Sound is a form of energy which stimulates our sense of hearing. Everyday, we hear many sounds all around us all the time: the radio blaring, the telephone bell ringing, students, friends’ talking to each other and so on. How is sound produced? If we touch a silent bicycle bell we do not find anything special in it. If we ring the bell and touch it gently, we’ll find that the bell is vibrating continuously till such time that it stops ringing. As it is ringing if we hold it tightly, the vibrations will stop and no sound will be heard. This clearly tells us that sound is produced by vibrating bodies. The vibrating body has a certain amount of energy which travels in the form of sound waves. This energy is provided to it by some outside source.
Sound can be produced by the following methods:
(i) by vibrating strings as in veena, violin, etc., known as string instruments.
(ii) by vibrating air as in flute, trumpets, etc., known as wind instruments.
(iii) by vibrating membranes as in tabla, drums, etc., known as percussion instruments.
(iv) by vibrating metal plates as in bells, etc.
Propagation of Sound in Various Media
A wave is a disturbance that moves through a medium when the particles of the medium set the neighbouring particles in motion. These in turn produce similar motion in others. The particles themselves do not move but the disturbance is carried forward. Wave motion is produced due to the repeated vibrations of the particles of the medium. This is what happens when sound is propagated through a medium.
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 source to our ears. There is no actual movement of the particles from the source to our ears. The particles only vibrate back and forth and transfer sound energy from one to another till it reaches our ears. The material through which sound travels is known as the medium. The medium can be a solid, liquid or a gas. Sound needs a medium to travel. It cannot travel through vacuum.If a train is very far away and we cannot see it, we cannot hear the sound through air. But if we place our ears on the railway track, we’ll hear the sound of the approaching train. This shows that sound can travel through solids.
Place a squeaking toy in a plastic bag and immerse it in water. Ask your friend to place his / her ears on the side of the bucket of water and squeeze the toy. Your friend will be able to hear the toy squeak. This shows that sound can travel through liquids also. This fact has been used in the detection of submarines or any other submerged objects under sea. You are able to hear your teacher speak or your friends speak to you because air is the medium through which the sound waves are traveling in this case. Sound cannot travel through vacuum. The bell jar experiment helps to prove this.
Sound Waves are Longitudinal Waves
Take a slinky. Mark a dot on it. Fix one end to a wall. Hold the other end. Give it a sharp push towards the wall. You will notice the dot move back and forth, parallel to the direction of propagation of the disturbance. There are regions where the coils of the slinky are pressed close to one another (compressions) followed by regions where the coils are further apart (rarefactions). This is how a sound wave also travels.The particles of the medium just oscillate about their mean position. Hence sound waves are longitudinal in nature.Another type of wave is called transverse wave. Light rays, electromagnetic waves are all transverse in nature. In this type of wave, the particles of the medium move about their mean position, in a direction perpendicular to the direction of propagation of the wave.Both longitudinal and transverse waves are mechanical waves.
Speed of Sound
Sound takes some time to travel from the sound-producing body to our ears. The speed of sound tells us the rate at which sound travels from the sound-producing body to our ears. The speed of sound depends on a number of factors. These are given below:
1. The speed of sound depends on the nature of material (or medium) through which it travels. The speed of sound is different in different materials (or different media). For example, the speed of sound in different materials like air, water andiron is different. At room temperature, the speed of sound in air is 344 m/s; the speed of sound in water is about 1500 m/s; and the speed of sound in iron is 5130 m/s. In general, sound travels slowest in gases, faster in liquids and fastest in solids.
2. The speed of sound depends on the temperature. For example, the speed of sound in air at a temperature of 0° C is 332 m/s but the speed of sound in air at a temperature of 20 C is 344 m/s. In fact, as the temperature of air rises, the speed of sound in it increases. Thus the speed of sound in air will be more on a hot day than on a cold day.
3. The speed of sound depends on the humidity of air. For example, the speed of sound is less in dry air but more in humid air. In other words, sound travels slower in dry air but faster in humid air. In fact, as the humidity of air increases, the speed of sound through it also increases.
