There are three sources of sound of equal intensity with fr…

Physics Wave and Sound Mix MCQ (Single Correct)

There are three sources of sound of equal intensity with frequencies 400, 401 and 402 vib/sec. The number of beats heard per second is

Text Solution

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The correct answer is:

Let n – 1 (= 400), n (= 401) and n + 1 (= 402) be the frequencies of the three waves. If a be the amplitude of each then and

Resultant displacement due to all three waves is

⇒ ⇒

This is the resultant wave having amplitude

For maximum amplitude cos 2 π π t = 1 ⇒ ⇒ 2 π π t = 2m π π where m = 0, 1, 2, 3, ...

⇒ ⇒ t = 0, 1, 2, 3 ...

Hence time interval between two successive maximum is 1 sec. So beat frequency = 1

Also for minimum amplitude (2cos 2 π π t) = 0

Hence time interval between two successive minima is 1 sec so, number of beats per second = 1

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