Clearly there's a dip around 100Hz (at least at the listening position). Although the Radio Shack SPL meter is not linear (we make a CD that corrects for it's non-linearities)--it's not THAT far off. Now the question is why do you have that dip? Chances are it is a null at your listening point. You can measure this by setting your CD player to repeat the 100Hz tone. Then walk around the room and plot the response of the 100Hz at different points. If you're listening point is in a null, the solution is pretty simple--you need to do some shifting of speakers and listening position (I say simple, but it really isn't that simple, because it will likely cause a null at another frequency). CARA software can help with this, they look for optimum positioning with the least nulls and peaks. You can also use the rules of 1/3 and 1/5 that tend (not always) minimize these problems as well. Lastly, our website offers some help on speaker placement at www.rivesaudio.com It's in the Acoustical Issues Section--go to the listening room and select speakers. There's a download there for a short paper on placement, but I have to admit it does not have a lot about peaks and nulls (we should probably update it some).
If your problem is not caused by a peak or null, then it could be the room construction. Some rooms flex in such a manor that they actually create their own notch filter. Fortunately, this is typically not too difficult to fix. First you need to find out which part is creating the notch. In my experience it's usually the ceiling or floor of an older type house. The older construction allows it to sag and flex and essentially soak up a particular frequency--kind of like a resonator. Basically what ever it is needs to be re-enforced and made rigid. This can be easy in some houses, but difficult in others.
Hopefully, the problem is just speaker and listening position.
If your problem is not caused by a peak or null, then it could be the room construction. Some rooms flex in such a manor that they actually create their own notch filter. Fortunately, this is typically not too difficult to fix. First you need to find out which part is creating the notch. In my experience it's usually the ceiling or floor of an older type house. The older construction allows it to sag and flex and essentially soak up a particular frequency--kind of like a resonator. Basically what ever it is needs to be re-enforced and made rigid. This can be easy in some houses, but difficult in others.
Hopefully, the problem is just speaker and listening position.