Roman Saini is teaching live on Unacademy Plus
Class 8 Science NCERT-Physics Lesson 3
Chapter 13: Sound
Importance of Sound: * Sound plays an important role in our life. It helps us to communicate with one another. We hear a variety of sounds in our surroundings. A school bell vibrates when it is rung. The to and fro or back and forth motion of an object is termed as vibration. When a tightly stretched band is plucked, it vibrates and produces sound. When it stops vibrating, it does not produce any sound * The musical instruments like manjira (cymbals), the ghatam, the noot (mudpots) and the kartal are commonly used in many parts of our country. These musical instruments are simply beaten or struck to produce sound. When we pluck the string of an instrument, like the sitar, the sound that we hear is not only that of the string. The whole instrument is forced to vibrate, and it is the sound of the vibration of the instrument that we hear
** Similarly, when we strike the membrane of a mridangam, the sound that we hear is not only that of the membrane but of the whole body of the instrument. Sound Produced by Human * When we speak loudly for a while or sing a song, or buzz like a bee, we feel our s: throat vibrating. In humans, the sound is produced by the voice box or the larynx A hard bump on the throat that seems to move when we swallow is known as the voice box. It is at the upper end of the windpipe. Two vocal cords, are stretched across the voice box or larynx in such a way that it leaves a narrow slit between them for the passage of air. * When the lungs force air through the slit, the vocal cords vibrate, producing sound. Muscles attached to the vocal cords can make the cords tight or loose. When the vocal cords are tight and thin, the type or quality of voice is different from that when they are loose and thick
Hard palate Nasa cavity Teeth Vocal folds are closed when we swallow. Soft palate Tongue Vocal folds Lips Air causes vocal folds to vibrate between open and closed positions when we talk Trachea Larynx (voice box) Esophagus (food channel) Trachea (windpipe) Air from lungs Vocal folds are open when we breathe quietly
Propagation of sound: * Sound needs a medium to travel. When air is removed completely from a vessel, it * On the other hand, sound can travel through wood or metal. In fact, sound can * In our childhood, we must have made a tov telephone. It shows that the sound can is said that there is vacuum in the vessel. The sound cannot travel through vacuum travel through any solid. travel through strings. travels down a canal at the end of which a thin membrane is stretched tightly. It is * The shape of the outer part of the ear is like a funnel. When sound enters in it, it called the eardrum *$ The eardrum is like a stretched rubber sheet. Sound vibrations make the eardrum vibrate. The eardrum sends vibrations to the inner ear. From there, the signal goes brain. That is how we hear.
Stapes (attached to oval window) Semicircular Canals Incus Malleus Vestibular Nerve Cochlear Nerve Cochlea External Auditory Canal mpaniC avity Eustachian ube ympanic Membrane Round Window
Amplitude, Time Period And Freauencv Of A Vibration: $ we know that the to and fro motion of an object is known as vibration. This motion is also called oscillatory motion. * The number of oscillations per second is called the frequency of oscillation. Frequency is expressed in hertz. Its symbol is Hz. A frequency of 1 Hz is one oscillation per second * Amplitude and frequency are two important properties of any sound. We can differentiate sounds on the basis of their amplitudes and frequencies. The frequency determines the shrillness or pitch of a sound. If the frequency of vibration is higher we say that the sound is shrill and has a higher pitch If the frequency of vibration is lower, we say that the sound has a lower pitch. For example, a drum vibrates with a low frequency. Therefore, it produces a low-pitched sound
On the other hand, a whistle has a high frequency and therefore, produces a sound of higher pitch. A bird makes a high-pitched sound whereas a lion makes a low-pitched roar. However, the roar of a lion is very loud while the sound of the bird is quite feeble. Loudness of sound: * Loudness of sound is proportional to the square of the amplitude of the vibration producing the sound. For example, if the amplitude becomes twice, the loudness increases by a factor of 4 The loudness of sound depends on its amplitude. When the amplitude of vibration is is feeble The loudness is expressed in a unit called decibel (dB). ed i oud. When the aund prakised
Every day we hear the voices of children and adults. Their voices are different. The frequency of the voice of a child is higher than that of an adult. Usually the voice of a woman has a higher frequency and is shriller than that of a man Auddible and Inaudible Sounds: *We know that we need a vibrating body for the production of sound. But we cannot hear the sound of all vibrating bodies. The fact is that sounds of frequencies less than about 20 vibrations per second (20 Hz) cannot be detected by the human ear. Such sounds are called inaudible. *On the higher side, sounds of frequencies higher than about 20,000 vibrations per second (20 kHz) are also not audible to the human ear. Thus, for human ear, the range of audible frequencies is roughly from 20 to 20,000 Hz.
Hearing Impairment: ** The total hearing impairment, which is rare, is usually from birth itself. Partial disability is generally the result of a disease, injury or age Children with impaired hearing need special care. By learning sign language, such children can communicate effectively. Because speech develops as the direct result of hearing, a child with a hearing loss may have defective speech also. *Tec es for the hearing-impaired have made it possible for such persons to improve their quality of life. * Society can do much to improve the living environment for the hearing-impaired and help them live normal lives