A "transmission line" speaker has a labyrinth at the rear of the driver(s), usually to augment bass response.
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The length of the port is usually determined by what frequency the cabinet is tuned to. Port length is usually a multiple of the sine wave that represents the frequency, for example, 35 Hz. The wave length of that particular frequency is divided usually by 4 or 8 to give port length. That is, the port length is 1/4 or 1/8 as long as the sine wave of 35 Hz. Hope this helped, it might just be confusing.
There are MANY variations on what most people call a "transmission line" nowadays. To try and make things simple, the sound coming out of the back of a woofer is typically loaded into the speaker box. Directly behind the woofer is a chamber that feeds into what one might think looks something like a maze or "labyrinth". At the end of the labyrinth, one would have an opening in the box. By varying the overall length, width, taper and density ( achieved by various amounts of "acoustic stuffing" ) of the labyrinth along with the size of the opening, you can effectively tune the output of the box and the velocity of sound exiting it.
As a side note, ALL vent's of ANY design have a point of maximum flow and will operate at various levels of efficiency above and below that point. As such, one really needs to calculate the amount of airflow required to achieve the desired listening levels and then tune their vents with that in mind. There are newer ports on the market with flared ends which help to broaden the efficiency range of their operation, making the vent less susceptible to over and under flow conditions.
The end result of all of the work that goes into a TL is a vented speaker that does not suffer from most of the side effects of "normal" i.e. ported or passive radiator designs. However, TL's do introduce side effects of their own into the system. Compared to most vented speakers, transmission line bass is typically much cleaner with better attack, definition and less ringing. It sounds very similar to that of a good sealed box woofer with a low Q. On top of that, the amplifier can load into the driver much easier and control it with less effort compared to most vented speakers due to the very noticeably smooth impedance curve. Most vented designs have one very huge impedance peak or two lesser ( but still quite high ) impedance peaks whereas TL's remain VERY smooth and even in impedance, even at resonance. The high impedance peaks of most vented designs are what make the woofers uncontrollable, resulting in the typical "bloat" that most vented designs suffer from. After all, how much power can most amps deliver into a load that is somewhere between 40 - 90 ohms* ??? Obviously, not much and that is why most vented designs tend to ring and lack definition. Most sealed and TL designs keep their impedance peak at the point of resonance somewhere at or below 20 ohms. That is, if they are designed properly. TL's typically are slightly better ( read as "lower" ) in terms of impedance at resonance than most sealed designs. I have seen TL's that never varied below 6 ohms or above 10 ohms across their entire operating range. Obviously, this makes for a VERY nice and controllable load for most amps.
Due to the fact that the driver does not suffer from severe oscillation at resonance, this gives the TL a slightly slower roll-off rate as compared to a sealed box and what is MUCH better than that of a comparable ported or passive radiator design. Most people tend to hear and think of this as very good "extension" of low frequencies. The bass fundamentals are still reproduced pretty solidly, but maybe not at the greatest levels. Distortion is also typically pretty low IF the labyrinth or "line" is properly tuned and designed.
Another "benefit" of a typical TL design is that, due to having to build the "tunnel" within the speaker cabinet, the cabinet has a lot of bracing that most other cabinets would never receive. The effect is that of having a very sturdy cabinet with minimal resonances. The drawback is that you also have all of that wood inside the box, so they are typically pretty heavy and more costly to produce compared to most other speakers of similar size.
The one major drawback to a TL design is that they are typically LESS efficient than a sealed design. Venting a woofer typically increases the output by 2-3 dB's over the same woofer in a sealed box and that is why most manufacturers do it. Taking that one step backwards, TL's might end up being 4-6 dB's LESS efficient than if the same woofer was used in a box with a port or passive radiator. As such, TL's typically require quite a bit of power to play loud. However, due to their very clean and extended bass ( if using a good sized driver and a well tuned cabinet ), one can listen at slightly lower levels than what one is accustomed to and still experience what one would call a very "full" and powerful sound.
The reason that you don't see a lot of TL's floating around is that they are MUCH harder to design, require a lot of fine tuning on an individual basis, are hard to mass produce and cost more to build and ship due to the greater amount of materials and weight. As such, they have found greater support from DIYer's that are willing to put forth the effort and cost that it takes to one of these designs "right".
Hope this gave you a good basic run-down on TL's. They are worth checking into if your prefer quality over quantity. If you like sealed designs, you'll probably like TL's too. They might be TOO tight, clean or "dry" if you are a big fan of "normal" vented designs. As with anything though, you can find TL's that will sound more like a vented design than that a sealed design. If that is what floats your boat, so be it : ) Sean
Outstanding post Sean!!
I've been living happily with my Kinetic Audio Labyrinths (http://www.kineticaudio.com/audiophile.com ... those are
my set of speakers in the picture!!). They are similar in
concept to Bud Fried's old IMF designs.
These are true 1/4 wavelength TLs. A 12" woofer with a 17hz
fs at the end of a 9' folded line with a 10" round terminus
on the rear of each speaker. f3 of the woofer when in the
line is 16hz. No subwoofers needed in my room!
The pros and cons of these speakers are as per Sean's post:
1) Big and heavy. 48"(h) x 18"(d) x 15"(w) at 250lbs each.
Pass the "knuckle rap" test with no problems.
2) Relatively inefficient. Woofer has 92db sensitivity
free-air but is lowered to approx 86-87db in the line. I
vertically biamp to maximize current flow.
3) Much easier to place than vented-box speakers since the
terminus does not (or should not in a proper TL design)
radiate any sound. Although not optimal for imaging, I've
got my "Labs" right up against the front wall due to lack
of space (although plenty of space to the sides). No bass
problems (bloating, etc) whatsoever.
