Sound Notes PDF - Class 9th Physics

 PRASHANT KIRAD SOUND Sound: A sound is a form of energy which produces a sensation of hearing in our ears. It is produced due to vibrations of different objects. It travels in the form of waves. Product...

 PRASHANT KIRAD SOUND Sound: A sound is a form of energy which produces a sensation of hearing in our ears. It is produced due to vibrations of different objects. It travels in the form of waves. Production of Sound: Sound is produced by the vibrations of objects. Vibrations create compressions and rarefactions in the surrounding air, leading to the formation of a sound wave. The energy required to make an object vibrate and produce sound is provided by some outside source (like our hand, wind etc.). 10 Examples:- & Sound of our voice is produced by the vibration of two vocal cords in 9 our throat. B Sound of a drum or tabla is produced by vibration of its membrane U when struck. PH EX Sound can be produced by following methods: By vibrating string (sitar) By vibrating air (flute) By vibrating membrane (tabla, drum) By vibrating plates (bicycle bell) By friction JOSH METER? By scratching or scrubbing the objects etc.  PRASHANT KIRAD Propagation of Sound The matter or substance through which sound is transmitted is called a medium. It can be solid, liquid or gas. Air is the most common medium for sound propagation. A wave is a disturbance that moves through a medium when the particles of the medium set neighboring particles into motion. 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 creating a region of high pressure. This region is called a compression (C).This compression starts to move away from the vibrating object. When the vibrating object moves backwards, it creates a region of low pressure called rarefaction (R). 10 & 9 B U PH EX Sound waves as longitudinal Waves: Compressions and Rarefactions: Compressions (C): Regions where particles are close together. Rarefactions (R): Regions where particles are spread apart. Sound propagates through these alternating compressions and rarefactions. Longitudinal Waves: In longitudinal waves, particles of the medium move parallel to the direction of wave propagation. Particles oscillate back and forth around their rest position, not traveling with the wave. Transverse Waves: Particles oscillate perpendicular to the direction of wave propagation. Example: Light waves (non-mechanical).  PRASHANT KIRAD Characteristics of Sound Waves: Wavelength (λ): The distance between two consecutive compressions or refractions is called Wavelength. SI unit: metre (m) Frequency (f): The number of oscillations per unit time is called the Frequency of a Wave (Number of compressions + the number of refractions per unit time) SI unit: Hertz (Hz) Amplitude (A): The maximum displacement of the particle of the medium from their original undisturbed position is called amplitude of the wave. SI unit: metre (m). Time period (T): The time taken between two consecutive compressions or refractions to cross a fixed point is called Time Period of the Wave. In other words, the time taken for one complete oscillation through a medium is called a Time Period. 10 SI unit: second (s) & Relationship between Frequency and Time period: f = 1/T 9 B U PH EX Pitch: Pitch of a sound depends upon: 1. the frequency of the sound 2. size of the object producing the sound 3. type of the object producing the sound Timbre: The timbre or quality of sound is a characteristic with which we can differentiate between different sounds even if they have same pitch and amplitude.. “ saari definitions acche se dekhelna.”  PRASHANT KIRAD Loudness: It depends on the amplitude of the sound wave. Two sounds with same intensity can vary in loudness only because we can detect one sound easier than the other Velocity: The distance travelled by a wave in one second is called velocity of the wave. SI unit: metre per second (m/s). 10 & 9 This is the wave equation. B U Speed of Sound in Various Mediums: PH Sound cannot travel at the same speed in different mediums. EX The speed of sound in a medium is affected by three things: The density of the medium. For instance, speed of sound is the maximum through solids. The temperature of the medium. As the temperature increases, the sound propagates easily. Humidity in the air also affects the travel of sound. As the humidity increases, so does the propagation of sound Sonic Boom: When an object travels in the air with a speed greater than that of the sound, it produces a sound with high energy. These objects exert a large amount of pressure on the air which causes the production of shock waves in the air. These shock waves produce extremely large and loud sound waves which are called Sonic booms. Some aircrafts, bullets, rockets etc. have ‘supersonic speed’.  