CBSE Class 9 Physics Sound Worksheet

INTRODUCTION - SOUND

• Sound is a form of energy like heat energy, light energy, potential energy and kinetic energy. It causes a sensation of hearing in our ears. Th
• The SI unit of sound is decibel (dB) representing sound pressure level (SPL). Decibel is a generic term used to represent the ratio of two quantities on a logarithmic scale as follows: dB = 10 log (X1/X2
• Sound cannot be created nor destroyed but can be changed from one form to another. E.g., when we clap, a sound is produced. Here, muscular energy is converted into sound energy. This is in accordance with the law of conservation of energy.
Similarly, in an electric bell, when connected to electricity starts producing sound. Here, again electrical energy is converted into sound energy.

PRODUCTION OF SOUND:

• Sound is produced due to the vibration of objects.
• The motion of materials or objects causes vibration.
• Vibration is a kind of rapid to and fro motion of an object about a central position. It is also referred to as oscillation.
For example, a stretched rubber band when plucked vibrates and produces sound.

Activity to show that vibrating bodies produces sound: - Strike the prongs of a tuning fork on a rubber pad and bring it near the ear. We can hear a sound. If a suspended table tennis ball is touched with the vibrating prong, the ball is pushed away repeatedly. This shows that the prong is vibrating and vibrating objects produces sound.

(A tuning fork is an instrument used in laboratories to perform sound related experiments).

Examples of how certain sounds are produced -
In every musical instrument, there is a vibrating part which produces sound. In case of musical instruments like flute, the air column vibrates to produce sound.

In stringed musical instruments like sitar, veena etc, there is a tightly stretched string which vibrates to produce sound. In humans, sound is produced by the voice box or the Larynx. Vocal cords in the larynx vibrate and produce sound. We hear sound with our ears. When a bird flaps its wings, a sound is produced.

PROPAGATION OF SOUND

• The travelling of sound is called propagation of sound.
• Sound is propagated by the to and fro motion of particles of the medium.
• When an object vibrates, it sets the particles of the medium around it vibrating. Each particle disturbs the particle. Thus, the disturbance is carried from the source to the listener.
• The disturbance produced by the vibrating body travels through the medium but the particles do not move forward themselves.

SOUND NEEDS A MEDIUM TO PROPAGATE:

• A medium is necessary for the propagation of sound waves.
• The matter or substance through which sound is transmitted is called a medium. The medium can be solid, liquid or gas.
• Sound cannot travel in vacuum. A true vacuum refers to the complete absence of matter. Sound waves can travel only through matter. So, sound needs a physical medium in order to travel anywhere.
• Wave that requires medium to propagate is called Mechanical Wave.For example - sound wave. Sound cannot travel in the absence of a medium.

Activity to show that sound needs a material medium for its propagation. Suspend an electric bell in an air tight bell jar. Connect the bell jar to a vacuum pump. If the switch is pressed, we can hear the sound of the bell. If air is pumped out through the vacuum pump, we cannot hear the sound of the bell. This shows that sound needs a medium to travel and sound cannot travel in vacuum

Question and answers based on the above topics :-

Question. How does the sound produced by a vibrating object in a medium reach your ear?
Answer. When an object vibrates, it necessitates the surrounding particles of the medium to vibrate. The particles that are adjacent to vibrating particles are forced to vibrate. Hence the sound produced by a vibrating object in a medium is transferred from particle to particle till it reaches your ear

Question. Explain how sound is produced by your school bell.
Answer. When the school bell is hit with a hammer, it moves forward and backwards producing compression and rarefaction due to vibrations. This is how sound is produced by the school bell.

Question. Why are sound waves called mechanical waves?
Answer. Sound waves require a medium to propagate to interact with the particles present in it. Therefore sound waves are called mechanical waves.

Question. Suppose you and your friend are on the moon. Will you be able to hear any sound produced by your friend?
Answer. No. Sound waves require a medium to propagate. Due to the absence of atmosphere on the moon and since sound cannot travel in vacuum, I will not be able to hear any sound produced by my friend.

Question. What is sound and how is it produced?
Answer. Sound is produced due to vibrations. When a body vibrates, it forces the adjacent particles of the medium to vibrate. This results in a disturbance in the medium, which travels as waves and reaches the ear. Hence sound is produced.

Question. Describe with the help of a diagram, how compressions and rarefactions are produced in the air near a source of sound.
Answer. When the school bell is hit with a hammer, it moves forward and backwards producing compression and rarefaction due to vibrations. When it moves forward, it creates high pressure in its surrounding area. This high-pressure region is known as compression. When it moves backwards, it creates a low-pressure region in its surrounding. This region is called rarefaction.

