need help-how to build sound absorber, deffusive

What are the dimensions of your room? You could have a standing wave problem that is very difficult to correct with absorbers and diffusers. Most absorbers do not work well below about 80-100Hz (even the so called "bass traps"). If you have room that is square, or 2 dimensions are multiples of each other you could have a pretty serious problem that needs to be corrected electronically. I would get a radioshack or other sound pressure meter and measure the room (and speaker) response first using something like Stereophile's test CD. Once you know what frequencies are giving you the problem, it will be easier to figure out how to deal with the problem.

Ltt:
Your dimensions aren't too bad. You probably have a bump around 40 Hz, but I doubt it's so serious that you can't tame it (at least most of the way) with passive devices. I would still recommend that before you begin, chart the frequency response and find out how bad the bump is. Then you can see how effective any changes you make are. The measurements don't replace listening of course (and sometimes a flat response sounds just like that--flat--not involving), but I would still do it to have an objective measure to help guide you. Once you get close to eliminating the bass problem, you will need to fine tune things with your ears, not with measurements.