An organ pipe is open at both ends. It is producing sound at its third harmonic, the frequency of which is 406 Hz. The speed of sound is 343 m/s. What is the length of the pipe?

Answer 1

Read http://www.studyphysics.ca/newnotes/20/unit03_mechanicalwaves/chp141516_waves/lesson51.htm
to convince yourswelf that the third harmonic wavelength is in this case (3/2)L, where L is the pipe length.

In this case, the wavelength is
lambda = V/f = 343/406 = 0.8448 m

The tube length must be 2/3 of that which would be 0.563 m