Wayne,
> Why in the world would you want
absorption for a frequency where you have a
null? <
Eric nailed it. Nulls are caused by reflections, so adding absorption reduces the severity of the nulls.
Eric: Building a bass trap tuned to 40 Hz is not trivial because it has to be very large and deep. You'll find detailed plans for wood panel bass traps on my personal web site:
www.ethanwiner.com/articles.html
Scroll about 1/3 of the way down the page and find "Build a Better Bass Trap." Note that the deep-bass trap there is tuned to around 90 Hz. To move its center frequency down to 40 Hz you'll want to increase the plywood thickness to 3/8 inch and also increase the cavity depth to about 8 inches. I think you'll also want to make the rigid fiberglass at least twice as thick to two inches, or maybe even four inches thick. I've never built a trap tuned that low so I can't say for sure.
--Ethan