Question. Cite an experiment to show that sound needs a material medium for its propagation.
Answer. Take an electric bell and hang it inside an empty bell-jar which is fitted with a vacuum pump (as shown in the figure below). Initially, one can hear the sound of the ringing bell. Now, pump out some air from the bell-jar using the vacuum pump. You will realize that the sound of the ringing bell decreases. If you keep on pumping the air out of the bell-jar, then glass-jar will be devoid of any air after some time. Now try to ring the bell. No sound is heard but you can see bell prong is still vibrating. When there is no air present in the bell jar, a vacuum is produced. Sound cannot travel through vacuum. Therefore, this experiment shows that sound needs a material medium for its propagation.

Question. Why sound wave is called a longitudinal wave?
Answer. The vibration of the medium that travels parallel to the direction of the wave or along in the direction of the wave, is called a longitudinal wave. The direction of particles of the medium vibrates parallel to the direction of the propagation of disturbance. Therefore, a sound wave is called a longitudinal wave.

Frequently Asked Questions on Sound

Question. How does the sound produced by a vibrating object in a medium reach your ear?
Answer. When an object vibrates, it necessitates the surrounding particles of the medium to vibrate. The particles that are adjacent to vibrating particles are forced to vibrate. Hence the sound produced by a vibrating object in a medium is transferred from particle to particle till it reaches your ear.

Question. Explain how sound is produced by your school bell ?
Answer. When the school bell is hit with a hammer, it moves forward and backwards producing compression and rarefaction due to vibrations. This is how sound is produced by the school bell.

Question.Why are sound waves called mechanical waves?
Answer. Sound waves require a medium to propagate to interact with the particles present in it. Therefore sound waves are called mechanical waves.

Question. Suppose you and your friend are on the moon. Will you be able to hear any sound produced by your friend?
Answer. No. Sound waves require a medium to propagate. Due to the absence of atmosphere on the moon and since sound cannot travel in vacuum, I will not be able to hear any sound produced by my friend

Question. What is sound and how is it produced?
Answer. Sound is produced due to vibrations. When a body vibrates, it forces the adjacent particles of the medium to vibrate. This results in a disturbance in the medium, which travels as waves and reaches the ear. Hence sound is produced.

Exam Questions NCERT Class 9 Science Chapter 12 Sound

Question. Where is the density of air higher; at compressions or at rarefactions?
Answer. At the compression the density of air is higher.

Question. Name the quantity that represents the length of one complete wave.
Answer. Wavelength represents the length of one complete wave.

Question. What is relation between time period and frequency?
Answer. Frequency Time period = 1

Question. Name two animals that communicate using infrasound?
Answer. Rhinoceroses and whales communicate using infrasound.

Question. Name the waves used by bats while flying in the dark.
Answer. Bats use ultrasonic waves while flying in the dark.

Question. What is the distance between two consecutive crests in a wave called?
Answer. Wavelength is the distance between two consecutive crests in a wave.

Question. Is the amplitude of a wave the same, as the amplitude of the vibrating body producing the wave?
Answer. Yes, the amplitude of a wave is same, as the amplitude of the vibrating body producing the wave.

Question. What is the range of frequencies associated with :
(a) Infrasound
(b) Ultrasound
Answer. (a) Infrasound : Sound waves between the frequencies 1 to 20 Hz.
(b) Ultrasound : Sound waves of the frequencies above 20,000 Hz.

Question. What is a stethoscope? Name the principle on which a stethoscope works.
Answer. Stethoscope is a medical instrument used for listening sounds produced within the body, chiefly in the heart or lungs. Stethoscope works on the principle of multiple reflection of sound.

Question. How moths of certain families are able to escape captures from bats? What is the range of frequencies associated with :
(a) infrasound?
(b) ultrasound?
Answer. They have very sensitive hearing equipment, that can hear the squeaks (ultrasound) of bat and know when a bat is flying nearby.
(a) Less than 20 Hz.
(b) More than 20,000 Hz.

Question. A person fires a gun standing at a distance of 55 m from a wall. If the speed of sound is 330 ms–1, find the time for an echo to be heard.
Answer. Given d = 55 m, v = 330 ms–1, t = ?
                 2d = v × t
or t = 2d/v== 2×55/330= 0.3

Question. What are wavelength, frequency, time period and amplitude of a sound wave?
Answer. Wavelength : It is the linear distance between two consecutive compressions or two consecutive rarefactions.
Frequency : The number of compressions or rarefactions taken together passing through a point in one second is called frequency.
Time period : It is the time taken by two consecutive compressions or rarefactions to cross a point.
Amplitude : It is the magnitude of maximum displacement of a vibrating particle about its mean position.

