## Question

A pipe of length 20 cm is closed at one end. Which harmonic mode of the pipe is resonantly excited by a 425 Hz source? The speed of sound = 340 ms^{ –1}.

### Solution

First harmonic

Let N be the frequency of the source and *V _{p}* that of the

*p*th harmonic of a closed pipe. The source will resonantly excite that harmonic mode of the pipe for which

For any value of p = 1, 3, 5, … Now for a closed pipe we know that

Therefore, for resonance,

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