room treatment effectiveness

I agree with most of what Shadome said except the last statement. It turns out when you are dealing with modal characteristic it affects it everywhere except the null points, which you don't care about anyway. While you correct for a particular location it has a positive effect just about everywhere--though the most positive effect is at the point where it's measured.

The issue you described with the bass traps only lowering the bump 0.5 db is no surprise. This is basically lack of knowledge on how to use the bass traps properly and tune to the specific frequency. Shadome brings up the 1/4 wavelength cancellation principle. When you use the theory applied to traps--absorptive ones you realize the traps have to be signficantly out in the room to affect that frequency and you need a large trap. There are more effective traps for specific frequency. The NEW RPG modex traps are very effective. For broadband there are ways of building traps extremely effective.

The other aspect you bring up, which is a very good one, is how do you know what to buy. It's not easy and the other caveate is if 2 absorption panels are good, that doesn't mean 6 more will be better. This is why we provide the services we do. Our fees typically save the clients money on not buying unneeded acoustical treatment, or often saving a lot of money for clients that will build their own devices based on our designs.

2chnben: You understand the problem well. Not only does the size increase as the frequency decreases and I do not agree with the diameters that the ASC describe--they need to be significantly larger. But the size increase AND the efficiency decreases. As the frequency gets lower, the problem gets tougher. Now 200 Hz is not that tough--that can be dealt with passively in most (not all) situations, unless there are some other serious restrictions (like no corners or space to work in). Usually the really problematic issues are 100 hz and lower.