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CBSE Class 9 Answered

why some materials have the property to absorb sound especially cotton
Asked by Uzair Chanda | 20 Dec, 2013, 07:36: PM
answered-by-expert Expert Answer
There are general properties that the best sound absorbers have, regardless of material. Any item that will allow air to pass into it and then be absorbed by a cellular structure will make a good sound absorber.

Answered by | 20 Dec, 2013, 11:22: PM

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CBSE 9 - Physics
(3 X 3 = 9) CASE STUDY QUESTION 26. Ultrasounds can be used to detect cracks and flaws in metal blocks. Metallic components that are generally used in construction of big structures like buildings, machines and also scientific equipment. The cracks or holes inside metal blocks, which are invisible from outside, reduce the strength of the structure. Ultrasonic waves are allowed to pass through the metal block and detectors are used to detect the transmitted waves. If there is even a small defect, the ultrasound gets reflected back indicating the presence of flaw or defect. Clearly indicated in the figure Defect or flaw Ultrasound Detectors Metal Block (a) The transmitter sends an ultrasound wave of speed 5960m/s through a long steel bridge and a block reflects it back after 0.02s. how far the block in the metal bridge is from the transmitter. (b) With the above property of ultrasound waves, Can ultrasonic waves be used in the medical field? If so, how and give one use in the medical field to justify your answer? (c) Give two practical applications of reflection of sound waves?
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