Question. The pulse rate of a man is 80 beats in one minute. Calculate its frequency.
Answer. No. of beats per minute = 80
         No. of beats per second = 60 .
                                            = 80 = 1 3
                        So, frequency = 1.3 Hz

Question. What is echo? Explain the conditions that have to be satisfied to hear an echo?
Answer. Reflection of sound wave from a large obstacle is called an echo. The most important condition for hearing an echo is that the reflected sound should reach the ear only after a lapse of at least 0.1 second after the original sound is off and the obstacle is at least at a distance of 17 m.

Question. Explain, why can echoes not be heard in a small room?
Answer. For hearing echo, there should be at least a distance of 17 m between the source of sound and the body from which sound is reflected. In small rooms this is not the case, hence, echoes are not heard.

Question. Which wave property determines :
(i) loudness (ii) pitch
Answer. (i) The amplitude of the wave determines the loudness. More the amplitude of a wave, more is the loudness produced.
(ii) The pitch is determined by the frequency of the wave. Higher the frequency of a wave, more is its pitch and shriller is the sound.

Question. When a sound is reflected from a distant object, an echo is produced. Let the distance between the reflecting surface and the source of sound production
remains the same. Do you hear echo sound on a hotter day?
Answer. If the temperature rises, the speed of sound will increase. This in turn will increase the minimum distance required for hearing an echo. No echo is heard because the distance between the source of sound and reflecting body does not increase.

Question. Distinguish between transverse and longitudinal waves (three points).
Answer. Transverse waves :
(i) Particles the medium vibrate at right angles.
(ii) Alternate crests and troughs formed.
(iii) e.g., water waves.
Longitudinal waves :
(i) Particles vibrate parallel to the direction of waves.
(ii) Alternate compressions, rarefaction formed.
(iii) e.g., sound waves.

Question. Does sound follow the same laws of reflection as light does? Explain.
Answer. Yes, sound and light follow the same laws of reflection that are given below :
(a) Angle of incidence at the point of incidence =Angle of reflection. ^+i = +rh
(b) At the point of incidence, the incident sound wave, the normal and the reflected sound wave lie in the same plane.

Question. What is a trough?
Answer. A trough is a depression in a wave, i.e., maximum displacement in the negative direction (below the mean position).

Question. What do you understand by the term infrasonic vibrations?
Answer. The sounds of frequency lower than 20 Hz are called the infrasonics or subsonics.

Question. What is the reflection of sound?
Answer. When sound travels in a given medium, it strikes the surface of another medium and bounces back in some other direction, this phenomenon is called the reflection of sound. The waves are called the incident and reflected sound waves.

Question. What type of surfaces are the best for reflecting sound?
Answer. The best surface for the reflection of sound is polished or rough and big obstacle is necessary.

Question. What are good and bad reflectors of sound?
Answer. Good reflectors means which reflect the sound clearly. Bad reflectors means which do not reflect the sound clearly.

Question. A baby recognizes her mother by her voice. Name the characteristic of sound involved
Answer. The characteristic of sound involved in uniqueness of the sound is quality of sound or timber.

Question. What is SONAR? For what it is used?
Answer. SONAR is Sound Navigation And Ranging. It is a technique used to measure the depth of the sea, locate the sunken ships or icebergs and submarines.

Question. In which of the three media, air, water or iron, does sound travel the fastest at a particular temperature?
Answer. Sound travels faster in solids when compared to any other medium. Therefore, at a particular temperature, sound travels fastest in iron and slowest in gas.

Question. Does sound follow the same laws of reflection as light does? Explain.
Answer. Yes. Sound follows the same laws of reflection as light. The reflected sound wave and the incident sound wave make an equal angle with the normal to the surface at the point of incidence. Also, the reflected sound wave, the normal to the point of incidence, and the incident sound wave all lie in the same plane.

Question. Give two practical applications of reflection of sound waves.
Answer. (i) Reflection of sound is used to measure the speed and distance of underwater objects. This method is called SONAR. (ii) Working of a stethoscope – the sound of patient’s heartbeat reaches the doctor’s ear through multiple reflections of sound.

