Sound
Sound is a form of energy which makes us hear and travels in the form of waves. Sound is produced by various sources. Though sound is mechanical in nature, its perception is, of course, largely physiological.
1.1 PRODUCTION OF SOUND
When you clap your hands, a sound is produced. In order to know how sound is produced, let us perform the following two simple experiments.
Experiment – 1
Stretch a metallic wire AB between two nails fixed on a table as shown in the figure. When we pluck the wire a sound is heard. If a V-shaped small paper rider (R) is placed near the centre of the wire, it starts vibrating. If the rider is placed at the centre of the wire, it flies off.
Experiment – 2
On striking a bell B with a hammer, it produces sound. On touching it with finger, we feel that the bell is in a state of vibration. A pith-ball, P, suspended near the bell moves from its equilibrium position P to P as shown in figure.
In the above activities we have produced sound by plucking and striking. We can also produce sound by scratching, rubbing, blowing, shaking, etc.
Thus, we conclude that sound is produced by setting the objects in vibration. Vibration means a kind of rapid to and fro motion of an object.
1.2 PROPAGATION OF SOUND
The sound produced by vibrating objects reaches the listener only when it passes through a medium which may be a liquid, a solid or a gas. Let us now briefly discuss as to how sound travels from the source point of generation to the listener.
(i) The vibration of the object sets the particles of the medium around it vibrating.
(ii) The particles do not travel from the vibrating object to the ear. A particle of the medium in contact with the vibrating object is first displaced from its equilibrium position as shown in the figure. This particle exerts a force on a neighboring particle (2) which is displaced from its equilibrium position and also starts moving. After displacing particle (2), particle (1) comes back to its mean position. This process continues till the particle near the listener’s ear starts vibrating.
Thus, sound created by the source reaches the listener through the particles of the medium without any net transport of the medium.
1.2.1 Wave Nature of Sound
A wave is a vibratory disturbance in a medium which carries energy from one point to another, without there being a direct contact between the two points.
(a) Type of waves on the basis of material medium
(i) Elastic waves or mechanical waves : Those waves which need a material medium for their propagation are called elastic waves or mechanical waves e.g., sound waves and water waves are elastic or mechanical waves.
(ii) Electromagnetic waves : Those waves which do not need a material medium for their propagation are called electromagnetic waves. These waves can travel through vacuum as well as through medium e.g. light waves and radio waves are electromagnetic waves.
(iii) Seismic waves : The disturbances inside the earth which cause waves moving in all direction are called seismic waves. It is these waves which cause earth quakes.
(b) Types of waves on the basis of its direction of propagation
(i) Transverse waves : The waves in which the particles of the medium vibrate up and down ‘at right angles’ to the direction in which the wave is moving, are called transverse waves. Eg., light waves, radio waves, water waves etc. The water waves (or ripples) formed on the surface of water in a pond are transverse waves. This is because of the fact that in a water wave, the molecules of water move up and down in the vertical direction when the wave travels in the horizontal direction along the water surface. When a stone is dropped in a pond of water, transverse water waves are produced on the surface of water. |
The ‘elevation’ or ‘hump’ in a transverse wave is called crest. In other words, a crest is that part of the transverse wave which is above the line of zero disturbance of the medium. In figure, XY is the line of zero disturbance and A and C are the two crests of the transverse water waves.
The ‘depression’ or ‘hollow’ in a transverse wave is called trough. In other words, a trough is that part of the transverse wave (B and D) which is below the line of zero disturbance.
(ii) Longitudinal waves: 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 longitudinal wave. Eg. – Sound waves.
Wave motion is a form of disturbance (a mode of energy transfer) which is due to repeated vibrations of the particles of the medium about their mean positions and the motion is handed over from one particle to the other without any net transport of the medium. |
1.2.2 Sound Waves Are Longitudinal Waves
Before we study about sound waves, let us understand the nature of longitudinal waves with the help of a slinky AB (It is a toy in the form of a long flexible spring which can be very easily extended or compressed).
The slinky is arranged in the horizontal position with its end B fixed. Initially when the slinky is neither compressed nor stretched, there is a fixed distance between its loops.
If the free end of slinky is pushed forward a few loops near it are compressed. This region where the loops are closer to each other than the normal distance is called a compression.
Now, if the slinky is pulled outwards, a few loops near it are pulled away from each other. This region where the loops of the slinky are farther apart than the normal distance is called a rarefaction.
If pulling and pushing of the slinky is done at regular intervals, a series of compression and rarefactions are set up in the slinky.
Production of compressions and rarefactions near a source of sound
Air is the most common medium through which sound travels and it does so with the help of intervening layers of air. As discussed earlier, a source of sound puts the particles of the medium into vibratory motion. Let us consider a vibrating tuning fork as a source of sound.
