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Sound is a form of energy which produces a sensation of hearing in our ears.
Sound has a history, from the first sound ever, to the first sound heard by an animal, to the first recorded by a human being.
The first sound ever was the sound of the Big Bang.
The first organism to be able to hear things was probably the bony fish, which appeared on this planet about 400 million years ago. The first sound that we recorded as a species was gathered by a device called a phonautograph, invented by a man named Édouard-Léon Scott de Martinville in 1857. In 1875, Alexander Graham Bell transmitted the first sound vibrations between two receivers. The first radio broadcast, speech transmitted without wires, on December 23, 1900. The first cellphone call was made on April 3, 1973, by Martin Cooper. | ||||||||||||||||||||||||||||||||||||
| Production of Sound
Sound is produced due to the vibration of objects. Vibration is rapid to and fro motion of the particles of an elastic body about a central position. The vibrating body causes the medium (water, air, etc.) around it to vibrate. For example,
When the head of a drum is hit with a mallet, the drumhead vibrates and produces sound waves.
Stretched strings of a guitar vibrate to produce sound. | ||||||||||||||||||||||||||||||||||||
| Humans Produce Sound Humans have a voice box which produces sound which is also known as larynx. It is present on the top of the wind-pipe (or trachea). The larynx contains two ligaments known as vocal cords. The vocal cords are type of string. When it vibrates sound is produced. | ||||||||||||||||||||||||||||||||||||
| Propagation of sound
Sound is produced by vibrating objects and moves through a medium from the point of generation to the listener's ear as a wave. The matter or substance through which sound is transmitted is called a medium. It can be solid, liquid, or gas.
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| Sound Needs a Medium to Propagate Sound is a mechanical wave and needs a material medium like solid, liquid, or gas to travel and be heard. It cannot travel through a vacuum. Sound can travel through solid, liquid and gasses because the molecules of solid, liquid and gasses carry sound waves from one place to another. But sound cannot travel through vacuum because it has no molecules which can vibrate and carry sound waves. | ||||||||||||||||||||||||||||||||||||
| Sound on Outer Space Sound cannot be heard on the surface of moon because there is no air on the moon to carry the sound waves. Astronauts who land on moon talk to each other through wireless sets using radio waves. This is because radio waves can travel through vacuum. | ||||||||||||||||||||||||||||||||||||
| Sound Wave
A wave is a disturbance that moves through a medium when the particles of the medium oscillate about a stable equilibrium position and set neighbouring particles into motion.
They in turn produce similar motion in others. The particles of the medium do not
move forward themselves, but the disturbance is carried forward. In wave, energy is transferred from one point to another without any direct contact between them. Sound waves are characterised by the
motion of particles in the medium and are called mechanical waves.
On the basis of direction of propagation, mechanical waves are categorized into two parts:
(i) Longitudinal waves
Another type of waves which do not require any medium for propagation are called electromagnetic waves. These waves can travel through vacuum also. For example,
(ii) Transverse waves
Light waves, X-rays etc.
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| Longitudinal waves
The wave in which the individual particles of the medium move in a direction parallel to the direction of propagation of the disturbance is called a longitudinal wave.
For example,
Sound wave, ultrasound waves, seismic P-waves.
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| Transverse waves
The wave in which the individual particles of the medium move about their mean positions in a direction perpendicular to the direction of wave propagation is called a transverse wave.
For example,
Ripples on the surface of water, vibrations in a guitar string, waves produced in a stretched string,
electromagnetic waves – eg light waves, microwaves, radio waves, seismic S-waves.
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| Compressions and Rarefactions
The material through which the waves travel is called the medium. Air is the most common medium through which sound travels. All sounds are caused by vibrations. When a vibrating object moves forward, it pushes and compresses the air in front of it creating a region of high pressure. This region is called a compression.
Compression is the part of the wave in which particles of the medium are closer to one another forming the region of high pressure and density. Compressions are represented by the upper portion of the curve called the crest.
The peak represents the region of maximum compression.
As the object moves back and forth rapidly, a series of compressions and rarefactions is created in the air. These make the sound wave that propagates through the medium. Propagation of sound can be visualised as propagation of density variations or pressure variations in the medium. | ||||||||||||||||||||||||||||||||||||
| Characteristics of a sound wave
Sound waves have following characteristics:
(i) Amplitude
(ii) wavelength (iii) Frequency (iv) Time period (v) Speed | ||||||||||||||||||||||||||||||||||||
| Amplitude
The maximum displacement of each particle from its mean position is called amplitude.