Characteristics of a Sound Wave
A wave can be completely described by certain terms like wavelength, wave velocity, wave frequency, time period, amplitude, etc. A sound wave is represented graphically in the following figure. Density and Pressure varies when the sound wave moves in the medium. The density and pressure of the medium at a given point of time varies with distance above and below the average values for both. Compressions are the regions where the particles are close together and are represented by the upper portion of the curve in the graph. The peak represents the maximum compression. Hence compressions are the regions where pressure and density are very high. Rarefactions, on the other hand, are low pressure regions where the particles of the medium are spread apart. In the below figure they are represented by the valleys in the lower portion of the curve. The peak forms the crest and the valley is the trough of a wave.
The distance between two consecutive compressions or two consecutive rarefactions is called a wavelength. This is shown in the figure. It is represented by the Greek letter lambda (λ). The SI unit for wavelength is m.Frequency tells us rate at which the waves are produced by their source. 'The
number of complete waves (or cycles) produced in one second or the number of vibrations per second' is called frequency of the wave. The unit of frequency is hertz (Hz). One hertz is one vibration per second. Frequency is denoted by the Greek letter 'nu' (n).'Time period of a wave is the time taken by two consecutive compressions or two consecutive rarefactions to cross a fixed point'. Or the time taken for one complete oscillation in the density of the medium ( move from the maximum value to the minimum value and back to the maximum value) is known as time period of the wave. It is denoted by T and the SI unit is second (s). 'The frequency of a wave and its time period are reciprocal of one another'. Suppose the time period of a wave is T seconds.In T seconds, the number of wave produced is 1.Then in 1 s the number of waves produced = 1/T. But the number of waves produced in 1 second is called the frequency of the wave. Therefore,frequency and time period are reciprocal of one another.The maximum displacement of the particles of the medium from their mean position, when a wave passes through the medium,is called the amplitude of the wave. It measures the height of a crest or the depth of a trough. It is represented by 'A'. The force with which an object is made to vibrate determines the amplitude of vibrations.The distance travelled by a wave in one second is known as wave velocity. It is denoted by V. The SI unit for wave velocity is m/s. Speed = Distance / Time
Therefore V = λ/T
= λ x 1/T
= λ ν
Characteristics of Sound
Any sound has the following characteristics:
Its loudness – which depends on the amplitude of vibrations. Greater the amplitude the louder the sound. Loud sounds can travel larger distances as they have more energy.
Its pitch – which depends on the frequency of vibrations. The faster the vibrations, the higher the frequency and so the higher the pitch. A high pitch sound corresponds to more number of compressions and rarefactions passing a fixed point per unit of time. Different objects of varying sizes and conditions vibrate at different frequencies to produce sounds of different
pitch.
Its quality or timbre – this helps us to identify one sound from another, one voice from another having the same pitch and loudness. Pleasant sounds have rich quality.
Its intensity – it is the amount of energy that passes through unit area each second. Loudness and intensity are not the same. Loudness is a measure of the response of the ear to the sound. Two sounds may be of the same intensity but we may be able to hear one as louder than the other because our ear detects it better.
Reflection of Sound
We have studied the reflection of light in earlier classes. Just like light, sound can also be made to change its direction and bounce back. The bouncing back of sound when it strikes a hard surface is called reflection of sound. Hard surfaced like walls, metal sheets, plywood, etc., reflect sound waves. Sound is reflected in the way as light. The laws of reflection of light are obeyed during the reflection of sound. The reflection of sound, however, does not require a smooth and shining surface like that of a mirror. Sound can be reflected from any hard surface,whether smooth or rough.The reflection of sound follows the law "angle of incidence equals angle of reflection", sometimes called the law of reflection. The same behavior is observed
with light and other waves, and by the bounce of a billiard ball off the bank of a table. The reflected waves can interfere with incident waves, producing patterns of constructive and destructive interference. This can lead to resonance called standing waves in rooms. It also means that the sound intensity near a hard surface is enhanced because the reflected wave adds to the incident wave, giving a pressure amplitude that is twice as great in a thin "Pressure Zone" near the
surface.This is used in pressure zone microphones to increase sensitivity. The doubling of pressure gives a 6 decibel increase in the signal picked up by the microphone.