4) Bass transients are so good it's scary with no overhang
at all. Reproduction of kick drums at high SPLs is matched
by only a very small number of other loudspeakers.
5) Woofer distortion is extremely low, although the low 70hz
crossover certainly plays a significant part in that. I
think this characteristic more than anything is what
sometimes gets TL bass labeled as "dry" or "sterile."
For the curious, my system (2-channel only):
Kinetic Audio Labyrinth loudspeakers
Wadia 860 CDP
Panasonic SV-3700 DAT (use 860 as playback DAC)
Fanfare FT-1A FM tuner
Sony DVD-V (2-channel output to pre for movies).
NEC S-VHS (output to pre for TV shows).
ARC LS-5mkII preamp
2 ARC D-400mkII amps (vertically biamped)
All cables and ICs DIY: Canare/Neutrik balanced ICs,
Canare/Canare/Neutrik RCA->XLR hybrid IC (for DVD->pre and
SVHS->pre), Canare/Neutrik AES/EBU digital cable
(DAT->CDP), Canare 4S11 speaker cable with ST Connections
dual gold bananas.
Steve from HiFi Farm: Owning a couple pairs of Ohm F's, i've never heard these speakers refered to as "transmission lines". Nor does their mode of operation resemble anything close to what is typically considered a "transmission line" speaker as set forth in any technical manuals regarding speaker design. I would be curious as to how you arrived at that description and definition ???
Meisterkleef: In a "classic" transmission line, the diameter of the line is equivalent to the driver size and remains so all the way to the exit. As such, a 12" driver would make use of a 12" diameter path and a 12" vent. The Kinetic Audio design is actually a TATL i.e. a "tapered acoustic transmission line". This varies from a "classic" design in that the line becomes smaller in diameter ( tapers ) as you get further away from the driver. This approach causes a slight increase in the "air spring" of the box, making it work a little more like a sealed box than the "classic" TL design.
As to Piet's question about tuning and Meisterkleef's comments about the vent contributing output, i have FELT massive amounts of air being pumped out of various TL designs. Some TL's make use of various amounts of stuffing / damping material in the line itself. The amount and density of the stuffing will not only affect the apparent line length, but the amount of sound that the backwave actually contributes to the output level. As such, one can somewhat tune the line in terms of quantity of output and frequency of resonance. Sean
The sound coming out of ANY vent can never be in phase with the front wave of a driver at all frequencies at all times. That is why a sealed design is always superior in this respect. The TL is not nearly as "bad" as most other designs, as the entire length of the labyrinth is typically lined ( and sometimes even stuffed ) with damping material. As such, the amount of out of phase leakage across a wide frequency range is quite minimal when compared to a port that allows sound to flow freely through an open hole and / or a passive radiator that "talks" at many various frequencies. Theoretically, the TL should also be better in this respect than a "stuffed port" or "Aperiodic" design due to the length of the line / absorption losses. Dynaudio calls their version of a "resistive" or "stuffed" port tuning a "vario-vent".
As far as efficiency levels go, everything that i've ever read / seen / heard tells me that TL's are not real efficient i.e. probably less than 90 dB's or so. One could build something with a higher sensitivity, but this would be done at the expense of extension. Getting high sensitivity with extreme bottom end capability becomes very expesive and requires a big cabinet. This is why many manufacturers fall prey to the laws of diminishing return and opt for some type of venting to augment what they can't get out of the driver naturally. Like anything else though, there are trade-offs involved when you take the "less than optimum / cost IS an object" approach. Sean
Agree with Sean on virtually all of his points. I also never heard the Steve@HiFiFarm definition of Transmission Line.
In response to a few of the others' questions above, the original true Transmission Line was a cabinet design to "get rid" of the back-wave, and have a better loading of the woofer at the point that it would normally have an impedance spike. The original Transmission Lines did not have any output form the port, at all, and were designed specifically so they would not. Tapered transmission lines that taper down toward the port have better low end extension and linearity than straight pipes, or tapering wider toward the port. Sounds that do exit the port may be in phase, but are not "in-phase at all frequencies" with the direct radiations of the cone. Cancellation and addition, known as "comb-filtering" can occur. Since most modern "transmission line" speakers are quite a departure from the original idea, they should more aptly be called "modified transmission lines" or maybe some "catchy" name for marketing purposes.
I use a TQWT Voigt Pipe enclosure which is strange combination of bass-reflex, transmission-line, and back-horn. It makes use of some of the characteristics of all these designs in a "blend" that looks to be horrible on paper, but sounds great in the listening room.
Transmission line design is still a time consuming job that needs alot of work to get it right, and doesn't really lend itself to mass-market applications.
Sean, here is a link with the dimensions that I used. This guy made some for an 8" driver, and then put a cover over the hole, and used some 4" instead. BTW, the "wings" he uses have no resemblance to the "wings" I used. Mine are "swinging doors" on the sides that go from top to bottom.
These plans are taken from the Lowther Club of Norway design which is no longer posted on their site. Download these dimensions if you are interested, because it's getting hard to find them. These pipes work great with Lowthers.
I use Green Mountain Audio Diamantes that are a quarter wave transmission line. The speakers are cast marble and two pieces each. The upper part contains the speakers and crossover while the bottom/base contains the folded network for the transmission line. The tube is many feet long and filled with long fiber sheeps wool to slow the air flow. I have experimented with slightly changing the damping by altering the length/amount of wool by only 1.5 ft or less makes a very noticeable difference. I had some left over from an update from series 1 to series 2 many years ago which also had a crossover change included. The speakers are rated at about 87db but sound best with a lot of power and cranked up loud like over 90 db average. The bass is very quick and dynamic, no residual warmth, extends to the low 30hz range in my room but very little below that. It is a very different sound than my B&W's.