(a) When the right prong moves from left (L) to right (R), it compresses the layer of air in front of it. This results in the increase of the pressure as well as density of this layer. This layer (or region) of compressed air is called a compression.
(b) When the prong moves from its right extreme (R) to left extreme (L), the air in front of it expands. As a result, the pressure (as well as density) of this layer decreases. This region of rarefied air is called a rarefaction.
Thus,
A sound wave which propagates as a series of compressions and rarefactions is a longitudinal wave.
A sound wave can be considered as propagation of pressure or density variations in the medium.
1.2.3 Characteristics of a Sound Wave
As already discussed, sound waves are produced due to variations in pressure and density of the medium. The various other characteristics are:
(a) Compression and rarefaction
A compression is formed when particles of the medium lie closer to each other whereas a rarefaction is formed when the particles of the medium lie farther apart than the normal distance.
(i) Compression : A portion of the medium where a temporary decrease in volume and consequently a increase in density takes place when a sound wave passes through the medium, is called a compression or a condensation.
(ii) Rarefaction : A portion of the medium where a temporary increase in volume and consequently a decrease in density takes place when a sound wave passes through the medium, is called a rarefaction.
(b) Graphical representation of sound wave
The graphical representation of sound wave is given below :
(i) Crest : The portion of the medium where the density (or pressure) has a value larger than its average value is called a crest.
(ii) Trough : The portion of the medium where the density (or pressure) has a value smaller than the average value is called a trough.
The points of maximum density (or pressure) and minimum density (or pressure) are also called crests and troughs respectively.
(iii) Amplitude (A) : The magnitude of the maximum disturbance in the medium on either side of the mean position is called the amplitude of the wave. It is usually represented by the letter A. In case of sound, the unit of A is the same as that of density or pressure.
(iv) Oscillation : As is clear from the graph, the density (or pressure) of the medium oscillates between a maximum value and a minimum value. The change in density (or pressure) from maximum value to the minimum value and again to the maximum value is called an oscillation.
(v) Frequency (v) : The frequency of a sound wave is defined as the number of complete oscillations per second. It is denoted by the symbol (Greek letter, nu).
Unit of frequency is cycle per second (cps) or s1 or hertz (Hz) which is named after Heinrich Hertz (1857-1894).
Bigger units of frequency are kilohertz (kHz, 103Hz) and megahertz (MHz, 106 Hz).
(vi) Time Period (T) : The time taken for one complete oscillation in density (or pressure) of the medium is called the time period of the wave.
Time period of the wave is also defined as the time taken by its two consecutive compressions or rarefactions to cross a fixed point.
(vii) Wavelength () : The distance between two consecutive compressions or two consecutive rarefactions is called the wavelength of the wave. It is denoted by the symbol (Greek letter, lambda). Wavelength of a sound wave is also equal to the distance travelled by it in its periodic time (T). Unit of wavelength is metre (m).
(c) Relation between frequency and time period
If the frequency of the wave is , then from the definition of frequency,
Time taken for completing oscillations = 1 second
And time taken for completing 1 oscillation = second
(d) Relation between speed of sound, frequency and wavelength
Speed of sound is the distance travelled by the sound wave per unit time. It is denoted by v and is measured in metre/second (m/s).
From the definition of wavelength,
Thus, to describe a sound wave, we need to know :
(a) Speed
(b) Frequency (or wavelength) and
(c) Amplitude
1.3 SOUND NEEDS A MEDIUM TO TRAVEL
Sound is a mechanical wave and needs a material medium like air, water, steel etc. for its propagation. It cannot travel through vacuum, which can be demonstrated by the following experiment. Take an electric bell and an airtight glass bell jar. The electric bell is suspended inside the airtight bell jar. The bell jar is connected to vacuum pump, as shown in figure. If you press the switch you will be able to hear the bell. Now start the vacuum pump. When the air in jar is pumped out gradually, the sound becomes fainter. When the air is completely ejected from the bell jar, no sound is being heard. Although, the bell is working alright and same current is passed through it, but no sound is being heard. It clearly shows that sound cannot travel without a material medium. |
Two astronauts cannot talk to each other on the moon as they do on the Earth. Why? |
1.4 CHARACTERISTICS OF SOUND
When a violin and a piano are played together in an orchestra, sounds produced by these instruments travel through the air (medium) and reach the ear at the same time which shows that sounds received by the ear are of different nature.
