COMPLETE: Sound Class 8

ww.cbselectures.com/topic/sound-part-i/ Website: https://www.cbselectures.com Buy Remaining Videos: https://www.cbselectures.com/product/class-8th-science-tutorials/ SOUND Sound is a form of energy that enables us to hear. Every object has its special sound. Production of sound Sound is produced because of vibration of an object. In other words, a vibrating object produces sound. Sound can be produced by following methods: By vibrating strings By vibrating air columns By vibrating membranes By vibrating plates Sound production in humans A human being produces sound because of vibration of his voice box. The voice box is also known as larynx. There are two stretched membranes (Vocal cords) attached in larynx; with a narrow slit between them. When air passes through vocal cords they produce different types of sound with different combinations of stretching. Propagation of sound The traveling of sound is called propagation of sound. Sound cannot propagate in the absence of a medium. Sound travels through solid, liquid and gas. Case of moon We cannot hear sound on moon because there is no air on moon to carry the sound waves or sound variations. Speed of sound Speed of sound is 340 meters per second in air. Sound travels slowest in gases, faster in liquids and fastest in solids. THE HUMAN EAR Sound travels in the form of waves of vibrating air molecules. Outer Ear: It consists of the pinna, ear canal, and the eardrum or the tympanum. The pinna gathers the sound waves and then leads to the ear canal from where they strike the eardrum. Middle Ear: The vibrations from the eardrum are transmitted to three closely-packed bones called the hammer, anvil and stirrup. These three bones finally transmit the vibrations to the inner ear. Inner Ear: This consists of cochlea, which is a long coiled tube. It contains a fluid and has numerous cells with hair. These sensitive hair cells transmit the vibrations to the brain through the auditory nerve, for the brain to register the sound. AMPLITUDE The maximum displacement of vibration of the vibrating body from its mean position is known as its amplitude. Frequency: The number of vibrations made by the vibrating body in one second is known as its frequency. The SI unit of frequency is the hertz (Hz). Time period: The time taken by the vibrating body for one complete vibration is known as the time period of vibration. It is denoted by T. T=1/f and f=1/T Time period and frequency are reciprocals of each other. Question: A pendulum oscillates 40 times in 4 seconds. Find its time period and frequency. Answer Since time period is time taken to complete one oscillation. Characteristics of sound 1. Loudness: Based on the amplitude of a sound wave, we can determine the loudness of the sound. Loudness is proportional to the square of the amplitude. Shrillness or pitch Pitch denotes the shrillness or flatness of a sound. Sound can be high or low. A woman’s voice g