TL Definition

Definition Transmission Line

Many people still seem to be confused about this term merely meaning a long rear path which exits to supplement the bass. That is a reflex, labyrinth or horn - a TL is conceptually opposite

TL is a mathematical term. it describes a wave-guide transmitting energy in a line. If waves reflect within the enclosure it is not a line. If the load(drive unit) is not balanced to the line impedance the load reacts against the line and is inefficient. for a line to be invisible to it's load(and thereby not interfere with it's motion) it must have no resonances and be of little resistance. The perfect match occurs when the wave is absorbed along a long line gradually, and a perfect loudspeaker when the line end emits no sound.

Think of a fiber-optic cable across the Atlantic, there is little loss because although the sides are transparent, the angle of reflection along  the inside is so slight. If a line changed it's shape along it's length, it would cause losses, in a loudspeaker this would manifest itself as enclosure resonances.

Normal loudspeaker enclosures are a perfect disaster for creating complex modulation of the driver from internal reflections and converting rear-sound energy into kinetic energy of the enclosure.

A TL is a perfect solution for eliminating internal reflection and converting the rear-sound energy evenly and completely into heat.

The longer the line, the more efficient the driver and the lower the bass potential. To reproduce below 27hz, you must have at least an 8 foot un-terminated, stuffed line.

 

a TL is not the same as a mere labyrinth. a labyrinth or reflex neither absorb all the sound, or prevent it exiting the  enclosure. The UK Patent office confirm this definition. a TL is a completely stuffed line that has constant cross-section area, terminating in an open end, through which no audible sound emits.

 

The TransmissionLine has its design roots in the Stromberg-Carlson acoustic labyrinth (1930).

It first consisted of a log pipe, (open at one end and the driver mounted at the other) with a cross-sectional area about the same as that driver.

The line length was made about 25% of the driver resonance's wavelength.

Working with the same basic concept in the early 1960s, A.R.Bailey expermented with different damping materials and techniques in folded lines.

This work has since become the basic bible for most TL designs.

 

With TLs, the enclosure functions as a low-pass filter with a 90degrees phase shift, absorbing all the rear wave energy of the woofer except for very low frequencies

TLs can be characterized by:

    Low cabinet resonance

    Relativley loud deep bass (below 50Hz).

    Highly damped impedance peak.

    Decreased cone motion in the 40Hz region.

    Low degree of mid-bass coloration.


Note: If the end of the pipe emits any sound at

Controversial ?

all, it is not a TL, but REFLEX.

Labyrinth is not the same as TL; labyrinth just meanse convoluted path where the sound is bent round bends, TL means rear wave is completely attenuated.

Beware certain manuafacturers love miss-using the TL term as a 'low-tuned-reflex' doesnt sound as expensive as a 'Transmission Line', which we should all know, is less likely to provide extended and even bass, unlike the exageratted, limited and distorted bass which comes out of combining port sound with driver sound.

In the development of loudspeakers, manufacturers have given up on TLs, because the public demand a 'thump -thump' bass for their rocky taste, and so they pretend to be making a TL, but in fact by removing most of the stuffing, they are making a reflex speaker.

The Resolution, as far as I know is the ONLY manufacturer in the world, to offer a true wideband (20-20khz) Transmission Line Loudspeaker.

links:

http://www.6moons.com/industryfeatures/tls/tls.html

F o r m u la s T r a n
Calculation Line

Driver Parameter:

fs:27 Hz

 


Calculating Length

 

Vline = 0,8 * 344 = 276 m/s

L = 276/27 = 10,2 m

 

L(amda)=L/4

10,2/4 = 2,55 m

 

Source: The Loudspeaker Design Cookbook, Vance Dickason

 

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