These sounds are distinguished by the following characteristics :
Loudness
Intensity
Pitch or frequency
Quality or timbre
1.4.1 Loudness
The sensation produced in the ear which enables us to distinguish between a loud and a faint sound is called loudness. If a tuning fork is first struck softly and then hard, we hear a faint sound and a loud sound respectively. Since these sounds are produced by the same tuning fork, they have same frequency. But as shown in the figure the faint sound has a small amplitude whereas the loud sound has a large amplitude. Thus, loudness of sound is : |
Directly proportional to as the square of the distance from the source of sound, i.e., closer the source to the listener, louder the sound.
Directly proportional to the surface area of the vibrating body, i.e., larger the size of the vibrating body, louder the sound.
Directly proportional to the density of the medium.
1.4.2 Intensity of sound
The intensity of sound at any point in space is defined as the amount of energy passing per unit area in a direction perpendicular to the area.
Thus, intensity =
Unit of intensity = watt / metre2 (W/m2)
Thus, intensity is a physical quantity that can be easily measured. Though loudness and intensity are closely related to each other, they are not one and the same thing.
S. No. | Loudness | Intensity |
1. | It is not an entirely physical quantity. | It is a physical quantity which can be accurately measured. |
2. | It depends upon (i) sensitivity of the ear and (ii) intensity of sound. | It does not depend upon the sensitivity of the ear. |
1.4.3 Pitch or frequency
Pitch is that property of sound which help in differentiating between a shrill sound and a grave (flat or dull) sound. High pitched sounds are called treble and low pitched sounds are called bass. Pitch is directly proportional to the frequency. The voice of a lady is shriller than that of a man because the frequency of a women’s ordinary voice is around 280 Hz and that of man is around 140 Hz. |
1.4.5 Music and noise
All sounds are categorized into :
(a) Musical sound or music : These sounds have a pleasant effect on the listener. Sounds produced by a tuning fork, musical instruments, singing of sounds, etc are few examples of musical sound. A musical sound consists of a series of sound impulses following each other at regular intervals of time without sudden changes in amplitude (i.e., loudness). Such sounds are usually of high frequency.
(b) Noise : These sounds have an unpleasant (disagreeable or boring) effect on the listener. Rustling of leaves, murmuring of students, etc are some examples of noise. A noise consists of a series of sound impulses following each other at regular intervals of time and there are sudden changes in amplitude (i.e. loudness). Such sounds are of low frequency.
Both musical sound and noise are based on psychological response of the ear and brain to various types of sound. There is no clear line of demarcation between them on the basis of sensation these produce on the ear of the listener.
Differences between musical sound and noise
S. No. | Musical Sound | Noise |
| 1. | It has a pleasant effect on the ear. | It has an unpleasant effect on the ear. |
| 2. | It consists of a series of sound impulses which follow one another regularly. | The sound impulses do not follow one another regularly. |
| 3. | The frequency of musical sound is high. | The frequency of a noise is low. |
| 4. | There are no sudden changes in amplitude (loudness) of the waves constituting a musical sound. | There are usually sudden changes in amplitude (loudness) of the waves forming a noise. |
1.5 SPEED OF SOUND IN DIFFERENT MEDIA
When we strike an object with a hammer the sound of hammering is heard a short while after the actual impact. Similarly, the sound of cracker is heard only after it has exploded.
The flash of lightning is seen first and thunder is heard later on. These observations reveal that
However, in certain solids, the speed (v) of sound is much less than that even in gases as v (for vulcanized rubber) = 54 m/s and v (for hydrogen) = 1284 m/s. The speed of sound in lead (a solid) = 1332 m/s and in sea water (a liquid) = 1531 m/s. The speed of sound in methyl alcohol (a liquid) = 1103 m/s and in hydrogen (a gas), its value = 1284 m/s. |
The speed of sound increases with increase in temperature of the medium. In air, it increases roughly by 0.61 m/s with rise of 1C in temperature. The speed of sound in air at 0C is 331 m/s and at 22C, it is 344 m/s.
Sonic boom or shock waves
Source that move faster than the speed of sound are said to have supersonic speeds. Bullets, jet aircrafts, etc, often travel at supersonic speeds. When a sound producing source moves with a speed higher than that of sound, the energy emitted is unable to move in front of the source and is concentrated on the sides. This concentration of energy as it travels outward is called shock wave. It is not necessary for an object to have a vibrating source of sound in order to create a shock wave. The only condition necessary is that the object moves with a speed greater than the speed of sound.
Sonic boom is an example of a shock wave produced by a distant supersonic aircraft and is clearly heard by an observer at rest on the ground. The energy carried by a sonic boom lasts only for a fraction of a second and is sometimes sufficient to crack glass windows and shatter buildings.
Question: How does the sound produced by a vibrating object in a medium reach your ear?
Answer: Sound produced by a vibrating object reaches our ear through sound waves which propagate through the medium as a series of compressions and rarefactions.
Question: Explain how sound is produced by your school bell?