It is denoted by A.
Its SI unit is metre (m).
The loudness or softness of a sound is determined by its amplitude. The sound is perceived as louder if the amplitude increases, and softer if the amplitude decreases. The amplitude of the sound wave depends upon the force with which an object is made to vibrate. | ||||||||||||||||||||||||||||||||||||
| Wavelength The distance between two consecutive crests or two consecutive troughs is called the wavelength. It is denoted by the Greek letter lambda (λ). Its SI unit is meter (m). | ||||||||||||||||||||||||||||||||||||
| Frequency The number of oscillations per unit time is called frequency. It is usually denoted by the Greek letter nu (⋎). It is also denoted by the symbol n or f. Its SI unit is hertz (symbol, Hz). The SI unit of frequency was named as hertz in the honour of Heinrich Rudolph Hertz. | ||||||||||||||||||||||||||||||||||||
| Time period
The time taken for one complete oscillation is called time period.
It is denoted by the letter T.
Its SI unit is second (s).
The frequency of a wave is the reciprocal of the time period.
Frequency =
1/Time Period i.e., ⋎ = 1/T Or, ⋎ T = 1 | ||||||||||||||||||||||||||||||||||||
| Speed
The distance travelled by a wave in one second is called speed of the wave.
Its S.I. unit is metres per second (m/s).
Speed =
The speed of sound remains almost the
same for all frequencies in a given medium
under the same physical conditions.
Distance/Time v = λ/T Where λ = wavelength of the wave travelled in one time period T. v = ⋎ λ (∵ ⋎ = 1/T )Thus, Velocity = Wavelength × Frequency v = ⋎ λ is called the wave equation. | ||||||||||||||||||||||||||||||||||||
| Characteristics of Sound
The basic characteristics of sound are:
(i) Pitch
(ii) Loudness (iii) Tone | ||||||||||||||||||||||||||||||||||||
| Pitch of Sound
The frequency of a sound wave, how the brain interprets it, is called its pitch. A higher frequency sound, faster the vibration of the source has a higher pitch, and a lower frequency sound has a lower pitch.
A high pitch sound corresponds to more number of compressions and rarefactions passing a fixed point per unit time.
Objects of different sizes and conditions vibrate at different frequencies to produce sounds of different pitches.
(i) It represents shrillness or flatness of sound.
(ii) It depends on the frequency of vibration. (iii) Pitch may be different for the different listeners for same frequency. (iv) Pitch depends on the number of waves perceived by the observer (listener). | ||||||||||||||||||||||||||||||||||||
| Loudness of Sound
The loudness or softness of a sound is determined basically by its amplitude. A larger amplitude means a louder sound, and a smaller amplitude means a softer sound. The amplitude of the sound wave depends upon the force with which an object is made to vibrate.
If we strike a steel pot lightly, we hear a soft sound because we produce a sound wave of less energy (amplitude). If we hit the pot hard we hear a louder sound. The loudness of a sound is also determined by the sensitivity of the ear. The human ear is most sensitive to frequencies between 2,000 and 5,000 Hz. The individual hearing range varies according to the general condition of a human's ears and nervous system. The volume we receive thus depends on both the amplitude of a sound wave and whether its frequency lies in a region where the ear is more or less sensitive.
(i) It depends on the amplitude of the sound wave.
(ii) Amplitude, as well as loudness, decreases with distance. (iii) Louder sound has higher energy. (iv) Louder sound can travel a larger distance. (v) Loudness of sound is measured in bel and decibels (dB). | ||||||||||||||||||||||||||||||||||||
| Music And Noise
The quality or timbre of a sound is the feature that enables us to distinguish one sound from another having the same pitch and loudness. The sound that is more pleasant is of richer quality.
Both music and noise are a mixture of sound waves of different frequencies. Music is the sound that is pleasant to hear and is of rich quality. Music is ordered sound. For example,
The sound emanating from a musical instrument.
Noise is the sound that is unpleasant to the ear and is of poor quality. Noise is a disordered sound. For example,
Children screaming or yelling. The sound produced by vehicles.
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| Tone and Note
A sound of single frequency is called a tone. A tone is represented by a single wave form. For example,
A musical note has tones of various frequencies (sounds of different pitch) and amplitudes (loudness).
The sound which is produced when several frequencies are mixed is called a note.
It is pleasant to listen to. A note is represented by many component waves in it.
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| Intensity of Sound
The amount of sound energy passing each second through unit area in a direction perpendicular to that area is called the
intensity of sound. It is also called acoustic intensity.
