Harmonic sound waves are produced inside the pipe whenever a multiple of half the wavelength of the sound waves matches the length of the pipe:
[tex]n(\frac{\lambda}{2})=L[/tex]Where L is the length of the pipe, n is an integer and λ is the wavelength of the sound waves.
The smaller the integer n is, the greater the wavelength will be. Then, to find the second longest harmonic wavelength, set n=2:
[tex]\begin{gathered} \Rightarrow2(\frac{\lambda}{2})=L \\ \Rightarrow\lambda=L \end{gathered}[/tex]Substitute L=3m to find the second longest wavelength for sound waves in the pipe:
[tex]\lambda=3m[/tex]Therefore, the second longest harmonic wavelength for sound waves in that pipe is:
[tex]3m[/tex]