Nuuk It shows you need more absorption that reaches down to this resonance.
Andre is probably right that you're speaking about the 1st order y axis mode.
But as long as you don't provide yourself with some measurement posibilities, people can give you advice with the best of intent without avail.
What probably happens is what you see on those animations:
http://web.archive.org/web/200108092000 ... terOne.asp
You should try again and set your bass drum on the line you know don't hear it.
And then bring the bass drum to the corner and try again.
When reading between the lines I don't think that the styrofoam has anything to do with it. If the insulation is well fixed I expect any possible resonance to be higher in frequency.
But now that A LOT of modes (e.g. ALL odd y-modes axial modes, ALL [x,ODD,0] tangential modes, all [x,ODD,x] oblique modes have this nul on this center line.
If it's really 1 explicite 1st order y-axis axial mode one could think about a panel trap on the long walls.
But that one can calculate about but must tune it in practice. Then combined with this existing insulation (the gypsum cladded polystyrene) it will make a triple leaf system which will shift the theoretical resonance frequency of the added panel trap.
But if there are still other problems than altering this wall in a huge paneltrap can cause problems.
At a certain moment one can not talk a project together without one single measured data.
The safest thing is adding more corner absorption (horizontal corners). But then you need to make sure what frequency you're talking about. If it's lower than the standard chunks can handle then they must be enlarged.
What is the thickness of this ceiling absorption and the cavity behind it? If it's only 5 cm than it probably doesn't do a thing on this mode.
Diffusers won't help you solve such a low frequent modal problem.