1130 ÷ 17 = 66.5 (close enough to 67Hz)
Can you move your subwoofer to one of the quarter points of the 17ft room width (i.e., 4¼ feet from the side wall) and re-measure? Placing the sub in a null might help reduce the ringing.
I know the issue of risers as bass traps/broadband absorbers has been brought up a couple of times. And the end of the discussion usually is that it is complicated and can have negative as well as positive influences on the acoustics of a room.
The conversation usually starts with whether cutting holes into a riser to allow the sound waves to interact with the fiberglass or other absorber that lies within the riser is beneficial. And then it evolves into how many holes. How big should the holes be. Where should the holes be. Etc.
I would like to phrase a related question from a slightly different perspective. I can understand how cutting holes in the riser may help higher frequencies gain access to the absorbent material. But for some reason, I have this impression in the back of my mind that says that lower frequencies (either because of their wavelength or amplitude?) are not easily "stopped" by ordinary construction materials like drywall, plywood, OSB, etc. This explains in part why it is so hard to "soundproof" a home theater from the rest of the house and the supports the whole soundproofing industry of clips, channel, drywall, green glue, etc.
So my question is whether a riser, constructed of ordinary construction materials like 2x4's to 2x12's, 1/2-3/4" plywood or OSB, nails or screws, and maybe drywall or carpet or 3/8-5/8" wood flooring for example, would really pose an impediment to said low frequencies interacting with any absorbent materials within.
At the end of the day, is my 4'x18'x1.5' riser filled with fiberglass already acting as a bass trap of sorts even though there are no deliberate holes cut into it for acoustic considerations? I have thought about cutting holes into it and seeing what would happen (and I may still do so anyways knowing that I can always just plug them up again before putting carpet down on the riser) but when I think more about it and the reports of the unpredictability of the net results, I've held off to date. But when I actually measure my room, I have a prolonged decay time at 18-20 Hz. and also between 70-150 Hz.
Yes, that was my worries as well!