The SI unit of intensity, which includes sound intensity, is the watt per square meter (Wm−2).
The terms "loudness" and "intensity" are sometimes used in place of each other, but they are not the same. Loudness is a measure of the response of the ear to the sound. Even when the two sounds are of equal intensity, we may hear one as louder than the other because our ears detect it better. | ||||||||||||||||||||||||||||||||||||
| Speed of Sound in Different Media
The speed of sound is defined as the distance travelled per unit time by a sound wave as it propagates through an elastic medium.
The sound travels about 15 times faster in iron (or steel) than in air.
Factors affecting the speed of sound:
(i) Properties of the medium through which it travels.
The speeds of sound at 25oC temperature in various media are as follows:(ii) Temperature of the medium. (iii) When we go from solid to gaseous state, the speed of sound decreases. (iv) The speed of sound increases with the increase in temperature of the medium through which it travels. (v) The speed of sound increases with the increase in humidity of air.
Speed of Sound in Air(v) = 331 + 0.59 × ToC ms−1
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| Sound Travels at Slower Than Light The speed of light is much greater than the speed of sound. The reason we see a flash of lightning before hearing thunder is that light travels faster than sound. Speed of light in air is 300,000,000 or 3 × 108 ms−1. | ||||||||||||||||||||||||||||||||||||
| Audible and Inaudible Sounds Infrasonic sound: The sounds having too low frequencies which cannot be heard by human ear are called infrasonic sounds. Thus, the sounds of frequencies less than 20Hz are called infrasonic sounds. The range of audible frequencies of sound for human hearing is from 20Hz to 20,000Hz. | ||||||||||||||||||||||||||||||||||||
| Ultrasonic sound The sounds having too high frequencies which cannot be heard by human ear are called ultrasonic sounds. thus, the sounds of frequencies greater than 20,000Hz is called ultrasonic. The human beings can neither produce ultrasonic sound nor can they hear ultrasonic sound. The sound having too high frequencies which cannot be heard by human beings is just called ultrasound. Some of the important uses of ultrasound are i. Ultrasound is used as a diagnostic tool in medical science to investigate inside of human body. ii. Ultrasound is used to study the growth of foetus (developing baby) inside the mother’s womb. iii. Ultrasound is used in treatment of muscular pain and a disease called arthisis. iv. Ultrasound is to measure the depth of sea. It is also used locate under water objects like shipwrecks, submarine, and shoals of fish, etc. | ||||||||||||||||||||||||||||||||||||
| Audible Frequency The audible range of human ear is 20 Hz and 20,000HZ, i.e., the human ears can hear only those waves whose freqnecy lies between 20 Hz and 20,000HZ. | ||||||||||||||||||||||||||||||||||||
| Reflection of sound
Sound bounces off a solid or a liquid like a rubber ball bounce off a wall. Like light, the sound gets reflected at the surface of a solid or liquid and follows the same laws of reflection. When sound waves strike a surface, they return back into the same medium. This phenomenon is called reflection.
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| Laws of Reflection
The law of reflection of sound waves states that:
(i) The angle of incidence is always equal to the angle of reflection.
(ii) The incident sound wave, the normal to the reflecting surface, and the reflected sound wave lie in the same plane at the point of incidence. ∠i = ∠r
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| Applications of Reflection of Sound
Many instruments work on the basis of multiple reflection principles of sound. For example,
Megaphone, loudspeakers, bulb horns, stethoscope, hearing aid, sound board, musical instruments like trumpets, shehnais etc.
Loudspeaker, Megaphone, and bulb horn are designed to send sound by multiple reflections in a particular direction without spreading the sound. In such devices, due to multiple reflections, the amplitude of the sound waves adds up to increase the loudness of sound.
In such devices, a funnel-like cone-shaped tube is used. The sound is introduced at the narrow end of the tube and allowed to come out from the wider end. The name ‘Loudspeaker’ is named as it is used to make the sound louder.
Stethoscope | ||||||||||||||||||||||||||||||||||||
| Echo
Phenomenon of hearing back our own sound is called an echo.
It is due to successive reflection of sound waves from the surfaces or obstacles of large size.
To hear an echo, there must be a time gap of 0.1 second in original sound and the reflected sound.
Necessary Conditions for the formation of an Echo
There are two conditions to experience the echo of sound -
(i) Time gap between the Sound must come back to the person after 0.1 second.