Reflection of waves in strings and air columns are essential to the production of resonant standing waves in those systems. Echoes are produced due to the reflection of sound.
Echo
When we stand in one corner of a big empty hall and shout the word ‘hello’, we will hear the word coming from the empty hall in the form of an echo a little while later.It appears as if the hall is repeating our ‘hello’. This happens because the sound of our ‘hello’ is reflected from the walls of the hall and this reflected sound forms the echo (which we hear as ‘hello’ coming from the empty hall). We can now say that'The repetition of sound caused by the reflection of sound waves is called an echo'.When a person shouts in a big empty hall, we first hear his original sound. After a little while, we hear the reflected sound of his shout. This ‘reflected sound’ is an‘echo’. Thus, an echo is simply a reflected sound. If we shout at a wall from 344 m away, the sound takes 1 second to reach the wall. The sound reflects from the wall, and takes 1 second to return. So, we hear the echo 2 seconds after we have shouted. We can now calculate the minimum distance from a sound-reflecting surface (like a wall), which is necessary to hear an echo.
Reverberation
The persistence of sound even after the source is stopped is called reverberation. Repeated reflections of a sound in a hall will persist until it is reduced to value where it is no longer heard by us. This repeated reflections that persists are called reverberation. In big theatres and auditoriums reverberations are not welcome. So,sound absorbing acoustic materials like fibre board, rough plaster or draperies are used to line the walls and roofs of these halls.The reverberant sound in an auditorium dies away with time as the sound energy is absorbed by multiple interactions with the surfaces of the room. In a more reflective room, it will take longer for the sound to die away and the room is said to be 'live'. In a very absorbent room, the sound will die away quickly and the room will be described as acoustically 'dead'. But the time for reverberation to completely die away will depend upon how loud the sound was to begin with, and will also depend upon the acuity of the hearing of the observer. In order to providea reproducible parameter, a standard reverberation time has been defined as the time for the sound to die away to a level 60 decibels below its original level. The reverberation time can be modeled to permit an approximate calculation. The picture given below is the reverberation room in which one can experience reverberations.
Uses of Multiple Reflection of Sound
We will now discuss some of the practical applications of the reflection of sound.The reflection of sound is utilized in the working of devices like megaphone, sound
boards and ear trumpet. These are described below:A megaphone (or speaking-tube) is a horn-shaped tube, which is used to address a]small gathering of people at places like tourist spots, fairs, and market places and during demonstrations. One end of the megaphone tube is narrow and its other end is quite wide. When a person speaks into the narrow end of the mega phonetube, the sound waves produced by his voice are prevented from spreading by successive reflections from the wider end of the megaphone tube. Due to this the sound of the voice of the person can be heard over a longer distance.Thus, a megaphone (or speaking-tube) works on the reflection of sound. The loudspeakers also have horn-shaped opening so that the sound of the voice of speaker (or music) can be heard by a large gathering over a considerable distance.The sound board is a concave board (curved board), which is placed behind the speaker in large halls or auditoriums so that his speech can be heard easily 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 sound board.The concave surface of the sound board 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 his speech easily. It is obvious that
the sound boards work on the reflection of sound.The ear trumpet is a hearing aid, which is used by the persons who are hard of
hearing. One end of the ear trumpet is wide, whereas its other end is narrow. The sound waves from a large surrounding area fall on the wide end of the trumpet,which reflects them into its narrower end that leads into the ear. In this way, more sound energy falls on the eardrum of the person and that leads into the ear. In this way, more sound energy falls on the eardrum of the person and improves his hearing capacity. Thus, an ear trumpet also works on the reflection of sound.In a stethoscope the heart beat of a patient reaches the doctor’s ears by multiple reflection of sound.
Uses of Sound
1. We use sound mainly to communicate with each other.
2. In SONAR (Sound Navigation and Ranging) sound is used to determine the
position of a submarine or the depth of the sea at any place. The principle used is
that of reflection of sound.
