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Sound SOUND Sound is a form of energy which makes us hear. The nature of sound produced depends upon the nature of material, the number of vibrations the vibrating body makes in one second. Sound is a wave motion. Production Of Sound Waves Sound is produced when an object vibrates. medium. Sound travel 5 times faster in water and 15 times faster in solid than air. Factors On Which The Speed Of Sound Depends 1. Temperature When the temperature increases, the speed of sound increases. 2.. Humidity Of Air As the humidity of air increases, the speed of sound through it also increases. Frequency Range Of Hearing In Humans It is between 20 HZ to 20000 Hz. Infrasonic Sound A sound with frequency less than 20 Hz. A vibrating object, which produces sound has a certain amount of energy which travels in the form of sound waves. The energy required to make an object vibrate and produce sound is provided by some outside source like our hand, wind etc. Sound can be produced by the following methods. 1. By vibrating membranes (in table) 2. By vibrating strings (in sitars) 3. by vibrating air (in flute) 4. by vibrating plates (in bicycle bell) Propagation Of Sound Sound waves are longitudinal waves. Sound waves in air consist of compressions and rarefactions. When an object vibrates then the air layers around it also start vibrating in exactly the same way and carry sound waves from the sound providing object to our ears. In the transmission of sound through air, there is no actual movement of air from the sound producing body to our ears. Nature Of Sound Waves Sound waves are mechanical waves i.e., they require a material medium to propagate. Sound waves can not travel through vacuum. Movement Of Sound Through Solids, Liquids And Gases The speed of sound is maximum in solids and minimum in gases and intermediate in liquids. Actually because sound wave are mechanical waves. They need material medium to travel. So a substance in which there is density of particles the speed of sound increases due to the increase in the transformation of energy by a large number of particles of the Ultrasonic Sound A sound with frequency greater than 20000 Hz. Nature Of The Sound Waves: Longitudinal Sound waves are longitudinal waves. In the longitudinal waves the individual particles of the medium move in a direction parallel to the direction of propagation of the disturbance. The particles do not move from one place to another but they simply oscillate back and forth about their position of rest. This is exactly how a sound wave propagates. Compression A compression is that part of a longitudinal wave in which the particles of the medium are closer to one another than they normally are. Or the portions of medium in which the particles are pushed closer having a large number of particles per unit volume. These are the regions where density as well as pressure is high. Rarefaction A rarefaction is that part of longitudinal wave in which the particles of the medium are farther apart than normal. Those portions in which the density of the particles and the pressure is lesser than normal. Transverse Waves There is another type of wave called transverse wave. In a transverse wave the particles do not oscillate along the line of wave propagation but oscillate up and down about their mean position as the wave travels. Light is an example of transverse wave in which the particles of the wave move in a direction perpendicular to the direction of propagation of wave. Characteristics Of The Sound Waves The (i) (ii) (iii) (iv) (v) characteristics of sound waves are: Frequency Amplitude Velocity Of Wave Wavelength Time period 1. Wavelength The distance between two nearest points in a wave which are in the same phase of vibration is called wavelength. In other words we can say that the distance between two nearest crests of a wave or the distance between two nearest troughs of a wave is called wavelength. Wavelength is denoted by (Lambda). S.I. unit of wavelength is m. 3. Frequency The number of waves produced per second. It is denoted by υ (nu). The S.I. unit of frequency is Hertz (Hz). The frequency of a work remains same in air, water, oil etc. Hertz is equal to vibrations per second. 4. Time Period The time taken to complete are vibration is called time period. Time period is denoted by T. Relation Between Time Period And Frequency The time required to produce one complete wave is called time period of the wave. Suppose time period of a wave is T seconds Now, In T Seconds, number of waves produced = 1 So, 1 In 1 second, number of waves produced = T But the number of waves produced in 1 second is called frequency. This means that frequency of wave of time period 1 . T will be T So, we can say that the frequency of a wave is the reciprocal of its time-period, i.e 1 Or f= T 1 Frequency = Time Period 5. Velocity Of Wave The distance traveled by a wave in one second is called velocity of wave. It is denoted by ν. S.I. Unit m/s Or Pitch, Loudness And Quality Pitch 2. Amplitude The maximum displacement of the particles of the medium from their original undisturbed positions, when a waves passes through the medium is called amplitude of the wave. This describe the size of the wave. The S.I. unit is metre. It is denoted by A. ms −1 Pitch is the characteristic of sound, which distinguishes between a shrill sound and a grave sound. Pitch is the interpretation of frequency of the emitted sound by the brain. Higher the pitch, higher is the frequency. A high pitch sound corresponds to more number of compressions and rarefactions passing through a point per unit time. Example Sound from a flute has high pitch whereas sound from a violin is of low pitch. Loudness Loudness of the sound is defined as the degree of the sensation produced on the ear. The loudness or softness of a sound depends upon amplitude of the sound. Amplitude of sound wave depends upon the force with which an object is made to vibrate. When the table is hit lightly soft sound is produced. When a table is hit hardly a loud sound is produced. A loud sound can travel a larger distance as they have more energy. As sound waves spread out from its source its amplitude as well as its loudness decreases. Quality The quality or timber of sound is that characteristic which enables us to distinguish one sound from another having the same pitch and loudness. The more the sound is pleasant, the rich is its quality. Tone A sound of single frequency