Using 2 subwoofers with different characteristics to improve overall performance?

This is a wild idea, far away from an implementation plan. I am just wondering if the idea is worth pursuing.

I love transmission line enclosures. But they have disadvantages like a dip at the 5th harmonic. Other minima can be mitigated with damping material. But more damping material has an adverse effect on the lower frequency range.

Currently I am using a large transmission line in my home cinema. The room is small (5x4 meters) and I have quite an uneven bass experience at different locations in the room.

In ancient times, we used to build stereo speaker systems which were left and right as equal as possible. If I recall, there were problems with bass amplification due to room resonances. But I don; t recall bass cancellations effects as bad as I have now with single sub woofers.

So I was thinking. What if I split up the current huge transmission line into 2 separate lines with about the same but unequal channel length, and half the channel area. Use 2 smaller drivers with a smaller area. So the channel area / driver area ratio would remain the same as recommended per design rules. The total volume of both sub woofers would be roughly equal to the volume of the single sub I have now.

Then, make the design of the transmission line length different for each of the drivers, so dips and peaks do not coincide. In addition, each sub would be placed at a different side of the room.

I assume this would lead to a more even bass reproduction throughout the room because of the different locations of the speakers. And a more even frequency response because the two transmission lines.

Or is this just a nonsense idea and should I better build 2 equal sub woofers and drive them as part of a stereo system using left/right amplifiers. Instead of combining both bass channels in a single sub channel?
 
In ancient times, we used to build stereo speaker systems which were left and right as equal as possible.
What do you mean 'we'? I ask rhetorically. ;) The pioneers taught me to design based on the needs of the app, which IME dictates a speaker's LF design based on the room 's acoustics and the speaker's location in it, though in reality it usually just meant critically damping the vent or box if sealed of one of a stereo pair and IIRC Dr. Geddes uses a variety of box alignment types for his dedicated sub system.

Re TL design, I have posted 'somewhere' that I've done this long ago for ~full height corner loading, one such that made inexpensive dual offset 8" drivers 'speak' (in pipe organ parlance ;)) to room filling loud down to a recording's typically 40 Hz, so ATM see no reason why your more advanced alignment(s) can't be made to work well too.
 

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@GM "we" as in what was common practice among speaker box builders.

Maybe my question was not well enough explained. It was not about damping or placement of a TL. The question is, would it be worth to build 2 transmission line enclosures with with different characteristics and locate them in different spots in the room. As opposed to one single TL enclosure with a larger driver and a larger line area and inherently installed in one single spot.
 
@jlinkels

Understood, hence a rhetorical question.

I understood it just fine and as I noted, if done right is well worth it IME same as in concept as Dr. Geddes' multiple woofer systems, though again, as I implied, yours will require (much?) more effort to design and/or experimentally arrive at a worthwhile solution.
 
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Placing them in different parts of the room is what gives you leverage to deal with the room.

Having them the same or different is of little importance. They would not be adjusted the same if you follow proper multi-sub procedures, and since they will be adjusted (EQed) they can start out how you like, so to speak.
 
@AllenB, @GM

Quite useful answers. I knew I am not the only one with the problem of unequal bass reproduction as a result of a single sub woofer. But the publications by Dr. Geddes are new for me. I need to study his paper and the comments before I start constructing. To know multi-sub is a viable approach is very valuable.
 
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There is a German DIY magazine which builds very good transmission lines and did some articles about how to construct them. Not sure if there are English versions around.
The position the driver at 1/3 and/or 1/5 of the line and additionally use 1-2 internal helmholtz ABSORBERS to dampen the remaining resonances. That's a pretty clever design and when your bandwidth is not to big (=woofer) you should get a response with very low resonances.


Or just use double the chassis in small closed volumes and tailor their position and response to your room ... but that would probably end your search for precise bass ... :geek:
 
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@IamJF I read German, no problem. My mother was Austrian :) Which is that magazine?

The question in this thread really was about placement and/or multiple sub woofers in the same system. What I experience in the room is clearly a position problem.

I did free-air measurements of the TL and those showed some different problems. So I am interested in TL articles.
 
Sorry, forgot the link o_O
http://www.hobby-hifi.de/
So if you want to build a good transimssion line you can find recources there.

To your room and sub positioning problem - I prefer a bunch of closed subs. You can Eq them to your room cause they paly without problems (just lower level) even under their resonance frequency and you can tailor them to your room and lostening needs. With a few of them you can couter room resonances. Or just do a bass array, even a single BA helps plenty. (search for DBA and SBA).
In my opinion it's the best solution - just not the cheapest cause you need a few more chassis and amp power for the same SPL level.
 
Start with Part 1, then move on to Part 2 and Part 3.

Once you have mastered those three articles, and completely understand the advantages of using REW, a calibrated microphone, and the asymmetrical distribution of low frequency sources (whether it be sealed, ported, transmission line, whatever), you are 99% ahead of everybody else and you have conquered the elephant, which is your room (at least for low frequencies <<300 Hz). You’ll also find that the potpourri of discussions regarding box alignments start becoming moot…

After that, you can go down the worm hole into SBA or DBA land (which I feel is more apropos for brand new constructions).

Best,
Anand.
 
OK, I get it. I did not know there was such extensive discussion and documentation about multi-sub systems. But that is why I asked the question here. And got answers.

Somehow I had a little bit uneasy feeling about these 2.1 systems. See, some 40 years years ago I used to build hi-fi (not the high end nonsense) speaker systems. Stereo amplifier, passive crossover. The CD had yet to be introduced and DSP's were far away. Could it be so easy to save on one bass enclosure?

So now in the 2010+ era with electronics much cheaper and active 2.1 systems in reach I wondered whether a 2.1 system could replace a classic 1+1 full range system. The first doubts occurred when in my living room I have an excellent vented sub woofer but also dead spots without any bass. And in my home theater a lesser TL sub and a lot of inaccurate, muddy and uneven bass.

So the classic 1+1 full range systems might not have the separate main and sub placement, adjustable delays, but in fact they were at least not a single sub system.

I can see now that a single sub is not sufficient. A 1+1 system is likely to be better. And a multiple sub system with adjustable delays and equalization might solve even more problems.

The other question (would it beneficial to use sub woofers with different characteristics) is also answered. No it is not. That is, different characteristics could be implemented with equalization not enclosure design.
 
One GOOD POSITIONED sub is nearly always better as a 2.0 system - cause you can't freely chose the position of the lof frequency drivers their, you need to stick to the stereo triangle. Has bad lambda/4 chancellations (SBIR).
But to find the perfect position ... you need a bunch of measurements and the freedom to place your sub (should be possible in a dedicated room).

After all just throwing a speaker in the room and hope for the best ... is not the best solution ;-). Get a cheap measurement mic and a free day and I'm sure you can improve your low frequencies a lot!

(btw - in my studio room one of the best solutions for a single woofer was directly in the corner, membrane close to the sidewall. No SBIR and resonances steady over the room so they where easy to EQ. But tis is a heavy treated room.)