PRASHANT KIRAD This energy is loud enough that it can break glasses or damage the buildings. The sound produced is similar to the sound of an explosion or thunderclap. Reflection of Sound Like light, sound also bounces back when it falls on a hard surface. It is called reflection of sound. The laws of reflection of light are also applicable to reflection of sound. i. The incident sound wave, the reflected sound wave and normal at the point of incidence lie in the same plane. ii. Angle of reflection of sound is always equal to the angle of incidence of sound. 10 Same laws of reflection of light jaise ! & 9 B Echo U PH The repetition of sound caused by the reflection of sound waves is called an echo e.g. Clapping or shouting near a tall building or mountain. EX To hear a distinct echo sound, the time interval between the original and reflected sound must be at least 0.1s, as sound persists in our brain for about 0.1s. The minimum distance for obstruction or reflective surface to hear an echo should be 17.2 m. Rolling of thunder is due to multiple reflection of sound of thunder from a number of reflecting surfaces such as clouds and the earth. Reverberation The persistence of sound in a big hall due to repeated or multiple reflections of sound from the walls, ceiling and floor of the hall is called reverberation. If its too long, sound becomes blurred, distorted and confusing. To reduce reverberation in big halls, heavy curtains are put on  PRASHANT KIRAD doors and windows, carpets are put on the floor and seats are made of sound absorbing material. Application of reflection of sound i. Megaphone, loudspeakers, bulb horns, etc. are designed to send sound in a particular direction without spreading all around. All these have funnel tube which reflects sound waves repeatedly towards audience. ii. Stethoscope - It is a medical instrument used for listening the sounds produced in human body mainly in heart and lungs. The sound of heartbeats reaches the doctor’s ears by the multiple reflection of the sound waves. iii. Sound board - In big halls, a curved board(cause multiple reflections) is placed behind the speakers so that his speech can be heard easily by audiences. iv. The ceiling of concert halls are made curved, so that the sound after reflection from ceiling reaches all the parts of the hall. 10 Range of hearing & i. Range of hearing in human is 20 Hz to 20000 Hz. 9 Children younger than 5 years and dogs can hear upto 25 KHz. B ii. The sounds of frequencies lower than 20 Hz are known as ‘infrasonic U sounds’. PH A vibrating simple pendulum produces infrasonic sounds. EX Rhinoceroses communicate with each other using frequencies as low as 5 Hz. Elephants and Whales produce infrasonic sounds. Earthquake produce infrasonic waves which some animals can hear and get disturbed. iii. The sounds of frequencies higher the 20 KHz are known as ‘ultrasonic’ waves. Dogs, dolphins, bats, and rats can hear ultrasonic sounds. Bats and rats can produce ultrasonic sounds Hearing Aid It is battery operated electronic device used by persons who are hard of hearing. Microphone convert sound into electrical signals, the those are amplified by amplifier. Amplified signals are send to the speaker of hearing aid. The speaker converts the amplified signal to sound and sends to ear for clear hearing.  PRASHANT KIRAD Application of ultrasound i. It is used to detect cracks in metal blocks in industries without damaging them. ii. It is used in industries to clean ‘hard to reach’ parts of objects such as spiral tubes, odd shaped machines. iii. It is used to investing the internal organs of human body such as liver, gall bladder, kidneys, uterus and heart. iv. Echocardiography: These waves are used to reflect the action of heart and its images are formed. v. Ultrasonography: The technique of obtaining pictures of internal organs of the body by using echoes of ultrasound waves. vi. Ultrasound is used to split tiny stones in kidneys into fine grains, which then get flushed out with time. “Class 9th Phodenge” 10 - Prashant Bhaiya & 9 B U PH EX