Acoustics: pipe resonance question, sub woofer position?

Question

I am planning to put a subwoofer into a pipe of resonance frequency 18hz, a 31' 2" pipe is resonant, so where at in the pipe should I place the sub

And if the sub is in the pipe, isn't the pipe a closed pipe?

Answer 1

The pipe you have described will fit about 1/2 wavelength inside the pipe, which would be right for a pipe open (or closed) at both ends.  If one end is open and the other closed, the pipe should be half this length.

If you make an airtight seal between the pipe and the speaker, it will then be a pipe closed at one end.  However, if you have a significant air gap between the end of the pipe and the speaker, that end of the pipe no longer has a displacement node.  Instead, a pressure node (displacement antinode) develops at the end of the (open) pipe.  One quarter of the diameter of the pipe is more than enough air gap to make that end of the pipe effectively open.