Answer: When the school bell is struck with a hammer, it starts vibrating and as a result of these vibrations, sound waves are produced.
Question: Suppose you and your friend are on the Moon. Will you be able to hear any sound produced by your friend?
Answer: Sound waves need a material medium for their propagation. Since there is no atmosphere on the Moon, one person cannot hear the sound produced by another person.
Question: Guess which has a higher pitch : a guitar or a car horn?
Answer: A guitar has a higher pitch than a car horn, provided the guitar is properly tuned.
Question: How are the wavelength and frequency of a sound wave related to its speed?
Answer: Speed of sound (v) = frequency () x wavelength ().
Question: Calculate the wavelength of a sound wave whose frequency is 220 Hz and speed is 440 m/s in a given medium.
Answer: Here, frequency of the sound wave, = 220 Hz
speed of the sound wave, v = 440 m/s
As v = , = = = 2 m
Question: In which of the three media : air, water or iron ; does sound travel the fastest at a particular temperature ?
Answer: Sound travels the fastest in iron which is a solid medium.
Question: Why are sound waves called mechanical waves?
Answer: Waves which need a material medium for propagation are called mechanical waves. Since sound waves also need a material medium for propagation, these are called mechanical waves. Sound waves are called mechanical waves or elastic waves as these are produced in a deformable or elastic medium. Unlike electromagnetic waves, sound waves need a medium to sustain them. Mechanical waves are governed by Newton's laws of motion.
Question: Which wave property determines (a) loudness (b) pitch?
Answer: (a) Loudness is determined by the amplitude of the sound wave which in turn depends on the force with which the object is made to vibrate.
(b) Pitch of a sound is determined by its frequency. (Apart from this, the pitch of a sound also depends upon the relative motion between the source of sound and the listener).
Question: A person is listening to a tone of 500 Hz sitting at a distance of 450 m from the source of the sound. What is the time interval between successive compressions from the source?
Answer: Here, frequency of the source, = 500 Hz
time period of the tone, T = = s
The time period between successive compressions from the source is equal to the time period of the tone, i.e., (1/500)s and it has nothing to do with the distance (450 m) of the person from the source provided the sound wave reaches the person.
Question: Which characteristic of the sound helps you to identify your friend by his voice while sitting with others in a dark room?
Answer: The quality (or timbre) of sound is that characteristic which enables us to distinguish one sound from the other even when these are of the same pitch and loudness. Each person has its own quality of sound and it is this characteristic which enables us to identify a person from others even without looking at him (i.e., in a dark room).
Question: Flash and thunder are produced simultaneously. But thunder is heard a few seconds after the flash is seen, why?
Answer: The speed of light (c) is greater than the speed of sound (v) by a factor of 106 as
c/v = = 106. Thus, the flash of light is seen earlier than the thunder of sound even though both are produced simultaneously.
Question: A person has a hearing range from 20 Hz to 20 kHz. What are the typical wavelengths of sound waves in air corresponding to these two frequencies? Take the speed of sound in air as 344 ms−1.
Answer: Here, ν1 = 20 Hz and ν2 = 20 kHz = 20 X 103 Hz
speed of sound, v = 344 m/s
∴ 17.2 m
and 0.0172 m
Question: A sound wave travels at a speed of 339 ms-1. If its wavelength is 1·5 cm, what is the frequency of the wave? Will it be audible?
Answer: Here, speed of sound wave, v = 339 m/s
wavelength of sound wave, = 1·5 cm = 1·5 x 10-2 m
frequency of the sound wave, = = = 22600 Hz.
The sound is not audible as its frequency lies beyond the audible range (20 Hz to 20,000 Hz).
Question: (a) Define intensity. Write its SI unit.
(b) Distinguish between loudness and intensity of sound.
Answer: The intensity of sound at any point in space is defined as the amount of energy passing per unit time per unit area in a direction perpendicular to the area.
In other words, intensity = =
Unit of intensity is watt/metre2 (W/m2).
Clearly, intensity is a physical quantity that can easily be measured.
Loudness | Intensity |
1. It is not an entirely physical quantity. 2. It depends upon (i) sensitivity of the ear and (ii) intensity of sound. | 1. It is a physical quantity which can be accurately measured. 2. It does not depend upon the sensitivity of the ear. |
Question: What are wavelength, frequency, time period and amplitude of a sound wave?
Answer: As said earlier, a sound wave is a longitudinal wave which travels in the form of compression and rarefactions which are defined as follows :
Wavelength () : The distance between two consecutive compressions or two consecutive rarefactions is called the wavelength of the wave. It is denoted by the symbol (Greek letter, lambda). Wavelength of a sound wave is also equal to the distance travelled by it in its periodic time (T).
