Given,
The length of the pipe, L=1.8 m
Speed of the sound, v=340 m/s
To form the standing wave with the longest wavelength, the number of nodes of the thus formed standing wave should be equal to 1.
The wavelength of the standing wave created in a closed-end pipe is given by,
[tex]\lambda=\frac{4}{1}L[/tex]On substituting the known values,
[tex]\begin{gathered} \lambda=\frac{4}{1}\times1.8 \\ =7.2\text{ m} \end{gathered}[/tex]Thus the longest possible wavelength is 7.2 m
The frequency of this standing wave is given by,
[tex]f=\frac{v}{\lambda}[/tex]On substituting the known values,
[tex]\begin{gathered} f=\frac{340}{7.2} \\ =47.22\text{ Hz} \end{gathered}[/tex]Thus the frequency of this standing wave is 47.22 Hz.