Transmission line

[bearjew]

Not really starting a DIY project at the moment, but this is more or less curiosity.  Has anyone designed a transmission line speaker before?  I have a vague knowledge and what they do and how they work, but the part that I still don't understand is how you decide what speaker you're using.  I mean, if I want to get 25Hz out of a transmission line, I have to use a speaker that puts out 25Hz, and then the transmission line amplifies it, right?

[Kingman]

I believe you are referencing an electrostatic? I have a source if this is correct....

[ataudio]

Hi bearjew.

The TL cabinet is one I have very limited experience with.  I've own several types of speakers that used them but never built one.  I did once order a transmission line speaker calculator program off the web but never got around to really building a final set.  But...the enclosure in any speaker system is used to control the Fs of the woofer in question.  It eliminates the wild hump at Fs...whether is a ported box, acoustic suspension, TL, open baffle, etc. The enclosure is a function of Fs and many other woofer parameters.  I suspect the usable bass, especially in the ultra low (near subwoofer) area, is really a function of the woofer in question.  You def want a good woofer, probably the lower Fs the better.  The lower the -3db point of the woofer the more usable lower bass. This will help reach those low freq.  Then comes the black art of "stuffing" the enclosure. Lots of voodoo, calculators and diff thoery on the web.  Is this for a sub? But in short the TL cabinet is built to 1/4 wavelength of the woofer resonant freq.  The cabinet is tuned to the woofer.  If not it wont dampen the Fs and you'll get a boomy bass.  Not too many commercial mfgs use TL.  From what I've read years ago its part math, part voodoo.  Most say its more than just a 1/4 wavelength cabinet.  I know this doesnt help too much.  I've long since lost that program I ordered.  I kinda gave up on TL enclosures.  My cabinet of choice is acoustic suspension types. 

[Kingman]

My cabinet of choice is acoustic suspension types. 

sometimes old school is best....

[bearjew]

Hi bearjew.

The TL cabinet is one I have very limited experience with.  I've own several types of speakers that used them but never built one.  I did once order a transmission line speaker calculator program off the web but never got around to really building a final set.  But...the enclosure in any speaker system is used to control the Fs of the woofer in question.  It eliminates the wild hump at Fs...whether is a ported box, acoustic suspension, TL, open baffle, etc. The enclosure is a function of Fs and many other woofer parameters.  I suspect the usable bass, especially in the ultra low (near subwoofer) area, is really a function of the woofer in question.  You def want a good woofer, probably the lower Fs the better.  The lower the -3db point of the woofer the more usable lower bass. This will help reach those low freq.  Then comes the black art of "stuffing" the enclosure. Lots of voodoo, calculators and diff thoery on the web.  Is this for a sub? But in short the TL cabinet is built to 1/4 wavelength of the woofer resonant freq.  The cabinet is tuned to the woofer.  If not it wont dampen the Fs and you'll get a boomy bass.  Not too many commercial mfgs use TL.  From what I've read years ago its part math, part voodoo.  Most say its more than just a 1/4 wavelength cabinet.  I know this doesnt help too much.  I've long since lost that program I ordered.  I kinda gave up on TL enclosures.  My cabinet of choice is acoustic suspension types.

thanks for the all the info.  long story short, the Acoustic Zen Adagio speakers are kind of a "dream" speaker, even though i did pass up an opportunity to own them about a year ago.  I never heard them, but the designer, Robert Lee, designed all of the True Image Research gear that you've seen floating around here, and it's all incredible.  The speakers are transmission line, and I don't really understand how the whole thing works.  The speakers are in a d'appolitto configuration, so are the woofers AND the tweeter going through the transmission line, or just one or both of the woofers?  The frequency response of the speakers are 30Hz - 25kHz.  Does that mean that the woofers go down to 30Hz without the transmission line, or what?

[Anders]

I remember an article on building a transmission line enclosure using a 3 inch speaker.
May have been in Speaker Builder magazine.

[ataudio]

I suspect both woofers share the same TL.  The tweeter really isnt a factor.  Typical, single box, enclosures dont account for the tweeter.  But there are mfg's that will build seperate tweeter enclosures such as B+W.  There they tune the enclosure for the tweeter.  Back to the TL...highly doubt the tweeter parameters are in place here.

 As far as your question of whether the woofers reach 30hz w/o the enclosure...I strongly suspect yes.  A woofer is pretty much usable down to its -3db point.  Then the ouput volume drops like a cliff. No cabinet can restore that.  Electronic EQ'ing will but it may push the limits of the woofer. 

30 hz for good woofer is not a problem.  Taming the Fs hump is where the magic of the cabinet comes into place.  Tame it or its BOOM BOOM BOOM and very unbalanced over all freq response. 

