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a low-pitched sound. On the other 10.7 Noise and Music hand, a whistle has a high frequency and therefore, produces a sound of We hear different types of sounds around us. Is the sound always higher pitch (Fig. 10.18). A bird makes pleasing? Does a sound sometimes a high-pitched sound whereas a lion makes a low-pitched roar. However, the cause discomfort to you? Some sounds roar of a lion is very loud while the are pleasant to the ear, whereas some are not. sound of the bird is quite feeble. Suppose construction work is going Every day you hear the voices of on in your neighbourhood. Are the children and adults. Do you find any difference in their voices? Can you say sounds coming from the construction site pleasing? Do you enjoy the sounds that the frequency of the voice of a childproduced by horns of buses and trucks? is higher than that of an adult? Such unpleasant sounds are called Usually the voice of a woman has noise. In a classroom, if all the students a higher frequency than that of a speak together, what would the sound man. produced be called? On the other hand you enjoy sounds 10.6 Audible and Inaudible from musical instruments. Musical Sounds sound is one which is pleasing to the We know that we need a vibrating body ear. Sound produced by a harmonium for the production of sound. Can we is a musical sound. The string of a sitar also gives out a musical sound. But, if hear the sound of all vibrating bodies? a musical sound becomes too loud, The fact is that sounds of frequencies less than about 20 vibrations per second would it remain melodious? (20Hz)cannotbedetectedbythe human 10.8 Noise Pollution ear. Such sounds are called inaudible. Onthehigherside,soundsof frequencies You already know about air pollution. higher than about 20,000 vibrations per Presence of unwanted gases and second (20 kHz) are also not audible to particles in air is called air pollution. Similarly, presence of excessive or the human ear. Thus, for human ear, the range of audible frequencies is unwanted sounds in the environment roughly from 20 to 20,000 Hz. is called noise pollution. Can you list some sources of noise pollution? Major Some animals can hear sounds of causes of noise pollution are sounds of vehicles, explosions including bursting frequencies higher than 20,000 Hz. Dogs have this ability. The police use of crackers, machines, loudspeakers etc. high frequency whistles which dogs can What sources in the home may lead to hear but humans cannot. noise? Television and transistor radio The ultrasound equipment, familiar at high volumes, some kitchen to us for investigating and tracking appliances, desert coolers, air many medical problems, works at frequencies higher than 20,000 Hz. conditioners, all contribute to noise pollution. 132 SCIENCE 2024-25