I have a huge hump at 30Hz

I would suggest putting a pair of 16” or 20” tube traps at the corners if you can afford them. After that, if you are using sub and are feeding the sub with line level signal from preamp, you can tamp the bass peaks with a low cost equalizer like the Behringer Ultra Curve Pro DSP 8024 or the newer ULTRACURVE PRO DEQ2496. But I would not put them in the signal path to equalize the main speakers.

Herman: Are you using any type of room treatments now ? How are you measuring your in-room response ? Sean
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If you want to avoid electronic intervention, you are probably limited to playing with speaker and listening position and/or helmholtz resonators. I have 5 helmholtz resonators in my room, custom designed for two different low frequency absorption areas. Rives may also be able to help guide you toward these or other passive solutions. I would certainly first try playing with listener and then speaker positioning if it is possible in your setting. Just mark your present locations, as you may create more problems than you solve. I have found changing distance from speakers by a matter of inches can alter the amplitude of a bass frequency tremendously. The same can be said for speaker positioning. These methods would obviously not change the total bass energy in your room, but would alter it at your listening position. In my room (almost 20x30) I have found that placing woffers closer to the wall, and closer together, increases bass in the 16-50hz range while reducing nodes in the 60hz area. I have found this same effect with many speaker systems, including dipoles. For better or worse, all of my systems have included separate subs and allowed for great experimentation. Best of luck.

Thanks so far. I am measuring with my modified (Sean instructions) Radio Shack SPL meter and confirmed by a local dealer with a nice RTA.

My room is treated with some damping but it is my understanding that 30 Hz is too low to be damped by any practical means. I have poked around on the ASC web site and it looks to me like their tube traps work across a wide range but still don't affect 30 Hz much.

The problem as I see it is the narrow range. A huge hump at 30 and almost flat at 25 and 40. Can a Helmholtz resonator be constructed to tame such a narrow peak? Rlapporte, when you say custom designed do you mean by yourself or someone else? Can you share this design?

I'll continue to play around with sub placement. That may be the key as suggested.

Herman, you are right, the 30 Hz can not dealt with correctly with absorption devices, particularly high Q factors. Even if you were able to absorb it you would bring down other frequencies as well. Your situation requires diaphramatic methods Helmhotz or capacitive style bass traps. Helmhotz can work quite well, but keep in mind they are tricky to build. High Q Helmhotz resonators are inefficient, which means they have to big and very very rigid. The best ones are poured out of concrete. I realize that's not likely a realistic option. You can do well with wood--I just wanted you to be aware of the issues. Placement of the resonators are very important as well. You want them at the lowest pressure zones, unlike absorbers that you want at the highest pressure zones. The capacitive traps can help as well, but you can't get very high Q factors with them (still they are much much better than absorption methods). They work much better to smooth out the frequency response across the listening room. For either of these methods, we can design these for you and figure out placement, it would be done on an hourly basis.

The other item you've mentioned is placement. There are a few tricks with placement that can help alot. Place the sub in the 1/4 wavelength cancellation point for 30 Hz. That will help quite a bit. This might not be very practical if you only have one sub.