Listening in: Threave gardens

I found this listening device in the gardens on the Threave Estate at Castle Douglas. It must be for listening to birds in the trees. All I could hear was a continuous low-pitched note. I know that 50Hz is about the lowest frequency we can hear from the signal generator in the lab. So suppose that a stationary wave is set up in the pipe by the wind vibrating the air. We say that since there is nothing to stop the air moving, there must be an anti-node at each end and therefore a node in the middle as shown below:

This means there is only a half wavelength in the pipe (two quarter waves back to back). If the pipe is 3 metres long, the full wavelength would be 6 metres. The wave equation is:

Wave speed = frequency x wavelength

We'll take the speed of sound in air as 330 m/s. So Frequency = 330/6 = 55 Hz. It could be right.