Frequency () : Frequency enables us to know as to how many times does a particular event occur in a given time. If you count your pulse, you may find that it throbs around 72 times per minute. This is expressed by saying that the frequency of the pulse is 72 time per minute. Similarly, the frequency of a sound wave is defined as the number of complete oscillations in density (or pressure of the medium) per second. It is denoted by the symbol (Greek letter, nu).
Time Period (T) : The time taken for one complete oscillation in density (or pressure) of the medium is called the time period of the wave.
Time period of the wave is also defined as the time taken by its two consecutive compressions or rarefactions to cross a fixed point.
Amplitude (A) : The magnitude of the maximum disturbance in the medium on either side of the mean position is called the amplitude of the wave. It is usually represented by the letter A. In case of sound, the unit of A is the same as that of density or pressure.
Q. 1 – 4 are of one mark each.
Q. 5 – 11 are of two marks each.
Q. 12 is of five marks.
1. Why can't we hear the sound of the explosions taking place on other planets?
2. How does loudness of sound decrease as one move away from the source of sound?
3. What is a noise?
4. What is a sound note?
5. A stone is dropped into a 40 m deep well. The sound of splash is heard 2.95 s after the stone is dropped. Find the speed of sound.
6. Anuj is viewing live telecast of an India-Pakistan cricket match on his TV screen. He saw a player hitting a boundary. He observed that he could see ball racing towards the boundary first and then heard the sound produced by ball striking the bat a bit later. Explain, how?
7. A soldier saw smoke first and heard the sound produced by firing of bullet from a gun afterwards. How will you explain it?
8. On a cloudy day, the sound of thunder was heard 4.5 s after the flash of light was seen. How far was the cloud? Given that speed of sound = 340 ms-1.
9. Define frequency of a wave. Give its unit. How is it related to time period?
10. Sound waves in air are also called pressure waves. Why?
11. How does velocity of sound change in air with (i) change in temperature and (ii) change in air pressure?
12. Describe in detail the various characteristics of sound.
5. 444.4 m/s
8. 1530 m
Sound waves, like light waves also get reflected when they fall on the surface of an obstacle. But unlike light wave, they do not necessarily require a polished surface for reflection. The following experiment establishes that reflection of sound follows the same laws as those for reflection of light.
(i) Place a large plane board, AB of a metal or wood in the vertical position as shown in the figure.
(ii) Take two hollow metallic tubes P and Q of same size and place them in the plane of the paper and in positions inclined to the board.
(iii) A cardboard screen S is placed between the two tubes so that the sound produced by the watch does not reach the ear directly.
(iv) Hold a small watch W at the free end of the tube P and try to hear the ticking sounds of the watch by positioning the ear at E.
(v) The position where the ticking sound of the clock is the loudest the tubes P and Q are found to be inclined to S at the same angle.
(vi) If the tube Q is lifted slightly vertically upwards, no sound is heard.
From the above experiment we obtain the following two laws for the reflection of sound waves. These laws are :
First law : The angle of reflection (r) is always equal to the angle of incidence (i)
i.e., r = i or i = r
Second law : The incident wave, the reflected wave and the normal (at the point of incidence, all lie in the same plane.
2.1 ECHO
An echo is defined as the phenomenon of repetition of sound of a source by reflection from an obstacle. It is a very common experience that when we utter a few words in a high domed hall the words are repeatedly heard on account of reflection from the original sound, the obstacle must be situated at a suitable distance. The sensation of sound lasts in our brain for (1/10) of a second. This property is called persistence of hearing. |
Thus, the time interval between the original sound and the reflected one must be at least 0.1s. The total distance covered by the sound from the point of generation to the reflecting surface and back should be at least 344 m/s 0.1s = 34.4 m. Thus, for hearing distinct echoes, the minimum distance of the obstacle from the source of sound must be half of this distance that is 17.2 m. This distance will change with the temperature of air.
2.1.1 Multiple echoes
When sound is repeatedly reflected from a number of obstacles at suitable distance, many echoes are heard one after the other. This constitutes multiple echoes.
Examples :
Multiple echoes are heard when some sound is produced between two distant buildings or cliffs.
The rolling sound of thunder is on account of multiple echoes due to successive reflections from a number of reflecting surfaces such as mountains, clouds, land, rocks and various layers of air of different densities.
Check Out- CBSE Class 9 Science Notes
(a) Uses of multiple reflection of sound : The phenomenon of multiple reflection of sound is put to many uses as described below :
(i) Megaphone : To confine the sound waves so that they travel in a particular direction, a megaphone is used. Sound waves which are now confined in a particular region by their multiple reflections from the walls of the tube travel larger distance than without the help of the tube. Horns, musical instruments like trumpets etc, are based on the principle. |
(ii) Ear trumpet : It is a sort of machine used by persons who are hard of hearing. The sound energy received by the wide end of the trumpet is concentrated into a much smaller area at the narrow end by multiple reflections. This makes the otherwise inaudible sound audible to the user.
