The choice
Almost all loudspeakers available today on the market are essentially based on two principles, pneumatic suspension, that is the closed box, and the bass reflex, which extends the low end of the frequency response through a tuned resonant port. Both solutions became very popular mainly due to the ease of their technical and industrial realization. However, they suffer from several sonic shortcomings like the poor performance at low frequencies of the closed box and the often artificial performance of the bass reflex. A solution which overcomes both limitations is offered by the transmission line, an acoustic layout exploiting a properly dimensioned duct to re-phase coherently the direct front emission of the speaker with the rear one. The result is a deep firm bass response, with controlled tonal color. The reasons why such a system is seldom employed by manufacturers are essentially twofold: transmission lines are nowadays still difficult to design and impose dimensional constraints for the drivers that render them exceedingly complex for large scale industrial production, except for small scale productions of extra-high level quality and design. In particular, with reference to the design, it should be noted that there is no simulation software commercially available today capable to analytically characterize the behavior of a real transmission line. The variables in this problem are so many that it is practically impossible to include all of them within a single general model, without incurring in gross approximations that would severely affect the predictions of the simulation itself. In other words, each transmission line would require a dedicated software specifically developed and adapted to its physical design.

Products
HL 2.2
Axcentia MKII

Documents
The reasons of a choice
The Albedo way in loudspeakers making
What is acoustical phase
The acoustical phase