Question. Flash and thunder are produced are produced simultaneously, but why the flash is seen first and then the sound is heard?
Answer. This is because the speed of sound in air is very less than the speed of light in air. With the increase in Density of the medium, the speed of sound also increases.

Question. A bucket kept under a running tap is getting filled with water. A person sitting at a distance is able to get an idea when the bucket is about to be filled. (i) What change takes place in the sound to give this idea? (ii) What causes the change in the sound?
Answer. (i) The frequency of sound to increases (f∝l1)when the length (l) of the air column is decreased, i.e., when the bucket is getting filled with water. (ii) Change in sound is caused because air column is decreased on the filling the water in bucket.

Question. How noise and music can change your mood/ mental happiness?
Answer. Music can relax the mind, energize the body, and even help people better manage pain. Whereas noise will put a negative effect on it .

Question. Define the following characteristics of a sound wave
a. Loudness b. Pitch. C. Quality of sound
Answer. A. Loudness: Loudness of sound depends upon the intensity of sound. It is found that. i.e. greater the amplitude, greater will be the intensity and so louder will be the sound. The Unit of loudness is decibels (dB) and The loudness of normal talks is about 60 dB.
B. Pitch: Pitch is the characteristic of a wave by which sound wave appears shrill or grave. It depends upon the frequency of the wave. Higher the frequency higher will be the pitch and vice-versa.
C. Quality : It is the sensation received by the ear by which, we are able to differentiate two sounds (even if they are of same pitch and loudness).

Question. What is the reflection of sound?
Answer. When sound travels in a given medium, it strikes the surface of another medium and bounces back in some other direction, this phenomenon is called the reflection of sound. The waves are called the incident and reflected sound waves.

Question. What type of surfaces are the best for reflecting sound?
Answer. The best surface for the reflection of sound is polished or rough and big obstacle is necessary.

Question. What are good and bad reflectors of sound?
Answer. Good reflectors means which reflect the sound clearly. Bad reflectors means which do not reflect the sound clearly.

Question. A baby recognizes her mother by her voice. Name the characteristic of sound involved
Answer. The characteristic of sound involved in uniqueness of the sound is quality of sound or timber.

Question. What is SONAR? For what it is used?
Answer. SONAR is Sound Navigation And Ranging. It is a technique used to measure the depth of the sea, locate the sunken ships or icebergs and submarines.

Question. In which of the three media, air, water or iron, does sound travel the fastest at a particular temperature?
Answer. Sound travels faster in solids when compared to any other medium. Therefore, at a particular temperature, sound travels fastest in iron and slowest in gas.

Question. Does sound follow the same laws of reflection as light does? Explain.
Answer. Yes. Sound follows the same laws of reflection as light. The reflected sound wave and the incident sound wave make an equal angle with the normal to the surface at the point of incidence. Also, the reflected sound wave, the normal to the point of incidence, and the incident sound wave all lie in the same plane.

Question. Give two practical applications of reflection of sound waves.
Answer. (i) Reflection of sound is used to measure the speed and distance of underwater objects. This method is called SONAR. (ii) Working of a stethoscope – the sound of patient’s heartbeat reaches the doctor’s ear through multiple reflections of sound.

Question. Flash and thunder are produced are produced simultaneously, but why the flash is seen first and then the sound is heard?
Answer. This is because the speed of sound in air is very less than the speed of light in air. With the increase in Density of the medium, the speed of sound also increases.

Question. A bucket kept under a running tap is getting filled with water. A person sitting at a distance is able to get an idea when the bucket is about to be filled.
(i) What change takes place in the sound to give this idea? (ii) What causes the change in the sound?
Answer. (i) The frequency of sound to increases (f∝l1)when the length (l) of the air column is decreased, i.e., when the bucket is getting filled with water.
(ii) Change in sound is caused because air column is decreased on the filling the water in bucket.

Question. How noise and music can change your mood/ mental happiness?
Answer. Music can relax the mind, energize the body, and even help people better manage pain. Whereas noise will put a negative effect on it .

Question. Define the following characteristics of a sound wave a. Loudness b. Pitch. C. Quality of sound
Answer. A. Loudness: Loudness of sound depends upon the intensity of sound. It is found that. i.e. greater the amplitude, greater will be the intensity and so louder will be the sound. The Unit of loudness is decibels (dB) and The loudness of normal talks is about 60 dB. B. Pitch: Pitch is the characteristic of a wave by which sound wave appears shrill or grave. It depends upon the frequency of the wave. Higher the frequency higher will be the pitch and vice-versa. C. Quality : It is the sensation received by the ear by which, we are able to differentiate two sounds (even if they are of same pitch and loudness).