(iii) Hearing aid : An ear trumpet is a mechanical device helpful only to a person with mild hearing loss. A hearing aid is an electronic device which is battery operated and is used by people with severe hearing loss. A hearing aid is fitted with a microphone which converts sound waves into electrical signals. An electronic amplifier amplifies these signals which are then fed to a speaker in the hearing aid. The speaker converts the amplified electrical signals into sound which is sent to the ear for clear hearing.
(iv) Stethoscope : It is a medical instrument used frequently by doctors for making a rough diagnosis of the diseases existing inside the body at places which are either inaccessible or accessible only through major operations. |
Working : The metal case C containing the diaphragm is gently pressed against the part of the body to be examined. The vibrations of this part are communicated to the diaphragm D which starts vibrating. As shown in figure, these vibrations suffer multiple reflections in the tubes R, T, T1 and T2 and ultimately reach the earphones. The original sound produced by the part of the human body is exactly reproduced in the earphones and a preliminary diagnosis of the ailment is made.
(v) Concert halls, Cinema halls and conference halls
The ceilings of these halls are curved. This enables the sound to reach all corners of the hall after reflection from the ceiling as shown in figure.
A sound board, which is a curved (parabolic or concave) sound reflecting surface is placed behind the stage, the source of sound is located at the focus of this reflecting surface.
2.2 REVERBERATION
When a sharp sound is made in a hall, the listener cannot hear it as such. It is found to get prolonged. The intensity of sound first reaches a maximum and then falls till it becomes inaudible. It is interesting to note that a sound wave suffers 300 reflections in a room of ordinary size before becoming inaudible.
Thus, the phenomenon of persistence or prolongation of audible sound after the source has stopped emitting sound is called reverberation. The time for which reverberation persists until it becomes inaudible is called reverberation time.
2.2.1 How is reverberation reduced?
Since reverberation is due to repeated reflections of sound waves from the ceiling, floor, walls etc. of a hall or an auditorium, we can reduce it by increasing the absorption of sound energy. To achieve this :
(i) The walls are covered with some sound absorbing material like felt, fiberboard, glass wool etc. or by heavy curtains with folds.
(ii) The floor is carpeted.
(iii) The furniture is upholstered.
(iv) False ceiling of a suitable sound absorbing material is used.
A certain amount of reverberation is always desirable. This would enrich the music played or speech delivered and makes it more pleasant. So the real job is to reduce reverberation to the right amount and not to completely eliminate it. |
There are three categories of longitudinal mechanical waves which cover different ranges of frequencies.
Sound waves or audible waves : These waves have frequencies which lie between 20 Hz to 20 kHz. This range of frequencies is called the audio-frequency (a.f.) range to which human ear is sensitive. These waves are generated in a variety of ways such as musical instruments, human vocal cords, insects and loudspeakers.
The frequency of sound waves emitted by a grown up male varies from 100 Hz to 250 Hz whereas that for children, it varies from 200 Hz to 450 Hz. The frequency of sound waves given by a honey bee is about 440 Hz, that of a mosquito around 500 Hz to 600 Hz and that of an ordinary housefly is around 350 Hz. |
Infrasonic waves or infrasound: Those longitudinal waves whose frequencies are below 20 Hz are called infrasonic. Earthquake waves are an example. During earthquakes, low frequency infrasound as low as 5 Hz is produced whereas whales and elephants also produce sound in the infrasound range. In some instances, however, infrasound has proved dangerous. It is found that at very low frequencies of 5 Hz to 10 Hz, certain organs of the body tend to resonate, leading to vibration-induced illness. This resonating of one organ leads to rubbing against another, thereby producing noticeable ill effects.
Ultrasonic waves or ultrasound: Those longitudinal waves whose frequencies lie above 20 kHz are called ultrasonic waves, ultrasonics or ultrasounds. Though human ear cannot detect these waves, certain creatures such as mosquito, fish, dog and bat show response to these frequencies.
3.1 APPLICATIONS OF ULTRASOUND
In contrary to audible sounds (which have lower frequencies), ultrasound can be obtained in the form of a narrow beam which can travel along well-defined paths even in the presence of obstacles. Such well-defined narrow beams of ultrasonic waves find application in many fields.