(ii) For above condition, the reflecting surface must be at a minimum distance of 17.2m. It also depends on temperature. | ||||||||||||||||||||||||||||||||||||
| Reverberation Persistence of sound wave for a long time because of repeated (multiple) reflections of sound are called reverberation. In big concert halls, due to the reverberation, sound may become blurred and distorted to be heard. To avoid reverberation, soft sound absorbent materials, such as curtains, plant fibre, compressed fireboard, carpets, etc. are used in the auditorium. | ||||||||||||||||||||||||||||||||||||
Infrasonic and Ultrasonic Waves
The human ear can detect a wide range of frequencies. Frequencies from 20 to 20 000 Hz are audible to the human ear. Any sound with a frequency below 20 Hz is known as an infrasound and any sound with a frequency above 20 000 Hz is known as an ultrasound.
Infrasonic Waves: The waves having freqnecy less than 20 Hz are infrasonic waves. A vibrating simple pendulum produces infrasonic sounds. Elephants and whales produces infrasonic waves. Earthquakes produces infrasonic waves Ultrasonic Waves: The waves having frequency more than 20,000 Hz are ultrasonic waves. Bats and rats can produce ultrasonic sounds. | ||||||||||||||||||||||||||||||||||||
| Applications of Ultrasound
These are the sound waves having frequency more than 20,000 Hz. Due to the high frequency, ultrasound is associated with more energy and can penetrate upto a large extent. This characteristic of ultrasound makes it very useful for many purposes. Some of its uses are stated below:
(i) They are used to detect any deformities (flaws, cracks) in metal blocks or sheets.
(ii) They are used to clean the hard to reach parts of machinery, like spiral tubes. (iii) Bats use ultrasoung waves tp find their prey. Bats produce high- pitched ultrasonic squeaks. These squeaks are reflected by objects such as prey’s and are returned to the bat's ear. This allows a bat to know the distance of its prey. (iv) Dolphins use ultrasound to find fish and to detect sharks that may attack them. (v) Ultrasound waves are commonly used for medical diagnosis and therapy, and also as a surgical tool. (vi) They are used to check the development of foetus during pregnancy to detect any abnormalities. (vii) They find their application in breaking stones formed in the kidneys into fine grains. Ecocardiography: These waves are used to reflect the action of heart and its images are formed. This technique is called echocardiography. Ultrasonography: The technique of obtaining pictures of internal organs of the body by using echoes of ultrasound waves is called ultrasonography. | ||||||||||||||||||||||||||||||||||||
| Sonar
Sonar stands for Sound Navigation And Ranging.
It is a device which is used to find distance, direction and speed of underwater objects like, water hills, valleys, icebergs, submarines, sunken ships etc.
Working of SONAR : SONAR consists of a transmitter and a receptor or detector and is installed at the bottom of a ship. The transmitter produces and transmits ultrasonic waves.These waves travel through sea water and after striking the objects on the bottom of sea, are reflected back and received and recorded by the detector. The sonar device measures the time taken by ultrasound waves to travel from ship to bottom of sea and back to ship. Half of this time gives the time taken by the ultrasound waves to travel from ship to sea bed. Let the time interval between transmission and reception of ultrasound signal is t.
Speed of sound through sea water is v
This method of measuring distance is also known as ‘echo-ranging’.
Total distance travelled by waves = 2d. Then, 2d = v × t. | ||||||||||||||||||||||||||||||||||||
| Human Ear
The human ear can be divided into three main parts:
(i) Outer Ear
Outer Ear:(ii) Middle Ear (iii) Inner Ear The outer ear is outside the body and is also called pinna. It extends into the ear canal. Ear canal is filled with air. Middle Ear: The middle ear is composed of the ear drum or tympanunn( it is an elastic membrane, circular in shape) and the bone ossicles. There are three bone ossicles, namely, the hammer, the anvil and the strirrup. Inner Ear: The internal ear is composed of a cochlea and three semi-circular canals. The cochlea is filled with liquid. The cochlea makes the hearing apparatus and the auditory nerve from it goes to the brain. Eardrum is the intersection of the outer and middle ear. Oval window is the intersection of middle and inner ear. Working of Human Ear: The outer ear called pinna collects the sound waves. The sound waves passes through the ear canal to a thin membrane called eardrum. The eardrum vibrates. The vibrations are amplified by the three bones of the middle ear called hammer, anvil and stirrup. The middle ear then transmits the sound waves to the inner ear. In the inner ear the sound waves are converted into electrical signals by the cochlea and sent to the brain through the auditory nerves. The brain then interprets the signals as sound. |
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