Question. Which of the following sound waves we can hear : 10 Hz, 500 Hz, 1500 Hz, 12000 Hz, 25000 Hz?
Answer. 500 Hz, 1500 Hz, 12000 Hz.


Question. What are longitudinal waves? Give two examples.
Answer. A wave in which the particles of the medium vibrate back and forth along the same direction, in which the wave is moving, is called a longitudinal wave.
Examples :
(a) The sound waves in air.
(b) The waves produced in air when a sitar wire is plucked.


Question. State three characteristics of a musical sound. Onwhat factors do they depend?
Answer. Characteristics of musical sound are :
(i) Loudness–Amplitude affects loudness–more amplitude, louder the sound and lesser the amplitude, softer is the sound.
(ii) Pitch-Frequency affects pitch-more frequency more pitch, less frequency less pitch.
(iii) Quality.


Question. How does the sound produced by a musical instrument, reach your ears? Astronauts need radio transmitter to talk to each other on Moon. Why?
Answer. The sound produced by the musical instrument makes the molecules of air vibrate. These vibrations are carried forward by the other molecules till they reach our ear. These then vibrate our eardrum to producesound. Since, sound requires a medium to propagate,
therefore, sound cannot travel between astronauts on the Moon, hence, they use radio transmitters.


Question. State any two characteristics of a wave motion.
Answer. The characteristics of wave motion are :
(i) It is a periodic disturbance.
(ii) Energy transfer takes place at a constant speed.

Question. Will the sound be audible if the string is set into vibration on the surface of the Moon? Give reason for your answer.
Answer. No, we will not hear any audible sound on the surface of the Moon. This is because sound requires a medium to propagate, since there is no atmosphere on the surface of Moon, therefore, the sound will not be heard.


Question. What change, if any, would you expect in the characteristics of musical sound when we increase :
(i) its frequency, and
(ii) its amplitude?
Answer.
(i) Pitch of sound will increases,
(ii) Loudness of sound will increases.


Question. The stem of a tuning fork is pressed against a table top. Answer the following questions :
(i) Would the above action produce any audible sound?
(ii) Does the above action cause the table to set into vibrations?
(iii) If the answer above is yes, what type of vibrations are they?
(iv) Under what conditions does the above action lead to resonance?
Answer.
(i) Yes, there is an audible sound produced.
(ii) Yes, the table top is set into ‘forced vibrations’ by this.
(iii) The vibrations are forced vibrations.
(iv) Pressing the stem of a vibrating tuning fork against a table top, would lead to resonance if the frequency of the tuning fork equals the natural frequency of oscillations of the table top.


Question. If the amplitude of a wave is doubled, what will be the effect on its loudness?
Answer. Loudness depends upon the square of the amplitude of the wave, therefore, when the amplitude of wave is doubled, the loudness becomes four times.


Question. How do the frequency and amplitudes affect a musical sound?
Answer. The ‘frequency’ of a musical sound affects its ‘pitch’. The more the frequency of a (musical) sound, the ‘sharper’ and ‘shriller’ the sound becomes.
The ‘amplitude’ of a musical sound affects its loudness, or intensity. The more the amplitude of the sound, the louder (or more intense) the sound is.


Question. Give one example each of natural vibration, forced vibration and resonance.
Answer. (i) Natural vibration : The vibrations of a simple pendulum about its mean position.
(ii) Forced vibration : A sonometer wire, under tension, vibrating under the influence of a vibrating tuning fork.
(iii) Resonance : A correctly adjusted length of a sonometer wire under proper tension, vibrating under the influence of a vibrating tuning fork.


Question. Mention one practical use of echoes.
Answer. Echoes are used in radars to estimate the distance of flying objects.


Question. How does a stretched string on being set into vibration, produce the audible sound?
Answer. On being set into vibrations, the stretched string, forces the surrounding air to vibrate. This vibrating air, in turn, affects our eardrum and produces an audible sound.


Question. Write conditions for the production of an echo.
Answer. Conditions for the production of an echo are :
(i) Time gap between the original sound and the reflected sound.
The echo will be heard if the original sound reflected by an obstacle reaches our ears after 0.1 s.
(ii) Distance between the source of sound and obstacle.
Thus, the minimum distance (in air at 25°C) between the observer and the obstacle for the echo to be heard clearly should be 17.2 m.
(iii) Nature of the obstacle : For the formation of an echo, the reflecting surface or the obstacle must be rigid such as a building, hill or a cliff.
(iv) Size of the obstacle : Echoes can be produced if the size of the obstacle reflecting the sound is quite large.