Industry
Medical science
Communication (SONAR)
3.1.1 Industrial uses of ultrasound
The various uses of ultrasound in industries are as follows :
(a) Cleaning instruments and electronic components : The cleaning is done by the method called cavitation or coldboiling. An instrument that needs cleaning but whose parts cannot be reached directly is placed in a liquid. The ultrasonic waves passing through the liquid produce tiny bubbles where the rarefaction of the ultrasonic wave reaches. When the compression of the wave reaches these bubbles, the bubbles are compressed until they implode (explode inward). This leads to the creation of several small localized shockwaves. These shock waves blast away any dirt or contamination from the unreachable portions, usually, frequencies in the range of 20 kHz to 30 kHz are used for this purpose.
(b) Plastic welding : Application of small pressures and ultrasonic vibration to two similar surfaces produce sufficient thermal energy to bond the surface together.
(c) Detecting flaws and cracks in metal blocks : To construct big structures like buildings, bridges, machines and scientific equipment, a large number of metallic blocks are assembled together. Cracks and holes within the blocks, which are invisible from outside, reduce the strength of a structure. To detect these flaws (cracks and holes) in a block, ultrasonic waves are passed through it.
3.1.2 Medical uses of ultrasound
(a) Echocardiography : It is used to study the heart-valve action. An image of the heart is obtained by getting ultrasonic waves reflected from various parts of the heart.
(b) Ultrasonography : It involves sending ultrasonic waves to various organs (like brain, liver, kidneys) in the body and looking at the reflected or transmitted waves. Using ultrasonography, stones in gall-bladder and kidneys or tumors in different organs can be detected. Ultrasonography is also used in prenatal examinations.
(c) Therapeutic uses : Ultrasound is used for treatment of neuralgic and rheumatic pains.
3.1.3 Sonar
It is an acronym which means SOund Navigation And Ranging.
A sonar is a device which measures the distance, direction and speed of objects lying under water using ultrasonic waves.
A sonar, which is installed in a ship or a boat, consists of (i) a transmitter and (ii) a detector. The ultrasonic waves produced by the transmitter travel through water. After getting reflected by the object on the seabed, these waves are picked up by the detector. The detector converts the reflected ultrasonic waves into electrical signals which are properly recorded.
Let t = time interval between the transmission and reception of the reflected ultrasound waves,
v = speed of sound through sea water,
d = distance of the object that reflected the ultrasound.
Clearly, total distance travelled by the ultrasound = 2 d
As distance = speed time
2 d = v t
or
The above method of finding the distance of an object is called echo-ranging as it is based on echo principle. The sonar technique is used to :
(i) Determine depth of the sea, called echo depth ranging.
(ii) Locate underwater hills, valleys, icebergs, submarines and sunken ships.
(iii) To locate the position of other ships or submarines.
(iv) Ship to ship communication.
The advantage of using ultra sonic waves is that these waves cannot be heard without the aid of special instruments.
We hear with an extremely sensitive device called the ear. It allows us to convert pressure vibrations in air with frequencies 20 Hz to 20 kHz into electric signals that travel to the brain via auditory nerve.
Auditory parts of human ear
The outer ear is called Pinna. It collects the sound from the surroundings. The collected sound passes through the auditory canal. At the end of the auditory canal, there is a thin membrane called the eardrum or tympanic membrane. When compression of the medium produced due to vibration of the object reaches the eardrums, the pressure on the outside of the membrane increases and forces the eardrum inward. Similarly the eardrum moves outward when a rarefaction reaches. In this way, the eardrum vibrates. The variations are amplified several times by the bones (the hammer, anvil and stirrup) in the middle ear which acts as levers. The middle ear transmits the amplified pressure vibrations received from the sound wave to the inner ear. In the inner ear, the pressure variations are turned into electrical signals by the cochlea. These electrical signals are sent to the brain via the auditory nerve, and the brain interprets them as sound.
Question: Why are the ceilings of concert halls curved?
Answer: The ceilings of the concert halls are curved to ensure that after reflection from the ceilings, sound reaches all corners of the hall.
Question: What is the audible range of the average human ear?
Answer: 20 Hz to 20,000 Hz.
Question: What is the range of frequencies associated with
(a) infrasound?
(b) ultrasound?
Answer: (a) Range of frequencies associated with infrasound : < 1 Hz to 20 Hz.
(b) Range of frequencies associated with ultrasound: > 20 KHz.
Question: Explain how bats use ultrasound to catch a prey.
Answer: The ultrasonic waves emitted by the bat are reflected from the prey (e.g., an insect) and are detected by bat’s ear. The nature of reflected waves tells the bat:
(i) the location and (ii) the nature of its prey.
Question: How is ultrasound used for cleaning?
Answer: The object to be cleaned is placed in a cleaning solution. When ultrasonic waves are passed through the solution, due to their high frequency, particles of dust, dirt and grease get detached even from the unreachable portions of the object and drop out in the solution.
