Question

Find the frequency of the 3rd harmonics of a closed pipe 400cm long if the velocity of sound in air is 350ms/1

Answer 1

The harmonics in the closed pipe are given as follows (see https://en.wikipedia.org/wiki/Standing_wave#Standing_wave_in_a_pipe):

$$f_n = \dfrac{mv}{2L}$$

where $n = 1, 2, 3...$ is a positive integer, $v = 350m/s$ is the speed of sound, and $L = 400cm = 4m$ is the length of the pipe. Substituting $n = 3$, obtain:

$$f_3 = \dfrac{3\cdot 350m/s}{2\cdot 4m} \approx 131Hz$$

Answer. 131 Hz.