Atlantic Technology AT-1 Loudspeaker The Way We Are
In the late 1960s, the primary speaker loading techniques were little different than they are today: horn, infinite baffle, acoustic suspension, bass reflex, and acoustic transmission line. There were boxless dipoles, too, but those were and are a relatively rare and exotic breed.
In those years, the mathematics needed to describe these designs were either complex and tedious to use or nonexistent. Cut and dry was usually the order of the day. Along the way to 2010, the bass reflex design, in particular, was studied, dissected, and finally characterized by a range of equations that greatly simplified the design process. Throw in the availability of affordable computing, and you have an explanation for why bass reflex designs account for the vast majority of today’s loudspeakers.
Nevertheless, a basic law of loudspeaker design, Hoffman’s Iron Law (named for Anthony Hoffman, one of the three founders of KLH), continued to hold fast. In the simplest terms, this rule of thumb states that of the three major low-frequency design goals—extension, small enclosure size, and acceptable efficiency—you can have any two, but not all three. Even a small driver in a tiny box can be designed to go down to 20 Hz, but it will be so inefficient that the power required to drive it to an acceptable level will likely blow it out before you can measure it.
H-PAS may not break the Iron Law, but it does succeed remarkably well in bending it. In a sit-down at last September’s 2010 CEDIA Expo, Boaz Shalev, the chief technology officer for H-PAS, attempted to describe to me some of the technical details behind the process.
Upon viewing a cross-section of the AT-1’s cabinet, a long-time audio fan would instantly call the design a transmission line (see diagram). Transmission-line bass loading has enjoyed an on-again, off-again history, popularized in some high-end British loudspeakers back in the ’60s. A few American manufacturers also gave it a go over the years. When properly designed, it offers purported benefits, including extended and free-breathing bass.
But an acoustic transmission-line design required a large, complex, and expensive enclosure, with line resonances that could be difficult to control. The design of the line and the optimum box stuffing were never characterized with the mathematical rigor that made other types of enclosure designs far more commercially practical.
When I looked at a cutaway diagram of H-PAS (slicing the box in half to have a look wasn’t in the cards), I saw the H-PAS as a short transmission line with a significant twist. At or near the exit of the line, there’s a port into a separate, sealed chamber. Atlantic claims that this chamber is tuned to dampen resonances within the line before they can exit the port and color the sound. According to Atlantic, the trusses with the holes in them behind the drivers provide bracing within the cabi- net to dampen vibration but without reducing airflow, which is critical in maintaining the air pressure within the cabinet in this design.
This is all fine and dandy, but a huge number of variables are involved. To attempt to account for all of them in a build-try-rinse-repeat mode would more likely lead to a flight over the cuckoo’s nest than to a great speaker system. The genius in the H-PAS design is the development of a set of differential equations that takes all the important factors into account. Atlantic’s Boaz Shalev showed me some of the equations, developed together with consultant Martin King, who has been researching transmission line design for years. The equa- tions made my head swim. When they are rolled into a computer algorithm combined with finite element analysis (an algorithm that, according to Atlantic, performs 2 million calculations), the human design effort involved is reduced to workable proportions—workable enough to be offered to other companies under licensing agreements managed by Atlantic.