CHAPTER 12 SOUND. Sound: Sound is a form of energy which produces a sensation of hearing in our ears.

Transcription

1 CHAPTER 12 SOUND Sound: Sound is a form of energy which produces a sensation of hearing in our ears. Production of Sound Sound is produced due to the vibration of objects. Vibration is the rapid to and fro motion of an object. Propagation of Sound: When an object vibrates, the particles around the medium vibrate. The particle in contact with the vibrating object is first displaced from its equilibrium position. The disturbance produced by the vibrating body travels through the medium but the particles do not move forward themselves. A wave is a disturbance which moves through a medium by the vibration of the particles of the medium. So sound is considered as a wave. Sound waves require medium for transmission. Sound waves are characterized by the motion of particles in the medium and are called mechanical waves. When a vibrating object moves forward, it pushes and compresses the air in front of it forming a region of high pressure called compression (C). When the vibrating object moves backward, it forms a region of low pressure called rarefaction (R). Note: 1. Compressions are the regions of high pressure and density where the particles are crowded and are represented by the upper portion of the curve called crest. 2. Rarefactions are the regions of low pressure and density where the particles are spread out and are represented by the lower portion of the curve called trough

2 Mechanical waves: A mechanical wave is a periodic disturbance which requires a material medium for its propagation. On the basis of motion of particles the mechanical waves are classified into two parts. (i) Longitudinal wave (ii) Transverse wave (i) Longitudinal wave: When the particles of the medium vibrate along the direction of propagation of the wave then the wave is known as the longitudinal wave. Eg: sound wave in air. (ii) Transverse wave: When the particles of the medium vibrate in a direction perpendicular to the direction of propagation of the wave, the wave is known as the transverse wave. Eg: 1. waves produced in a stretched string. 2. Light waves. CHARACTERISTICS OF A SOUND WAVE 1) Frequency: The number of vibrations per second is called frequency. It is represented by the symbol ٧ (Greek letter nu). Its SI unit is hertz (Hz). 2) Time period: The time taken to complete one vibration is called time period. Frequency, =

3 Ie, = or, T = 3) Amplitude of sound wave: The amplitude of sound wave is the height of the crest or tough. It is represented by the letter A. The SI unit is the same as that of density or pressure. 4) Wavelength: The distance between two nearest crests or troughs of a wave is called its wavelength. It is represented by the symbol λ (Greek letter lambda). Its SI unit is metre (m). Louder sound has shorter wavelength and softer sound has longer wavelength. 5) Velocity (or speed of the wave) of wave: The distance travelled by a wave in unit time is called velocity of the wave (or speed of the wave). Therefore, Speed = Let the distance travelled by the sound wave = λ And the time taken by the sound wave = T Speed, v = ie, v = v = ( ) v = ie, Speed = frequency wavelength Where, v speed of the wave Frequency of the wave Wavelength of the wave.

4 Note: The speed of sound is more in solids, less in liquids and least in gases. The speed of sound also depends on the temperature of the medium. If the temperature of the medium is more, the speed of sound is more CHARACTERISTICS OR PROPERTIES OF SOUND 1. Intensity: The amount of sound energy passing each second through unit area is intensity of sound. Its SI unit is Watt per metre square (W/m 2 ). 2. Loudness: loudness is a measure of the response of the ear to the sound. The loudness of sound depends upon the amplitude of vibration. Its SI unit is decibel ( db ) If the amplitude is bigger, more energy the sound wave contains and the sound is loud. If the amplitude is smaller, less energy the sound wave contains and the sound is soft. 3. Pitch: It is the sensation (brain interpretation) of the frequency of an emitted sound and is the characteristic which distinguishes a shrill (or sharp) sound from a grave (or flat) sound. The pitch of sound (shrillness or flatness) depends on the frequency of vibration. If the frequency is high, the sound has high pitch and If the frequency is low, the sound has low pitch.

5 4. Quality or Timber: It enables us to distinguish one sound from another having the same pitch and loudness. Note: Tone: A sound of single frequency is called a tone. Note: A Sound produced due to a mixture of several frequencies and is pleasant to listen. REFLECTION OF SOUND When sound waves strike a surface, they return back into the same medium. This phenomenon is called reflection. Laws of reflection: i) The angle of incidence is equal to the angle of reflection. ii) The incident ray, the reflected ray and normal at the point of incidence all lie in the same plane.

