Transmission Line ADD ONS

[caratje]

Hi All,

Like more people on this forum, I am (getting) intrigued by transmission line subwoofers. I have 2.5 way front (+ center) DIY speakers each using two 8" woofers (Vifa Carat and 2x Impuls for rear) and would like to add a subwoofer to my system for movie and stereo. After having read a good amount of all kind of available information, my preference would go to a transmission line based on two 10" or even 12" woofers (depending on power, size and in less extend price). That is I want something special, although I am not experieced, so I would not mind your opinion on my thoughts displayed in this post. Actually, it would be nice to be able to model the design I am going to build, but still depend on the experience of people that have already build one on more of the below mentioned types.

Two woofers not at the beginning of the line
- http://www.b0x.nl/sietse/tl-sietse.jpg
Principle not 100% clear to me (Pied Piper?!?!), but not stuffing needed

Helm Holtz resonance rooms
- http://img128.exs.cx/img128/4319/singular_isometric.jpg
As far as I understood to demp higher orders to avoid the TL dip (just like two woofers!!!)

Push-pull
- http://www.teresaudio.com/haven/subs/subs.html
To benefit from cancelations of imperfections (like PP?)

Push-push
- http://www.t-linespeakers.org/FALL/rtls/tline.html
Same as Push-pull?!?!?! Plus if the stereo signal is fed to the it some stereo picture in the higher frequencies of the subwoofer range.

What principle is most intriguing to you (and of course why?) and is there great benefit to combine them in one design?

Sorry for starting such an open discussion ;-)

Best regards,

Caratje

[DorinD]

Hi,
What's the intended crossover freqvency between the TL sub and the actual system? I ask you because I think this is the key.
I also use a strait&unstuffed TL sub of 2.5 meters long with 2x12" in isobaric, pasive crossed at 85Hz. I wish I have more eficiency and maybe more low extension. So, I'm studying another variant named Transflex with the same drivers but of 4 meters long (folded 2 meters), drivers side by side, triple volume and more eficiency, crossed @50Hz.
Somewhere I find the next phrase written by a TL guru:
"There is no reason to use a classic TL design. Using the mass-loading technique, with a taper ratio in excess of 4, you can go lower, smoother than a TL. You also maintain the pipe's ability to eat the back wave in the mid range, which gives you that open pure sound that is the reason to use a TL in the first place. An exception is for a bass TL that will be crossed over at or before the first dip. Then, an un-stuffed pipe, as large as you can stand is the way to go."

[caratje]

Hi Dorin,

That is a difficult question for me, because I actually don't know that well how low my Carats (and Impuls) are going:

http://www.speakerenco.nl/speaker2.php?id=59
http://www.speakerenco.nl/speaker2.php?id=12

There is stated 37 Hz, but I have my doubts about that... Eitherway, some flexibility will be preferred, I guess 100 or 150 Hz will for sure be high enough.

About the quote of a TL Guru. He states:
"There is no reason to use a classic TL design", but
"for a bass TL that will be crossed over at or before the first dip", then:
"an un-stuffed pipe, as large as you can stand is the way to go."

My TL-subwoofer will be mainly aimed at low-low bass. But because it is a game of balancing (space, unit price/size and power vs quality), I would not mind to be able to also produce some additional "boom" with it, of course asking for less quality. So concluding: Below 75 Hz for quality and sometimes for the fun up to 150 Hz.

But why did you go for a straight line? Easier to make? At the beginning of the line? Can you share your design?

Best regards,

Caratje

[caratje]

Hi Dorin,

I did some additional 'research' given your reply.

Do you mean the following with "TransFlex"

The FB1 (Floor Box 1) project started out purely as an experimental modification of the highly successful TB1 (Tiny Box 1) nearfield monitor. Designer Pete Thomas hadn't been able to try such a small bass driver in a true transmission-line system before, because the TB1 used a hybrid arrangement which he calls 'Transflex' -- a combination of reflex and short transmission-line elements (see 'The Transmission Line' box for an explanation of these designs).

http://www.soundonsound.com/sos/feb0...les/pmcfb1.htm

- When reading the whole article I conclude that TransFlex is less good than a real, well designed, transmission line. And for me the meantioned dimensions are not an actual bottle neck. TransFlex as mentioned above sounds a little bit like, I wanted a transmission like, but needed more boom, so I builded a reflex in parallel. But that of course doesn't have to be bad, bacisly you have more to tune. Note that the HelmHoltz resonator I refered to, doesn't have an external port.

Next to that I think I found the statement you quoted on the following site:
http://www.geocities.com/rbrines1/Pa...esonators.html

- Clear story and all in agreement with my current thinking. Not straight, woofer not at the end of the line and stuffing to damp higher orders. But how about two units, as far as I understood, damping is not needed any more?!?!?!

[caratje]

What do you think will work best:

[DorinD]

Hi Caratje,

I think B and C are the best, B for less magnetic interference and C for less distortions.
If you want a separate TL subwoofer (<80Hz) you need a long, unstuffed line and proper driver (very low Fs, Qt=0.3-0.4). And think of folding such a long tube.
Things like: tapering, stuffing, mass loading, etc. are for mid bass accuracy (80-600Hz) when the TL is part of a full range box.
Concerning Transflex I mean as it's defined in Jensen Technical Bulletin here:
Jensen Transflex

I choose a strait TL sub because of available tube and drivers, simplicity and need to rapid test it's potential. You can see some picture of it here:
http://www.hi-fi.ro/fhifi/viewtopic.php?t=5278&start=30

Now, I've got the courage to make the next step in TL design.

[caratje]

Hi Dorin,

Don't you think A will have a broader optimum, with two unequal offsets? (given MJ King's modelling) Or you assume that simply more pushing power at the optimum offset is more important/effective?

Tappering not needed? Hmm, that also sounds like I can also go for Bass reflex or closed box design...
I have also seen a subwoofer design which was basically using two separated TLs with slightly different length/offset. That was very big, as long as air is lineair I think I can combine them in one TL using different offsets.

Best regards,

Caratje

[theChris]

i built something like A for a car audio project. it somehow got the amp to oscillate at about 1000hz. i'm still a bit confused on that one!

[DorinD]

IMHO variant A with offset is good as long as you think about midbass.
For a real subwoofer the offset has no meaning, plus those two drivers are working in different conditions and could not be used to mecanical summing of left&right channels (what I wish I do with a pasive TL sub). Look here what I mean: http://www.visaton.com/english/bildg..._bweiche1.html

[caratje]

Hi Dorin,

Funny, I thought the filter design you refered to was in Dutch (links = left), but I guess it is actually in German...

Either way, I understand your decisions and can fully agree with them. The different offset per woofer would theoretically mainly benefit for mid bass range and for stereo sub you of course want both channels to be the same.

Hmm, I did not think about that during the design specification of my pre-amp (made by one of my best friends), so I have only one channel for sub. Too bad.

Thanks for sharing your opinion, it supported me in gaining a clearer picture on the material.

Best regards,

Caratje