The Subwoofer Conundrum

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A lot of you probably have already read this article (by Rod Elliott), but I just found it and thought it might be of interest to some. This article is well written IMO and had some good meat (so to speak).

I found this while searching for a Phase control circuit I could build for my newly installed IB. I truly love the sound and visceral impact of an IB (4-AV18’s) and would highly recommend one, but as in all things there is tradeoffs, nothing is perfect. The imperfection in my case is my room, which is nearly square. In the past, I have used 2 or more subs to tame the modes, and it has works pretty well, but again not without compromise. Pure wave prorogation vs. Pressure mode and the interaction or crossover between these two are becoming apparent to me now that I have spent a lot of time studying my personal bass problems. After reading this article the picture is far more clear on one end but more confused on the other. I am beginning to think that there is no solution for perfect or even very good bass response in a typical room. I really hoped that a Phase Control of some sort might help my massive 52Hz 16db dip at my listening position, but after reading this article I must wonder why any commercial subwoofer designer would ever put a phase control on a sub since it has a far greater chance of doing more damage then good. Obviously, placement is the first step of getting the bass right, but that is only the beginning, and if you room is bad and you have no room for treatment (like me) then you will have to accept a serious compromise.

My conclusion to my bass problem is to go as low as possible on the crossover point and as steep as possible on the slope. However, this is difficult with a typical processor’s bass management system (wish I could afford the Tact). First I will try reversing the polarity and re-testing my room and see if the 52Hz dip becomes a peak which may be easier to deal with and surly cause other problems. I’ll give the latter a try this weekend as it is the easiest to do, but I don’t expect a miracle.

So if anyone else is stuck trying to get the bass right in their room you might want to go over these articles to get a good scope on the bass picture from a slightly zoomed out POV.

The Subwoofer Conundrum

Phase Correction

My projects
The Rod Elliott article on phase 'correction' shows nothing below 600 Hz. So, even though he acheives a 12 dB improvement, he still calls the result deplorable, because it's in a bandwidth where a 6 dB phase-induced dip is very audible and the dip is relatively very wide.

Also, his testing is on 2 components of a single loudspeaker using the same active, analog crossover. He only touches on group delay. No testing done with seperate full range speakers and relatively very distant subwoofer(s), cascading filters, digital filters, digital delay is only touched on and nothing in the bandwidth you are interested in.

I would think that if you could improve your FR by 12 dB to (+/-)6 dB, you would be ecstatic.

It's important to use relative phase adjustments to know if the peaks and dips in your FR graph are reinforcements and cancellations due to relative phase problems between your sub(s) and satellites...or 'the room', or both.

A few graphs comparing polarity and relative phase changes to the sub's signal in 15-20 degree increments is a simple procedure. Since the mains (sadly, usually the only speakers tested with the sub in a FR sweep) share info with the sub down into the 30's Hz, their placement should be considered in relation to the sub as well as the sub's placement in trying to remedy something like a huge dip.

One more thing is to consider what frequency the dip is centered at and what the 'Q' of the dip is vs the western music scale (which is 1/12 octave, but doesn't correspond to 1/12 octave test tone CDs that I know of, so many folks have been testing tones that will never be played on their systems) to see exactly what the depth of the dip is regarding the actual frequencies you may be playing through your system.

Bottom line: the article by Mr. Elliott is very informative, but incomplete and only partially relative to your situation. If I had not had the success I've had in reconciling FR in-room by inserting a quality variable phase circuit in the sub's signal path, I might agree with you.

Just some thoughts. Don't give up on the relative phase/polarity tests, because they're simple to do and worth a try.

Thanks for the response, I'm taking the information in the article in general and not making it a benchmark in my mind, I have done much studding on this subject and glean a bit from everything I read, but its hard to know what to take to heart and what to dismiss for my situation, I have no real reference. Who has actually heard perfect bass in a room,. I know I haven’t, so what do you compare it to. Phase relationships between all the speakers would be very difficult to measure, even FR in a room is very difficult to measure properly, so I shotgun parts and move things around like everyone else. All my FR plots down below 100Hz is done with Sine Wave Test Tones approximately 1Hz spacing, so I'm getting good resolution I believe. I have been able to move the huge narrow dip out of the pass-band by boosting at 52, 63, and 74Hz, but I'm sure I'm getting some ringing at those frequencies. IOW I need a new plan or a formula for some small bass.
simon5 said:
What's the size of your room?

14'X14.5' with an opening at the right rear. The opening for the IB is in the front of the room about 1.5' to the right of center, and about 1.5' from the front wall. The front R/L corners have mid sub enclosures.

There is a few pictures that show some of the room relitive to the IB opening to give an idea of the layout. Sub/
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