subwoofer setup problem

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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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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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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Karls, i think that most people don't realize that a vent is only "tuned" or "effective" at the one specific frequency that it is resonated at. As such, they are an absolute "mess" anywhere outside of that range and typically do WAY more harm than good.

Of course, i'm strictly talking about sound quality here as one can show tons of impressive arguments on paper. That is, if all you want is an increase in efficiency and bass extension. Yes, the vent does give you a greater quantity of bass, but it is done at the expense of quality. This can be seen with all of the ringing / poor transient response that takes place with a vented design Then again, i'm sure that i'm preaching to the choir on this one.

Hopefully, this will stir up a few others to check into what we are talking about and bring in some new converts to what is considered "old" technology. What is hard to believe though is that vented designs actually pre-date sealed designs. I guess that after listening to all of those early bass reflex models, they had to do SOMETHING to make it listenable. Hence the "breakthrough" technology of a sealed box.

Thank God for Edgar Vilchur and Henry Kloss !!! Sean
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