[bearjew]

so the transmission line is more or less a way to smooth out the EQ curve of the speaker?  Is the bass still as accurate even though the sound has to travel through the TL?

[Resident]

Here is a whole page of TLine speaker projects that others have done.  Take what you can from that.  Lots of good information here.

http://t-linespeakers.org/projects/chris/index.html

http://p10hifi.net/tlinespeakers/FAL/box-plans/classicTL-revisited-WD.pdf

http://p10hifi.net/TLS/downloads/PushPush-B139-foldTL-map-300609.pdf

http://t-linespeakers.org/projects/tlB/index.html

[ataudio]

"so the transmission line is more or less a way to smooth out the EQ curve of the speaker?"

In short yes.  The woofer specifically.  Although some brands have seperate enclosures for the midrange as well.  Some midranges are open back and need protected from the back wave of the woofer....some midranges are sealed and can plopped into a cabinet with no further consideration.   The cabinets job is to tame (smooth out) the Fs of the woofer and make sure the the rear pressure wave never meets the front wave except in phase via a port, slot, horn, TL, etc. 

"is the bass still as accurate even though the sound has to travel through the TL?"


Oh yes.  No different than the air mass in a ported speaker, air mass in front of the throat of a horn, etc.  I've owed three sets of transmision line speakers.  Tight, tuneful bass, extended, etc.  Properly designed you would be hard pressed to say one style of speaker is any better than another.  IE: Ported VS. TL VS. Sealed.  Each has strengths and weak points.  My  TL speakers were the Meadowlark Kestrals...three diff models.
Loved them but now defunt. 

[geoffr]

My Meadowlark Kestrels are a tl design. You'd best be a decent cabinetmaker with a shop if you try to build one.

[AdamG]

T-lines are just a specially formed ported system essentially. There is a lot more engineering that goes into their design. Rather than reinforce only around the port tuning as a standard ported box does, a TLine reinforces a selected bandwidth. There is some ripple in this bandwidth that has to be taken into account with software. The size is highly dependent on the desired F3, you will get a slightly smaller size than a ported box for the same speaker, but still much larger than the sealed box. Certain woofers are better than others, it's a function of the QTC and Xmax from what I have picked up.

The reason they aren't more popular is because the extra calculations and effort required in designing them, and also the complexity of the build is much higher generally over a traditional sealed/ported box. You additionally have to be more careful about the high-pass on the system, similar to ported systems the Xmax of the system is rapidly approached as soon as you get below the tuning of the T-line. Most people go with active crossovers to take care of this, as well as take care of any major ripples in the line's response.

Your tweeters are all sealed-back, or else the woofer's pressure inside the box would blow out the tweeter's dome or radiating element. The independent midrange sections are required when your tweeter either is open backed or it leaks, because of this fact. Your tweeters will essentially become passive radiators to the woofers, with extremely limited x-max which will jump their gap and be ruined.

I've heard good things about T-lines response but have never personally built or heard one.

PS: On a MTM system the T-line calculations are especially difficult, because the distance from the woofer to each end of the line has to be taken into account, so each woofer's response would have to be calculated together, which can present a special set of difficulties, especially when you bring in center to center spacing between those woofers and the tweeter which you have to then shoehorn into the space remaining.

 

[ataudio]

My Meadowlark Kestrels are a tl design. You'd best be a decent cabinetmaker with a shop if you try to build one.

Neat....a fellow Kestral owner.  Great speakers.  So sad I sold mine.  I also see that
another forum member has a pair of Fourier 6's.  Another one of my favs.  Once
again...sold them too.  I cant understand why we keep selling off perfectly good gear

[schwarcw]

Here's an interesting project.  No specs on the performance.  Madisound makes some really nice kits. 

Without speakers:  https://www.madisoundspeakerstore.com/full-range-speaker-kits/fostex-bk-16-cabinet-only-each/

With speakers, does not need a crossover which is a good thing, performance down to 55 Hz: https://www.madisoundspeakerstore.com/full-range-speaker-kits/fostex-bk-16-folded-horn-kit-pair/

[AdamG]

Here's an interesting project.  No specs on the performance.  Madisound makes some really nice kits. 

Without speakers:  https://www.madisoundspeakerstore.com/full-range-speaker-kits/fostex-bk-16-cabinet-only-each/

With speakers, does not need a crossover which is a good thing, performance down to 55 Hz: https://www.madisoundspeakerstore.com/full-range-speaker-kits/fostex-bk-16-folded-horn-kit-pair/

Just a note, a folded horn is slightly different from a T-line, and even HARDER to make right! haha! They have some impressive gains though, when done properly!