subwoofer setup problem

Full agreement with Sugarbrie - you want a fair amount of "underlap". You might want to start with the crossover point close to half an octave below (or perhaps even a bit lower) where the mains start to rolloff. Trying the REL Strata with my Dunlavy SM-1s (-3dB somewhere in the mid-40s), the best crossover point seemed around 32Hz. In your system, you would need to be even lower, so it seems unlikely that the ACI would integrate as well as the REL, even though the ACI rolls off faster. You might have a slight suckout from setting the crossover in the mid-20s, but it won't degrade the sound of your Thiels.

Kadlec, does your preamp have a subwoofer output? Then you wouldn't need to spend any money on crossovers. You couldn't run the Thiels in full range of course. I would go this route before I chose (2) or (3).

Karls, I though Thiels were rolled at 6db? I don't actually know what the difference is (in technical terms) but it's the assumption I've been going under when reading other threads about roll off. I have Thiel 1.5s but I'm pretty sure Jim Thiel designs all his speakers very similarly.

I would try 3) first since it is relatively inexpensive, and ACI has a good return policy. I have also read ACI owners who have commented very favorably. Plus a simple capacitor (which is really all the filter is) is not going to cause much sonic degradation in frequencies above 65Hz.

If that doesn't work I'd bite the bullet and get a REL. I have a strata3 with the rolloff frequency set to 28Hz to complement Spica Angelus. If I set the frequency much above 30Hz it sounds awfully muddy ... could be just like you're describing.

I agree with Karls. Depending on the mains and their roll-off slope, it is typically best to "underlap" than to overlap. This is due to the fact that the mains and sub(s) will sum their total output by the time the sound gets to your listening position.

Even though the sub is not running at full steam ABOVE the cut-off frequency and the mains are not running at full steam BELOW the cut-off frequency, each would be contributing appr half of the total output. Kind of like how they take nearfield measurements from a woofer and then from the port / passive radiator and splice them together. Both are resonant within their own ranges but sum together to produce one final curve or output level.

I would also suggest keeping subs away from the corners of a room. All you'll get there is a boomy thud, massive room nodes and a complete lack of "quality" making it hard to blend to the mains. The only time that a sub should be corner mounted is if it has pathetically low output to begin with and you're trying to do "earthquake effects" for HT. Otherwise, forget about that type of installation for music reproduction.

Besides the "thuddiness", the low frequency sound waves now have a longer distance to travel coming all the way from the corner as compared to the mains that are probably situated closer to you. Due to the time delays involved, the bass would always seem to "lag behind" ( because it IS "lagging BEHIND" ) the rest of the music.

Try to situate the sub close to the mains and directly between them if at all possible. Not only does this minimize the reinforcement that occurs due to room gain in the corners, it tends to stabilize and focus the bass imagery. The amount of time delay between the various drivers is minimized, producing a more coherent sound wave that arrives at your ears within a few milliseconds of each other. Bass is far more natural due to relying on the drivers output rather than room reinforcement and blending to the mains is far simpler.

Once you get the basic package somewhat dialed in, you can experiment with moving the sub forward or back ( this affects the apparent bass "attack" and time of delivery ), fine tuning the crossover point, adjusting the output level, etc... Keep in mind that one adjustment will affect the other, so you'll have to decide how "picky" you want to get. Hope this helps... Sean
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My transmission-line ported Vandersteen 1Cs are rated to be 3dB down at 38 Hz. I got excellent integration of my Titan II LE (located slightly forward and to the right of the Vandys) with the sub's crossover set 24db down at about 40 Hz.

Mostly because of power handling concerns about the Vandersteens (when I wasn't around), I got ACI's 65 Hz passive filters for between my preamp and amp. I turned up one of the Titan II LE's crossovers to somewhere between 55 and 60 Hz and left the other crossover at around 40 Hz to achieve integration that's at least as good as it was before. It also resulted in a slightly improvement of the upper bass and midrange from my 2-way Vandersteens. There is increased open-ness and "air" in the music.