Question. A longitudinal wave is produced on a toy slinky. The wave travels at a speed of 30 cm/s and the frequency of the wave is 20 Hz. What is the minimum separation between the consecutive compressions of the slinky?
Answer. Wave speed,                 v = 30 cm/s
Frequency of the wave,          v = 20 Hz = 20 s^–1
The minimum separation between the consecutive
compressions is equal to the wavelength. Therefore,
                             Wavelength=30 cm s^–1/20s-1= 1.5 cm


Question. A bat can hear sound at frequencies up to 120 kHz. Determine the wavelength of sound in the air at this frequency. Take the speed of sound in the air as 344 m/s.
Answer. Frequency,n = 120 kHz = 120 × 103 Hz
                          = 120 × 103 s^–1
Velocity of sound in the air, v = 344 m/s
Wavelength of the sound wave = λ
We know,
Wavelength,λ= wave velocity/frequency
                   =344ms-1/120x103s^–1
                   = 2.87 × 10–3 m = 0.29 cm


Question. Give uses of multiple reflection of sound.
Answer. There are several uses of multiple reflection of sound :
(i) Megaphone is a device used to address public meetings. It is horn-shaped. When we speak through megaphone, sound waves are reflected by the megaphone. These reflected sound waves are directed towards the people (or audience) without much spreading.
(ii) The ceilings of concert halls and auditoriums are made curved. This is done so that the sound reaches all the parts of the hall after reflecting from the ceiling. Moreover, these ceilings are made up of sound absorbing materials to reduce the reverberation.
(iii) Stethoscope is a device used by doctors to listen the sound produced by heart and lungs. The sound produced by heart beat and lungs of a patient reaches the ears of a doctor due to multiple reflection of sound.
(iv) Sound boards are curved surfaces (concave) which are used in a big hall to direct the sound waves towards the people sitting in a hall. The speaker is (i.e., source of sound) placed at the focus of the sound board.
(v) Sound waves from the speaker are reflected by die sound board and these reflected waves are directed towards the people (or audience).
(vi) Hearing aid is used by a person who is hard of hearing. The sound waves falling on hearing aid are concentrated into a narrow beam of sound waves by reflection. This narrow beam of sound waves is made to fall on the diaphragm of the ear. Thus, diaphragm of the ear vibrates with large amplitude. Hence, the hearing power of the person is improved.


Question. Give application of ultrasound (ultrasonic waves).
Answer. Ultrasonic waves have number of uses :
(1) Ultrasonic vibrations are used for homogenising milk. These vibrations break down the larger particles of the fat present in milk to smaller particles.
(2) Ultrasonic vibrations are used in dish washing machines. The vibrating detergent particles rub against the dirty utensils and thus, clean them.
(3) Ultrasonic vibrations produce a sort of depression in rats and cockroaches.
(4) Ultrasonic vibrations are used to study the growth of foetus in mother’s womb.
(5) Ultrasonic vibrations are used in relieving pain in joints and muscles.
(6) Ultrasonic vibrations are used in detecting flaws in articles made from metals. They are also used in finding the thickness of various parts of a metallic component.


Question. A tuning fork produces 1024 waves in 4 seconds.Calculate the frequency to the tuning fork.
Answer. As the tuning fork produces 1024 waves in 4 seconds, hence
Frequency of tuning fork,
n = Number of vibration per second
= 1024/4 = 256 Hz


Question. A human heart, on an average, is found to beat 75 times a minute. Calculate its frequency.
Answer. No. of beats of human heart = 75 min^–1
                                                   = 75/1 min
                                                   = 75 /60 = 1.25 s^–1
So, average frequency of human heart beating
                                                    = 1.25 s^–1.


Question. A boat at anchor is rocked by waves whose consecutive crests are 100 m apart. The wave velocity of the moving crests is 20 m/s. What is the frequency of rocking of the boat?
Answer. Distance between two consecutive crests = 100 m
Wave velocity v = 20 m/s
The distance between two consecutive crests is equal to the wavelength of the wave. So,
        Frequency = Wave velocity/Wave length
                       =20 ms-1/100 m= 0.2 s^–1
So, the frequency of rocking of the boat is 0.2 s^–1.

Question. Do waves transport matter?
Ans : No.

