There are many methods to acoustically load a woofer's rear-wave output; among these loading techniques are sealed, (both acoustic suspension and infinite baffle), ported bass reflex, passive radiator, open-back dipole, and transmission line. Note that the term transmission line in electrical engineering describes a pathway to channel energy; many non-engineers confuse the transmission line speaker with the older design called a labyrinth; while it is true that several companies use the labyrinth design to achieve a transmission line, a labyrinth is not necessary to achieve true transmission line performance.

Question: What defines a transmission line speaker?

Answer: A non-resonant enclosure, with highly damped impedance peaks.

The labyrinth design of the 1930's, popularized by Stromberg Carlson, was revitalized by IMF and A.R Bailey in the UK, circa mid "60's. As old-fashioned woofers did not have low free-air resonances, a large labyrinth was utilized (with path lengths as long as 12 feet) to extend the deep bass response. To eliminate the resonances of the labyrinth, stuffing (often called a "fibrous tangle") was installed, made from rock wool, fiberglass, or sheep's wool. The stuffing was said to slow down the speed of sound in the line, enabling deep bass from a "quarter wave" path length; the stuffing also absorbed the midrange frequencies and reduced the cavity resonance, enhancing midbass/lower midrange clarity.

As the old-fashioned labyrinth is quite large and inefficient, Von Schweikert sought to reduce the size of the overall package, while also increasing the efficiency. Testing at Cal Tech in 1977 revealed that a long path length was not necessary to achieve deep bass, as a short pipe that is adequately damped with stuffing can extend down to the bottom octave using suitable woofers.

The engineering aspects of our Triple-chambered hybrid T-line design are:

1. Custom-engineered woofers with suitable Theile/Small parameters for a tuned pipe/short-line.
2. 100% stuffing density to eliminate cavity resonance and damp the impedance peaks;
   
3. An exit port that is tuned using Theile/Small parameters, modified to utilize the low Q of the short and highly damped line;
   
4. Three separate woofer loading chambers using our Distributed Resonance principle: one highly damped chamber for the upper woofer, a larger chamber for the second woofer utilizing 30% less density than the first chamber; the third chamber is formed by the combination of the two separate chambers, here is where we place the exit port tube. Note that these chambers are not formed by a section of labyrinth, they are formed by the inclusion of stuffing blankets.
   
   

VSA builds several different models using this proprietary design, which includes the VR-2 through VR-7. Electrical measurements prove that our hybrid line functions as a classic electrical transmission line: an impedance curve shows that the twin impedance peaks (generated by the enclosure and port) are highly damped; in fact, we used a large 12-ft line-path labyrinth we built at Cal Tech in 1977 as our benchmark for this impedance test. Another definitive test, the use of an impulse response, shows that the exit port does not ring like a bass reflex design, yet extends the bass down to the lowest octave.

Finally, a listening test shows that the VSA hybrid T-line design has very tight yet extended bass response, which is the main concern when all is said