Taming a bulkhead

My listening room has an 8' drywall ceiling with a drywall bulkhead containing the ac duct running between the right and left walls right over my listening position. The bulkhead comes down 11" from the ceiling. Should I be treating the 11" edge (facing my speakers) with an absorbent acoustic panel? I have a 2'x4' acoustic panel that I can readily cut down to make a 11" x 4' panel and attach to the bulkhead, but hate to ruin a nice panel if this is the wrong approach. Any thoughts?
Go to the Armstrong site and take a look at their 'wave' product, it will give you some ideas.
Hmm. It got me thinking.....and then my wife told me to stop thinking. Thanks for the suggestion.
I would not worry too much about it. Yes, it will result in a standing wave that can be calculated by the distance between the vertical surfaces and the front and rear walls. But that is probably a small factor next to the standing waves from the walls, ceiling and floors themselves if your room is rectangular. the reflections off of the vertical surface facing the speakers are gong to bounce up into the ceiling and then back towards the speakers. By the time that wavefront makes its way back to you it will be pretty low in level and far enough out in time that it will just be in the reverberant field.

I think you are probably ok. If it concerns you, just add some fuzzy product up there, but I don't think it will make a very big difference.
Appreciate the comments. I found an old DIY 2'x2' panel that I will modify to fit that space and see if it makes a difference.