is called a tone. Note The sound which is produced due to the mixture of several frequencies is called a note and is pleasant to listen to. Intensity The amount of sound energy passing each second through unit area is called intensity of sound. Sometimes the terms intensity and loudness is used interchangeably but they are not same. Loudness is a measure of the response of the ear to sound. Even when two sounds are of equal intensity, we may hear one sound louder than other simply because our ear detects it better. Reflection Of Sound Waves The returning back of sound waves after striking a hard surface is called reflection of sound. Sound can be reflected from any hard surface whether smooth or rough. The reflection of sound causes echoes. Echoes The repetition of sound caused by the reflection of sound wave is called on echo. The sensation of sound persists in our brain for about .1 s. To hear a distinct echo the time interval between the original sound and the reflected one must be at least .1 s. For hearing distinct echoes the minimum distance of the obstacle from the source of sound must be 17.2 m. This distance will change with the temperature of air. Echoes may be heard more than once due to successive or multiple reflections. Multiple Echoes When the sound is reflected repeatedly from a number of obstacles, more than one echoes are produced called multiple echoes. Minimum Distance For An Echo To Be Heard We know, Distance = Speed × Time Now, Speed of sound in air = 344 m/s 1 s) Time taken = 0.1 s ( 10 Because 0.1 is the minimum time interval between two sounds to be heard clearly. Therefore, Distance traveled = 344 × 0.1 = 34.4 m This distance is the total distance traveled by sound. But, our distance from the sound reflecting surface such as a wall to hear an echo should be half of the total distance traveled by sound, i.e. 34.4 = 17.2 m. 2 From this we conclude that the minimum distance from a sound reflecting surface to hear an echo is 17.2 m. Reverberation The repeated reflection that results in the persistence of sound is called reverberation. In an auditorium and a big hall excessive reverberation is highly undesirable, therefore to reduce it the walls of the auditorium are generally covered with sound absorbing materials like compressed fireboard, rough plaster or draperies. The seat materials are also selected on the basis of their sound absorbing properties. Uses Of Multiple reflection Of Sound 1. Megaphones and musical instruments like trumpet and shehnais have a tube followed by conical opening which reflects sound successively. 2. In stethoscope the sound of the patient’s heart beat reaches the doctor’s ears by multiple reflections. SONAR : Sound Navigation And Ranging Sonar is a device which is used to find the depth of a sea or to locate the under water things. Working Of SONAR In SONAR ultrasonic waves are sent out in all directions from the ship and reflected waves are received. By measuring the time between sending the sound waves and receiving the reflected waves the distance of the under water object from the ship can be calculated. 3. Generally the ceilings of the concert halls, conference halls and cinema halls are curved so that the sound after reflection reaches all corners of the hall. Apparatus Of SONAR The SONAR apparatus consists of two parts. 1. A transmitter which emits ultrasonic waves. 2. A receiver which revives the reflected ultrasonic waves. Range Of Hearing The audible range of sound for human beings extends from about 20 Hz to 20,000 Hz. Children under the age of five and some animals such as dogs can hear upto 25,000 Hz. Ultrasound The sound wave having frequency greater than 20000 Hz which can not be heard by human beings are called ultrasonic sound or ultrasound. Dogs, bats and dolphins can hear ultrasound. Due to its very high frequency ultrasound has a greater penetrating power. Supersonic Supersonic refers to sped of an object which is grater than speed of sound. Nowadays so many jet aeroplanes have been developed which have the speed 5 to 7 times greater than sound. Applications Of Ultrasound (i) Ultrasound is used to investigate inside the human body. (ii) Ultrasound is used in sonar to measure the depth of sea or ocean and to locate under water objects like shoals of fish, ship works, submarines, sea rocks and hidden ice bergs in sea. (iii) Ultrasound is used for finding the level of a liquid in a metal tank. (iv) Ultrasound is used in the industry for detecting flaws (faults) in metal blocks or sheets. (v) Ultrasound is used in the treatment of muscular pain and in treatment of disease called arthritis. (vi) Ultrasound may be employed to break small stones formed in the kidneys into fine grains. These grains later get flushed out with urine. (vii) Ultrasonography is used for the examination of the foetus during pregnancy to detect congenial defects and growth abnormalities. (viii) Ultrasound is generally used to clean parts located in hard to reach places. For example spiral tube, odd shaped parts, electronic components etc. (ix) Ultrasonic waves are used for ‘echocardiography’. (x) Ultrasound is used by bats to find their prey. Infrasound The sound frequencies below 20 Hz are called infrasonic sounds or infrasound. For example the vibrations of a pendulum. Structure And Working Of Human Ear The ear allows us to convert pressure variations in air with audible frequencies into electric signals that travel to brain via the auditory nerve. Human ear converts sound energy to mechanical energy. It allows us to perceive the loudness of sound by detection of the wave’s amplitude. The 1. 2. 3. human ear consists of three basic parts: The outer ear The middle ear The inner 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 ear drum or tympanic membrane. When a compression of the medium reaches the ear drum the pressure on the outside of the membrane increases and forces the eardrum inward. Similarly an eardrum moves outward when a rarefaction reaches. In this way the eardrum vibrates. The middle ear consists of three bones the hammer, anvil and stirrup. These three bones amplify the vibrations several times. The inner ear consists of cochlea. The middle ear transmits the amplified pressure variations received from the sound waves to the inner ear. In the inner ear the pressure variations are turned into electrical signals by the cochlea. These electr8ical signals are sent to the brain via the auditory nerve, and the brain interprets them as sound.