Question: A sonar device on a submarine sends out a signal and receives an echo 5 s later. Calculate the speed of sound in water if the distance of the object from the submarine is 3625 m.
Answer: Here, time interval between the transmission of the signal and its reception, t = 5 s distance of the object from the submarine, d = 3625 m
If v is the speed of sound in water, then
2 d = vt or v = = = 1450 m/s.
Question: An echo is returned in 3 s. What is the distance of the reflecting surface from the source, given that the speed of sound is 342 m/s.
Answer: Here, speed of sound, v = 342 m/s
time taken by echo to return, t = 3 s
If d is the distance between the source and the reflecting surface, distance covered by sound in time t = d + d = 2 d
(distance d while going to the reflecting surface and distance d while returning back)
As distance = speed (of sound) x time,
2 s = 342 (m/s) x 3 (s) = 1026 m
s = 513 m
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: The minimum distance (d) for the distinct echo to be heard (say at 22°C) is 17·2 m ( 2 d = vt = 344 x 0·1 = 17·2 m). On a hotter day, the temperature increases and the speed of sound in air also increases and as such 2 d = 356 x 0·1 = 35·6 m or d = 17·8 m. Thus, if the distance of the reflecting surface and the source of sound remains the same (i.e., 17·2 m), no echo is heard on the hotter day as the minimum distance now required is 17·8 m.
Question: A submarine emits a sonar pulse, which returns from an underwater cliff in 1·02 s. If the speed of sound in water is 1531 m/s, how far away is the cliff?
Answer: If d is the distance of the underwater cliff from the submarine, distance travelled by sonar pulse while going from the submarine and returning to it after getting reflected from the cliff = 2 d
Here, time taken by sonar pulse to return, t = 1·02 s
speed of sound in water, v = 1531 m/s
As total distance travelled by the sonar pulse = speed of sound x time,
2 d = vt
or 2 d =
i.e., d = = 780.81 m.
Question: Give two practical applications of reflection of sound waves.
Answer: (i) Megaphone and (ii) Ear trumpet.
1. Megaphone : Sometimes we want a given sound to travel a large distance before it becomes inaudible. This can be done if we avoid the wastage of sound energy by its transmission in all directions. We, therefore, confine the sound waves with the help of a speaking tube or a megaphone so that they travel in particular direction.
2. Ear Trumpet : It is a sort of machine used by persons who are hard of hearing. The sound energy received by the wide end of the trumpet is concentrated into a much smaller area at the narrow end by multiple reflections. The narrow end of the trumpet which is inserted in the ear delivers the entire amount of energy falling on the wide end which makes the otherwise inaudible sound audible to the user.
Question: What is reverberation? How can it be reduced?
Answer: The phenomenon of persistence or prolongation of audible sound after the source has stopped emitting sound is called reverberation. The time for which reverberation persists until it becomes inaudible is called reverberation time.
Since reverberation is due to repeated reflections of sound waves from the ceiling, floor, walls etc. of a hall or an auditorium, we can reduce it by increasing the absorption of sound energy. To achieve this :
(i) The walls are covered with some sound absorbing material like felt, fiberboard, glass wool etc. or by heavy curtains with folds.
(ii) The floor is carpeted.
(iii) The furniture is upholstered.
(iv) False ceiling of a suitable sound absorbing material is used.
Question: Explain how the human ear works.
Answer: (i) The outer ear collects sound waves which are conducted through the auditory canal.
(ii) These waves fall on the ear drum and set it into vibrations.
(iii) The middle ear which is of the size of a small marble and houses osscicles (three bones: hammer, anvil and stirrup) amplifies these oscillations about 60 times.
(iv) The inner ear which contains cochlea and is filled with a fluid converts these pressure variations into electrical signals.
(v) These electrical signals are conveyed to the brain via auditory nerve for interpretation.
Q. 1 and 2 are of one mark each.
Q. 3 – 5 are of two marks each.
Q. 6 – 8 are of three marks each.
1. What is the audible range of the average human ear?
2. Distinguish between echo and reverberation of sound.
3. A man shouts inside a deep well and hears the echo after 0.4 second after shouting. If the speed of sound is 340 ms-1, find the depth of the water level in the well.
4. A Sonar device attached to a ship sends ultrasonic waves in the sea. These waves are reflected from the bottom of the sea. If the ultrasonic waves take 4 sec to travel from ship to the bottom of sea, and back to ship. What is the depth of sea? (speed of sound in water = 1500 m/s).
5. Can multiple echoes of a single sound be produced? How?
6. Give two medical applications of ultrasound.
7. What is an echo? Why do we not get echo in all rooms? What is the minimum size of the room required for echo to be heard?
8. What is SONAR? What is the basic principle of its working? How is it used to determine the depth of a sea?
3. 68 m
4. 3000 m