Q26 of 36 Page 110

A pipe 20 cm long is closed at one end. Which harmonic mode of the pipe is resonantly excited by a source of 1237.5 Hz? (sound velocity in air = 330 m s-1)

Given:


Length of pipe l = 20 cm


Frequency of wave = 1237.5 Hz


We know that the for a pipe closed at one end the resonant frequency of the pipe is given by



where n is the harmonic, v is the speed of the wave and l is the length of the pipe. For first harmonic or fundamental frequency we have n = 1, then



for a frequency of 1237.5 Hz,




Therefore, the given frequency corresponds to the third harmonic.


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