subwoofer setup problem

For Leoturetsky: low-freq rolloff is a function of the physics of sealed vs. vented boxes, and it cannot be adjusted by the designer, it just is. Sealed boxes roll off at 12 dB/oct and vented boxes at 24 dB/oct. There are minor variations in the response right around the "knee" of the curve, depending on the "Q" of sealed boxes and the particular alignment of a vented box, but they all go to the same slope below that. Listening rooms add a boost of 6 dB/oct beginning when the wavelength equals the largest room dimension, and often this coincides approximately with the rolloff of the speakers. This will modify the actual in-room rolloff of a sealed box to 6 dB/oct and of a vented box to 18 dB/oct. This is why sealed boxes have a noticeably slower rolloff in the deep bass than vented ones.

Karls, please correct me if i'm wrong, but don't TL's ( transmission lines ) roll off at a different ( slower ) rate than a typical vented speaker ? I know that their characteristic resonance peaks respond VERY differently than a ported or passive radiator design. Then again, there are more than a few variations amongst TL designs too... Sean
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As a REL owner I should tell you to dump the ACI and buy REL, but I won't because I think that the ACI might work just as well.

The above threads are interesting physics descriptions ported vs vented etc etc but I think your best advice (from Sean I think, no not the same Sean) was to make sure the sub is well away from the corners ... I agree that subs in corners sound awful boom, boom boom).

I think you are correct to state that the overlap is the problem and I agree with all other posters that subs should underlap rather than overlap the mains in order to avoid muddy bass.

So I still think that 3) should be your first step for the following reasons :

1) ACI wouldn't sell the filters if they didn't think they work.
2) it's cheap and they are returnable if you don't like them.
3) It will stop any overlap, and might even improve the clarity of the main speakers since the drivers are not trying to reproduce low bass (leave it to the sub).
3) buying a different subwoofer is much more expensive, and may not be necessary after adding the filters.

I would try to resolve the issue with ACI before anything else. My REL sub is great but I'm sure your ACI is just as good if correctly set up.

Good luck, and please post back to let us know if you solve the problem.

Sean: TL's unfortunately haven't been mathematically modeled the same way closed and vented boxes have, so their rolloff is somewhat open to debate. I haven't seen a source I trust enough to say for sure. One author stated 18 dB/octave but I believe this was based on empirical data, not theory. In my mind, I would think that based on how TL's work, they will be lot closer to a vented rolloff than a closed box. The difference, of course, is that they are VASTLY better damped when done properly. TL's are still an untapped gold mine, IMO. They have so much potential and so little interest. And not even the so-called experts seem to be able to give good answers to even the most basic questions about them; there is an astonishing amount of disagreement and error in the literature about the most basic of concepts and equations. Someday....

I talked to my brother about TL's and his thoughts were that test data suggests an appr 6 dB per octave roll off. Obviously, this will vary with the specifics of each design. He commented that overall output levels are lower in efficiency than if the driver was used in a sealed design. He also stated that the woofer / mid-woofer increases in linear output as frequency rises i.e. the output on a graph looks more like a very linear but gradual decline as frequency decreases. There aren't any major "hills & valleys" ( impedance peaks ) like those found on most other vented designs.

While all of those comments tend to coincide with the measurements that i've seen and read about, the only one that i would question would be the one about roll-off. I did some digging and came across the following. This was taken directly from Vance Dickason's Loudspeaker Design Cookbook ( 5th edition ) page 73 / 74: "Looking at the response graph in fig 4.1, it is obvious that a TL's frequency response is similar to a sealed enclosure, but with an even more shallow rolloff, probably accounting for the opinion among TL aficionados that TL's are capable of producing very deep bass."

This does somewhat support my brothers' comments although it does not give a specific figure in terms of dB's per octave. He probably arrived at the 6 dB figure after studying various graphs & data.

Vance goes on to say that many proponents of TL's claim superior transient response to vented ( not hard to believe ) and even sealed designs. According to various testing, his conclusion is that they are extremely similar to a sealed box design with a Q of .7 or so. As you and others may be aware, most "fast & tight fanatics" will tell you that the "fastest & cleanest" bass occurs at a sealed Q of .5 or so ( ala Dunlavy and a select few others ). Obviously, TL's are still not quite as fast or clean as fully optimized sealed boxes but are still miles ahead of typical bass reflex designs.

This coincides with my experience in that i do like TL's, but still prefer a sealed design. If i absolutely HAD to choose some type of vented design though, it would be a TL. Sean
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