medium.
A rarefaction is that part of a longitudinal wave in which the particles of the medium are farther apart than normal, and there is a momentary increase in the volume of the medium.
Longitudinal waves consist of compressions and rarefactions.
(a) The ‘elevation’ or ‘hump’ in a transverse wave is called crest. It is that part of the transverse wave which is above the line of zero disturbance of the medium. The ‘depression’ or ‘hollow’ in a transverse wave is called trough. It is that part of the transverse wave which is below the line of zero disturbance of medium. A ransverse wave consists of crests and troughs.
(b) Speed of sound= 332m/s
Time =3 sec
(a) When we put our ear to a railway line, we can hear the sound of an approaching train even when the train is far off but its sound cannot be heard through the air. This is due to the fact that sound travels much more fast through the railway line made of steel than through air.
(b) There is no actual movement of air from the sound-producing body to our ear. The air layers only vibrate back and forth, and transfer the sound energy from one layer to the next layer till it reaches our ear.
This will be clear from an example: If we turn on a gas tap for a few seconds, a person standing a few metres away will hear the sound of escaping gas first and the smell of gas reaches him afterwards. The sound of gas travels through the vibrations of air layers so it reaches first, but the smell of gas reaches the person through the actual movement of the air layers, which takes more time. So, it is clear that the sound is not being transmitted by the actual movement of air from the gas tap to person, otherwise he would hear and smell the gas at the same time.
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(a) Given that there are four complete waves.
(b) Frequency = vibrations per sec x number of complete waves
= 30 x 4 =120 Hz
(c ) Speed = frequency x wavelength
= 120 x 0.05= 6m/s
Sound can travel through all the given materials.
(a) Z medium has no fixed shape and no fixed volume.
(b) W medium has a fixed volume but no fixed shape.
(c) Y medium has the same composition as that on the moon.
(d) X medium has a fixed shape and a fixed volume.
(i) The distance between two consecutive compressions or rarefactions is equal to its
wavelength is =20 cm= 0.20 m
(ii) Speed of wave =4 m/s
Speed of wave =frequency x wavelength
4 m/s = frequency x 0.20 m
The reflection of sound leads to formation of echoes
Echo is repetition of sound caused by the reflection of sound waves.
The persistence or sound in a big hall or auditorium is called reverberation.
a) Megaphone and bulb horn
c) Timbre or Quality
The loudness of sound is measured in decibel. Its symbol is dB.
Pitch helps us to distinguish between a man’s voice and a woman’s voice, even without seeing them.
Pitch of a sound is directly proportional to frequency. Higher the frequency, higher is the pitch of the sound.
Quality or timbre
Ears enable us to hear sounds.
Ear drum starts vibrating when outside sound falls on it.
There are three small bones in the middle ear- anvil, hammer and stirrup.
The function of three tiny bones in the ear is to increase the strength of vibrations coming from the ear drum before passing them onto the inner ear.
We should not put a pin or pencil or any other sharp pointed objects in our ears because they can damage the ear-drum and damaging of ear drum can make us deaf.
Ultrasound scans are used to monitor the growth of developing baby in the uterus of the mother.
An ultrasound scan for fetus is better than X-rays because X-rays can damage the delicate body cells of the fetus.
SONAR is used to find the depth of sea by using ultrasonic sound waves.
SO und Navigation And Ranging
Soundboard works on the principle of reflection of sound.
A megaphone is used to address a small gathering of people.
A stethoscope, based on the principle of reflection of sound, is used by doctors to listen to our heartbeats.
Soundboard is a concave board which is kept behind the speaker on the stage of a big hall.
Curtains and carpets can make our big room less echoey.
No we cannot hear infrasonic waves and ultrasonic waves. That’s because the frequencies of both these waves fall beyond the human audible range of frequencies.
Infrasonic sound waves
As the frequency increases the pitch of the sound also increases.
The loudness decreases with the decrease in the amplitude of sound.
Ultrasonic sound waves
An echo is heard sooner on a hot day because the speed of sound in air increases with temperature. So the speed of sound in air is more on a hot day, and an echo is heard sooner.
An echo is heard sooner in water because the speed of sound in water is higher than the speed of sound in air.
The persistence of sound in a big hall due to repeated reflections from the walls, ceiling and floor of the hall is called reverberation. If the reverberation time in a big hall is too long, then the sound becomes blurred, distorted and confusing due to overlapping.
Reverberations in a big hall or auditorium can be reduced by the following methods :
i) Panels made of sound absorbing materials are put on the walls and ceilings of hall and auditorium.
ii) Carpets are put on the floor to absorb sound and reduce reverberations
iii) Heavy curtains are put on doors and windows to absorb sound and reduce reverberations
iv) The seats in the hall are made from materials having sound absorbing properties
We hear more clearly in a room with curtains than in a room without curtains because curtains are bad reflectors of sound. They absorb most of the sound falling on them, and hence do not produce echoes. On the other hand, in rooms without curtains, there is a greater reflection of sound due to which some echoes are produced.
These echoes cause a hindrance to hearing.
A megaphone is a large, cone-shaped (or funnel-shaped) device for amplifying and directing the voice of a person who speaks into it. A megaphone works on the principle of multiple reflections of sound.
A bulb horn is a cone shaped wind instrument which used for signaling in bicycles, cars, buses, trucks and boats, etc. A bulb horn works on the principle of multiple
reflections of sound.
Stethoscope is a medical instrument used by the doctors for listening to the sounds produced within the human body, mainly in the heart and lungs. It works on the
principle of multiple reflections of sound.
The soundboard is a concave board (curved board) which is placed behind the speaker in large halls or auditoriums so that his speech can be easily heard even by the persons sitting at a considerable distance. The sound board works as follows: the speaker is made to stand at the focus of the concave soundboard. The concave surface of the soundboard reflects the sound waves of the speaker towards the audience (and hence prevents the spreading of sound in various directions). Due to this, sound is distributed uniformly throughout the hall and even the persons sitting at the back of the hall can hear the speech easily.
a) The loudness of sound is a measure of the sound energy reaching the ear per second. It depends on the amplitude of the sound waves.
(a)Pitch is that characteristic of the sound by which we can distinguish between different sounds of same loudness. It depends on the frequency of the sound waves