3. Sound finds many applications in entertainment electronics (TV, radio, cassette
and compact disc (CD) players, public address system, etc.)
4. Ultrasonics are used in the fields of engineering, industry, diagnostic, medicine,
surgery, etc.
5. Infrasonics are used in drilling deep oil wells.
Range of Hearing in Humans
Uses of Sound
If the frequency of the sound wave is less than 20 Hz or greater than 20,000 Hz, it cannot be heard by human beings. The human ear can hear sounds of frequencies between 20 Hz and 20,000 Hz. This is known as the frequency range of hearing or audible range in humans. Frequencies below 20 Hz are called infrasonic. e.g.earthquakes, volcanic eruptions, sounds made by some animals like whales etc. Frequencies over 20,000 Hz are called ultrasonic or ultrasound. Dogs can hear ultrasonic frequencies up to 50,000 Hz. That is why they are used for detective
purposes.
Ultrasound
The sounds having too high frequency which cannot be heard by human beings are called ultrasonic sound or ultrasound. In other words, the sounds having frequency greater than 20,000 hertz are called ultrasound. For example, a sound of frequency 100,000 hertz is an ultrasound. The ultrasound is reflected just like ordinary sound waves and produce echoes. But the echoes produced by ultrasound cannot be heard by our ears, they can only be detected by special equipment. Due to its very high frequency, ultrasound has a greater penetrating power than ordinary sound.So, it can be used to detect objects under the sea and organs inside the human body. Ultrasound is used for a large number of purposes these days.
The ultrasound machine transmits high-frequency (1 to 5 megahertz) sound pulses into your body using a probe.The sound waves travel into your body and hit a boundary between tissues(e.g. between fluid and soft tissue, soft tissue and bone).Some of the sound waves get reflected back to the probe, while some travel on further until they reach another boundary and get reflected. The reflected waves are picked up by the probe and relayed to the machine.The machine calculates the distance from the probe to the tissue or organ (boundaries) using the speed of sound in tissue (5,005 ft/s or 1,540 m/s) and the time of the each echo's return (usually on the order of millionths of a second).The machine displays the distances and intensities of the echoes on the screen,forming a two dimensional image.In a typical ultrasound, millions of pulses and echoes are sent and received each second. The probe can be moved along the surface of the body and angled to obtain various views.Ultrasound image of a growing fetus (approximately 12 weeks old) inside a mother's uterus. This is a side view of the baby, showing (right to left) the head,
neck, torso and legs.
Applications of Ultrasound
(i) Ultrasound is used in SONAR to measure the depth of the sea and to locate the underwater objects like the shoals of fish, shipwrecks, submarines, sea- rocks and ice- bergs in the sea.
(ii) Ultrasound is used to investigate inside the human body. Echocardiography helps the doctors to study the condition of the heart of a patient. Stones in the gall bladder, kidneys can be located with ultrasound. Ultra sonography helps to study the development of the fetus during pregnancy, to detect any abnormalities.
(iii) Ultrasound is used in industry for detecting flaws in metal blocks or sheets without damaging them. It is used to clean delicate machine parts or parts that are
located in hard-to-reach places. These are placed inside the cleaning solution which is then subjected to high frequency waves. These dislodge the dirt and dust particles and the parts get thoroughly cleaned.
A whale sends out its sounds and songs and (those sound waves reflect-off a fish swimming towards the whale) the whale uses those reflected sound waves to determine where the fish is and what direction it is swimming.
SONAR
SONAR stands for Sound Navigation and Ranging. It is a device used to find the depth of seas, to measure the distance, direction and speed of underwater objects.It consists of a transmitter and receiver both mounted on the ship.The transmitter produces ultrasonic waves which travel through water, strike the object on the sea bed and get reflected to the detector or receiver. Knowing the speed of sound in water and the time taken for the sound to be received, the depth can be calculated.
The total distance 2d = v x t
where v is the speed of sound in sea water and t is the time taken for the wave to travel to and fro. This method is called echo ranging.
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