Can you correct nulls with acoustic treatments.

Thats the whole idea. Well part of it anyway. To reduce standing waves (peaks and nulls), along with modal ringing, which is just as important. To absorb or diffuse sound in the room to avoid ringing and flutter echoes, and improve stereo imaging. This is some of the things that acoustic treatment helps with and which EQ cannot. Now about those acoustic resonators....
Bob

Hi Jim,
Welcome to the wonderful world of measurements. Its seems to me that you have two basic choices.
1. Move your speakers and listening position about until you get the best subjective results and then stop. Trust me, few do better.
2. Get serious about measuring and the post your results so others can coach you through the process of taking measurements and interpreting them. Expect a serious learning curve. You may eventually opt for room correction, but you should be properly informed before spending. New speakers may be far cheaper - or may do nothing to correct the problem. I went for a Tact unit and a sub with my dipoles. I am happy with the results, but it took a while.
regards,
Sam

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.

What I didn't expect was 10 to 15 hz. wide nulls (-10,-15,even -20 db.) at several other frequencies.

No need to panic. This is quite normal. Do as Plato suggests first (try speaker positioning).

You will need very serious room treatments for these null problem areas if they are below 60 HZ (and I doubt you can cure them entirely without turning your room into something like a set from Flash Gordon). Fortunately you don't need to cure it entirely. For starters, you can easily reduce the worst bumps with a PEQ. (Bumps are more of a problem due to their masking effect). Although nulls cannot be boosted (can't boost what is not there)....smoothing out the peaks will really help.

Try listening to Rebecca Pidgeon The Raven, "Spanish Harlem" and adjust so that you can hear the bass notes evenly and with roughly equal loudness. Good luck...

When I have these issues, I end up using two tools. The first one is a rough estimator and it's online.
CaraQuick

Just punch in the dimensions of the room, fill in a little bit about your room (number of chairs, etc.) and then pick a loudspeaker in their dropdown that is close enough to Maggies. Main thing here is you can move your listening positon and speakers around and see the nulls come and go.

Then I use a measurement device like the Velodyne SMS-1 ($600) to read my room realtime and deal with the small movements, even if you don't use the subwoofer EQ function. There are cheaper analyzers, but I use it cuz I also EQ my subs.

Start with the Cara and at least find out what you're dealing with.