Any experience in designing a Helmholtz bass trap?

Of course!

Thank you, Bill

Yes, you´re right. What I´m trying to kill is a SW that corresponds to the vertical first mode (about 7,5 ft from floor to ceiling). My DIY back-wall (air tight) wood panels perform very well and helped minimize 135 and 90 Hz SW my room had. But they failed below that freqs.
So I´m thinking about a tuned cylinder, I mean a Helmholtz resonator at 70 Hz or so.

I couldn´t find other that theoretic physics on that devices. Nothing to put hand on, I mean.

Any comment welcome.

Cheers.
Marcelo.
 
I guess we can assume that your speakers themselves are not responsible for excessive, high Q output at 72Hz?

I'll admit that I am somewhat surprised. Floor to ceiling standing waves are usually much less of a problem at the listening position than front to back standing waves. You didn't offer complete room dimensions so I can't be sure but you may have a tangential 72Hz mode that is adding to the problem. You may also have some peaking at that frequency due to room reflections. Often, moving the speakers as little as a few inches can solve these problems.

Before you start adding more devices in your room, I would suggest that, if practical, raising or lowering the driver that is responsible for reproducing 72Hz might offer a solution. You could always tilt the speaker to get back on axis.

You might be interested in our program, Visual Ears, which addresses these very problems and the use of which often removes the need for room treatment altogether. We have lots of completely satisfied users.

Full details can be found at www.kbacoustics.com

There is some useful information on Helmholtz resonators in the "Master Handbook of Acoustics" that you might want to refer to.

Hope this helps.