Question. Sound is produced due to a vibratory motion, then why a vibrating pendulum does not produce sound?
Ans : The frequency of the vibrating pendulum does not lie within the audible range (20 Hz to 20,000 Hz) and hence, it does not produce audible sound.

Question. Why do echoes produced in an empty auditorium usually decrease when it is full of audience?
Ans : When the hall is empty there are no obstacles in between to reflect the sound other than the walls.
When the hall is full of audiences, the sound produced undergoes multiple reflections from the people and so it overlaps with the sound produced. Hence, the listener is not able to distinguish between the original sound and the echo.

Question. What is a wave?
Ans : A wave is a disturbance that travels in a medium due to repeated periodic motion of particles about their mean position – such that the disturbance is handed over from one particle to the other without the actual motion of the medium.

Question. What is a transverse wave?
Ans : It is a wave in which the particles of the medium vibrate perpendicular to the direction of propagation of the wave.

Question. What is a longitudinal wave?
Ans : It is a wave in which the particles of the medium vibrate in the direction of propagation of the wave.

Question. What do you understand by the term ultrasonic vibrations?
Ans : Sounds of frequency higher than 20,000 Hz are called the ultrasonics.

Question. What do you understand by the term echo?
Ans : The sound heard after reflection from a rigid obstacle is called an echo.

Question. Do the particles of the medium move from one place to another in a medium?
Ans : No.

Question. Does the velocity of wave motion depend on the nature of the medium?
Ans : Yes.

Question. Does the velocity of wave motion depend on the nature or motion of the source?
Ans : No.

Question. What are transverse waves? Give two examples.
Ans : A wave in which the particles of the medium vibrate up and down at right angle to the direction in which the wave is moving.
Example : (i) The waves produced by moving one end of a long spring up and down rapidly.
(ii) Ripples formed on the surface of water in a pond.

Question. What are crests and troughs of a wave?
Ans : The elevation in a transverse wave is called crest. It is that part of transverse wave which s above the line of zero disturbance of the medium. The depression in a transverse wave is called trough. It is that part of the transverse wave which is below the line of zero disturbances.

Question. A Sitarist tries to adjust the tension and pluck the string suitably, before playing the orchestra in a musical concert. By doing so what is he adjusting?
Ans : He is adjusting frequency if the sitar string with the frequency of the other musical instrument.

Question. If the tension in the wire is increased four times, how will the velocity of wave in a string varies?
Ans : Velocity of the wave in string is directly proportional to the square root of the tension thus if tension is increased 4 times, the velocity will be doubled.

Question. Explain, how is the principle of echo used by the dolphin to locate small fish as its prey?
Ans : Dolphins are aquatic animals which send out ultrasonic sound to communicate with each other. They have a sound sensing system which enables them to find animals underwater with great accuracy due to the echo of the ultrasonic sound produced by them.

Question. Give two practical applications of the reflection of sound waves.
Ans : (i) In stethoscope the sound of patient’s heartbeat reaches the doctor’s ears by multiple reflections in the tubes.
(ii) Megaphones are designed to send sound waves in particular direction are based on the reflection of sound

Question. What is the other name of a long flexible spring?
Ans : Slinky is the other name of a long flexible spring.

Question. Distinguish between tone and note.
Ans : A pitch is a particular frequency of sound, for example : 440 Hz.
A note is a named pitch. For example : Western music generally refers to the 440 Hz pitch as A, specifically A4.

Question. How do you account for the fact that two strings can be used to give notes of the same pitch and loudness but of different quality?
Ans : The ‘quality’ of a given note is determined by the overall effect of the harmonics present in it. The harmonics are multiples of the fundamental or basic frequency of the ‘note’. Depending on the conditions under which vibrations are taking place, sometimes we get one set of harmonics and sometimes another set.
The quality of the two notes will, therefore, different even though their fundamental frequencies may be the same.

Question. Can you produce both types of waves (i.e., longitudinal and transverse) on a slinky?
Ans : Yes, we can produce both types of waves (i.e., longitudinal and transverse) on a slinky.

CHARACTERISTICS OF SOUND WAVES:

Sound waves have 4 characteristics
1. Amplitude 2. Wavelength 3. Frequency 4. Speed

• As the sound wave propagates in a medium, the density as well as the pressure of the medium at a given time varies with distance above and below the average value.
• Increase in density is not the same throughout compression. Maximum increase in density is seen at the centre of compression.

• i) Amplitude- The amplitude of sound wave is the height of the crest or tough. The amplitude is how high the crests are. In a sound wave, the maximum displacement associated with the particle constituting a wave is called its amplitude. It is represented by “A’. SI unit is metre.

