ESS (Electro-Static Sound) was a company that was based in Sacramento, California. Their original speaker designs were a hybrid of conventional woofers, passive radiators, and electrostatic tweeters in bookshelf and tower configurations. They were one of the only manufacturers of the patented Air Motion Transformer (AMT) speakers designed by Oskar Heil.
The first ESS flagship product was the ESS TransStatic-1. It used a design with electrostatic tweeters, an infinite baffle mid-range in a floor-loaded Transmission Line bass cabinet driven by the racetrack shaped KEF woofer. The transmission line bass cabinet unloaded onto the floor, resulting in astonishingly strong, deep bass output with crystalline clear highs from the functionally mass-less electrostatic tweeters. They were transparent and "airy." The TransStatic-1 was approximately 48" tall with a 14"x24" footprint. The TransStatic-1 sold for $999/pair in Walnut and $1,399/pair in Brazilian Rosewood in 1972.
There was a bookshelf variant with a KEF woofer, fewer tweeters and no transmission line. There was also a TransStatic-2 which was in essence four TransStatic-1 speakers, stacked two to a side with the top half flipped. This resulted in woofer cabinet exit points being at opposite sides of the stack.
ESS bought patent rights to the Air Motion Transformer (AMT) and their first product was the amt1. Looking like a truncated four sided pyramid; the system combined the amt driver with a 10 inch woofer and bass port. Several variations followed including the amt1a which replaced the 10" woofer with a 12" woofer and included a large passive radiator, giving the product better lower bass response characteristics, thusly reducing audible low frequency over-modulation. The amt1 tower was a loudspeaker design with 1 amt and a woofer and line transmission. The amt3 was a follow-up design with 1 amt, a 6.5 inch mid-range and two 10 inch woofers. Nelson Pass, currently of Passlabs (an amplifier builder) was employed by ESS during the production of the early amt-loaded speaker designs incorporating 10-inch woofers. The larger successors were a basic maturation of the pyramid styled amt1 and were always coupled with a radiator or, drone. Low frequency 10-inch drivers were subsequently replaced with a 12-inch woofer. Early designs incorporating ported transmission-line cavities were with Oskar Heil's blessing, while the 12-inch passive coupling and removal of ported cabinets from production were marketed as a logical distortion-free accompaniment. This succession of bass reproduction however, is claimed to be not fast enough to keep in with the speed of the amt and provide the listener with issues of severe harmonic over-production.
At the end of the 70s, ESS and Heil introduced the ESS Transar using one AMT for the high frequencies and a special mid-low frequency driver unit.
Until 2006, it was still possible to get ESS speakers from Sacramento on demand. In Germany, it was also possible to get ESS speakers (new euro-styling designs developed and manufactured by company Myro in Celle) as well as the AMT drivers alone, until 2008 when production was halted due to cost overruns.
In 2009, a recent reopening in Southern California has been met with consumer consternation as ESS has ventured into the car video arena offering headrest placed monitors typically in use with SUV's.
The most common use for the AMT driver is as a midrange-tweeter or tweeter in high-end multi-driver speakers, sometimes paired with horns, or in the case of Precide's speaker products, with an upward-firing woofer driver.
5 separate patents for the production and licensing of the Air-Motion Transformer (including any and all upgrades to its pleated membrane) expired sequentially in:
Jan. 1, 2002
Aug. 2, 2007
Nov. 1, 2007
Aug. 15, 2008
Jul. 10, 2009
There are a couple of companies producing Air-Motion Transformer style systems: Precide (Switzerland) who calls their version the AVT (Air Velocity Transformer), SoundTouch(China, www.audiopur.com) ELAC (Germany) who calls their version of the transducer JET or PI, ADAM (Germany).