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| Experiment to Demonstrate Interference of Sound Waves |
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| Interference of sound waves can be demonstrated using Quincke's tube. It consists of two U-tubes ABCDE and FGH, whose limbs can be inserted telescopically into each other. The tube ABCDE is provided with two side tubes at B and D. The side tube at D is fitted with a rubber tube. A tuning fork of high frequency (about 1000Hz) is sounded near the opening at B. Initially, the path lengths BCD and BGD are made equal. The rubber tube is held against the ear. The waves arriving at D along the two paths will be in phase. Hence, a loud sound is heard. Now, the tube FGH is gradually pulled out. When the
path difference between the waves becomes l/2,
the waves reaching D via the paths BCD and BGD will be 180o out of phase and cancel each other. The compression due to one wave will overlap with the rarefaction due to the other wave. On increasing the path difference, a loud sound is heard again, when the path difference i.e., BGD-BCD is l. This experiment, besides illustrating interference, provides a good laboratory method of measuring the velocity of sound in air. Initially, the path lengths are adjusted to be equal. The outer tube is pulled out till the sound intensity becomes minimum. The
distance through which the tube has been slided will be
l/4, since the path difference is l/2.
By measuring this distance using a scale, l can
be calculated. Assuming the frequency of the tuning fork to be f, the velocity of sound can be calculated using the relation v = f l. |
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