Speaker and/or listener placement along with capacitive style low frequency traps can help. The first thing is to figure out why you have the nulls. Are they uniform around the room (a suckout), are they caused by a piece of furniture (I've gotten my share of surprises here), or are they a basic function of room dimensions. I won't go into great detail for any of these, but rule out furniture first--remove anything that might be causing the problem and remeasure. Second is to map the room with your problem freuqencies--do they vary relative to the wavelength of that frequency? If so, it's a basic room mode issue. Placement can be a big help here as can the traps.
Now one thing you need to be aware of is there will always be one frequency at the centerline of the room that will be peaked, and another nulled. There is very little you can do about this with parallel walls. Calculate F1 and F2 for the width and that will tell you what those frequencies will be (this assumes you sit at the centerline of the room as most people do--while it has modal issues it typically has the best imaging). Now if there are other freuqencies you can see if you have an 1/4 wavelength cancellations. Are the speakers 1/4 of a wavelength off a boundary of that frequency? If so, they are cancelling that frequency out to some degree and should be moved to a distance that helps balance other peaked modes. Sounds easy--but it's a little more daunting because you'll find you will fix problem A reduce problem B and create a new set of problems.
Here is a link which may be of more help on this issue.
Rives Audio Listening room tutorial. There are also some good software programs such as Cara and RPG room optimizer that can help you as well.
Now one thing you need to be aware of is there will always be one frequency at the centerline of the room that will be peaked, and another nulled. There is very little you can do about this with parallel walls. Calculate F1 and F2 for the width and that will tell you what those frequencies will be (this assumes you sit at the centerline of the room as most people do--while it has modal issues it typically has the best imaging). Now if there are other freuqencies you can see if you have an 1/4 wavelength cancellations. Are the speakers 1/4 of a wavelength off a boundary of that frequency? If so, they are cancelling that frequency out to some degree and should be moved to a distance that helps balance other peaked modes. Sounds easy--but it's a little more daunting because you'll find you will fix problem A reduce problem B and create a new set of problems.
Here is a link which may be of more help on this issue.
Rives Audio Listening room tutorial. There are also some good software programs such as Cara and RPG room optimizer that can help you as well.