Amplitude depends upon the force with which an object vibrates . E.g., When we hit a table hard, a loud sound is produced due to its larger amplitude.
Similarly, if we hit the table slowly, the sound produced is low as its amplitude is small. Thus, loudness as well as soft sound is determined by its amplitude.

• ii) Wavelength- 

The wavelength is the distance between 2 consecutive compressions or 2 consecutive rarefaction is called wavelength n is represented as λ (lambda). Its SI unit is metre. Wavelength can also be considered as the distance over which graph/wave is repeated.

• iii) Frequency- The number of vibrations completed by a particle in one second is the frequency of the sound wave.
Frequency = Number of Oscillations / Total Time. =1/T

We can calculate the frequency of sound by calculating the number of compressions or rarefaction in one second. It is represented by a Greek letter (Greek letter nu). SI unit is Hertz. SI unit of frequency is named after Heinrich Rudolph Hertz who laid foundation 4 future development of radio, telephone, telegraph and TV.

• iv) Time period- The time taken by the particle of the medium for completing one oscillation/vibration is called the time period. It is represented by the symbol “T”. SI unit is second. Time period of a sound wave is the time between 2 successive compressions or 2 successive rarefactions

v) Velocity of sound wave- It is the distance travelled by a wave in one second. Speed is represented by V. Speed with which compression and rarefactions move ahead is called velocity. SI unit is meter per second (m/s).

The speed of sound is more in solids, less in liquids and least in gases

RELATIONSHIP BETWEEN SPEED V, FREQUENCY AND WAVELENGTH OF SOUND

Wave Velocity= Distance covered/ time taken = Wavelength/time taken
= λ /T......................(1)
As =1/T , eq(1), connecting V and in terms of frequency can be written as
V= λ×f......................................(2)
(Or)
Wave velocity= wavelength × Frequency

The velocity of sound remains almost same for all frequencies in a given medium under the same physical conditions.

Questions based on above topics:

Question. What is the formula of time period?
Solution: T= 1/f. Here f stands for frequency.

Question. A wave covers 25 oscillation with its crest and through from point A to B what is the time period of the wave from A to B?
Solution: As we know T= 1/f
Given f = 25 Hz.
( no. Of oscillation of wave = frequency and the SI unit of f is Hz)
T= 1/25 = 0.4Hz.

Question. What affects amplitude of a wave?
Solution: The amount of energy carried by a wave is related to the amplitude of the wave. A high energy wave is characterized by a high amplitude; a low energy wave is characterized by a low amplitude. The energy imparted to a pulse will only affect the amplitude of that pulse.

Question. Does amplitude decrease with distance?
Solution: The energy spreads out in a spherical shell, the energy density decreases as the square of the distance from the source. That's your amplitude decrease. However, the frequency and wavelength stay the same as long as it keeps traveling at the speed of light.

Question. Compute the amplitude of the wave if a wave travels a distance of 0.5 m and has a frequency of 5 Hz..
Solution: Given: Distance D = 0.5 m,
Frequency f = 5 Hz
The amplitude is given by
A= Distance /Frequency
A= 0.5/5 = 0.1 meter.

Question. How are the wavelength and frequency of a sound wave related to its speed?
Solution: Wavelength, speed, and frequency are related in the following way:
Speed = Wavelength x Frequency
v ( speed) = λ × f

Question. The frequency of a source of sound is 100 Hz. How many times does it vibrate in a minute?
Solution: Frequency = (Number of oscillations) / Total time
Number of oscillations = Frequency × Total time
Given, Frequency of sound = 100 Hz
Total time = 1 min (1 min = 60 s)
Number of oscillations or vibrations = 100 × 60 = 6000
The source vibrates 6000 times in a minute and produces a frequency of 100 Hz.

Question. Calculate the wavelength of a wave whose frequency is 220hz and speed is 440m/sec in medium?
Solution: frequency=velocity/wavelength
So, 220=440/wavelength
wavelength =2m.

Question. Frequency of sound is 100Hz. How many times does it vibrates in a minute?
Solution: Given f= 100 Hz
Time = 1 min
As we know f=1/T
1min = 60 second
F= 1/60
vibration in one minute =100 x 60=6000 times.

Question. A person is listening to a tone of 500Hz sitting at a distance of 450m from the source of sound. What is the time interval between successive compressions from the source?
Solution: Given f= 500 Hz.
T=1/f T=1/500
T=0.002 sec.