6 ECHO: If we shout or clap near a reflecting surface like tall building or a mountain, we hear the same sound again. This sound which we hear is called echo. It is caused due to the reflection of sound. To calculate minimum distance to hear an echo: Let d - distance between the source and the reflecting surface. t Time taken to hear an echo, and the minimum time required is 0.1 s v Speed of sound through the medium. We know, Speed of sound through medium = (1) Here, v = 344 m/s (The speed of sound through air) t = 0.01 s Total distance travelled by the sound wave = d + d = 2d Eqn (1) => speed of sound through air, v = 2d = vt d = d = Distance between the source and reflecting surface, d = 17.2 m. So to hear an echo clearly, the minimum distance from the reflecting surface should be 17.2 m. Conditions for the formation of Echoes The minimum distance between the source of sound and the reflecting body should be 17.2 m. The wavelength of sound should be less than the height of the reflecting body. The intensity of sound should be sufficient so that it can be heard after reflection. Reverberation Echoes may be heard more than once due to repeated or multiple reflections of sound from several reflecting surfaces. This causes persistence of sound called reverberation. Steps to reduce reverberation Use of sound absorbing materials like compressed fiberboards, rough plaster or draperies. Use of sound absorbing materials like heavy curtains, mats and cushioned seats. Use of sound absorbing materials like thermocol, hardboards etc.

7 Uses of Multiple Reflection of Sound Megaphones, horns, musical instruments like trumpets, etc. are deigned to send sound by multiple reflection in a particular direction without spreading in all directions. Doctors listen to sounds from the human body through a stethoscope. The sound of heartbeat reaches the doctor s ears by multiple reflection. The ceilings of cinema halls and auditoriums are curved so that sound after multiple reflection reaches all parts of the hall. Range of Hearing Human beings can hear sound frequencies between 20 Hz and 2000 Hz is called audible range. Sound whose frequency is less than 20 Hz is called infrasonic sound Sound whose frequency is more than Hz or 20 khz is called ultrasonic sound Note : 1 khz = 1000 Hz Animals which produces and hear infrasonic sound are Rhinoceroses, whale, elephants Animals which produces and hear Ultrasonic sound are Dolphins, bats and porpoises Hearing Aid : It is used by people with hearing loss. It is an electronic, battery operated device. Working: The hearing aid receives sound through a microphone. The microphone converts the sound waves to electrical signals. These electrical signals are amplified by an amplifier. The amplified electrical signals are given to a speaker of the hearing aid. The speaker converts the amplified electrical signal to sound and sends to the ear for clear hearing. Uses or Application of ultrasonic sound 1. Ultrasonic sound is used to clean objects like electronic components. The components to be cleaned are kept in a cleaning solution and ultrasonic waves are sent into the solution. Due to the high frequency, the dirt particles get detached from the components 2. Ultrasonic sound is used to detect cracks in metal blocks. Ultrasonic waves are sent through the metal blocks and if there are cracks, the waves are reflected back and the cracks can be detected. 3. Ultrasonic sound is used in ultra sound scanners for getting images of internal organs of the human body. 4. Ultrasonic sound is used to break small stones formed in the kidneys into fine grains so that they are removed through the urine. 5. Bats produce ultrasonic sound to catch the prey.

8 6. SONAR Sonar stands for Sound Navigation And Ranging. It is a device which uses ultrasonic waves to measure distance, direction and speed of underwater objects such as submarines. It is also used to measure the depth of sea and oceans. Working: Sonar has a transmitter and a detector installed in ships. The transmitter produces ultrasonic sound waves which travel through the water and after striking the object in the sea bed is reflected back to the detector. To calculate depth of the sea from a ship d = Where, d distance between sea bed and ship. v Speed of sound through sea water. t Total time taken. Structure of human ear The human ear consists of three parts the outer ear, middle ear and inner ear. Outer ear : This is also called pinna. It collects the sound from the surrounding and directs it towards auditory canal. Middle ear : The sound reaches the end of the auditory canal where there is a thin membrane called eardrum or tympanic membrane. The sound waves set this membrane to vibrate. These vibrations are amplified by three small boneshammer, anvil and stirrup. Inner ear : These vibration reach the cochlea in the inner ear and are converted into electrical signals which are sent to the brain by the auditory nerve, and the brain interprets them as sound.

9 Answer the following 1. Cite an experiment to show that sound needs a medium for its propagation. An electric bell suspended inside an airtight glass bell jar connected to a vacuum pump as shown in the fig. As the electric bell circuit is completed, the sound is heard. Now if the air is slowly removed from the bell jar by using a vacuum pump, the intensity of sound goes on decreasing and finally no sound is heard when all the air is drawn out no sound is heard. We could see that the hammer striking the gong repeatedly. This clearly proves that sound requires a medium for its propagation...