Last truly visible Acoustic Research from the beginning of the Teledyne ownership and before the subsequent various ownerships and fortunes.
Not that AR had been as successful in the 1970s as it had been throughout the 1960s but at least it had been present.
The 1980s and 1990s turned out to be more of a disappearing act improvised in several steps... even seeing an attempt at an AR3a reissue in Japan.
The AR9 is a very vertical 59kg four-way loudspeaker with five drivers.
The two 30cm drivers are placed on the sides of the enclosure to keep reflected sound-cancelling waves above 200Hz - big room necessary because that makes four 30cm drivers outputting long waves of sound !
The goal of this was to avoid the "wall dip" : where walls' reflections cancel out some of the bass output, making the transition between low mid and top of the bass range difficult and straining the amplifier's output.
The filter was also designed accordingly with two ranges taken into account. Neutral above the resonant frequency, it kicks in actively (so to say) below that frequency to keep damping as efficient as possble while maintaining output.
Nominal impedance is spec'ed at 4 Ohm with a big dip down to 3,2 Ohm.
It is naturally there, with the 'two way' filter, the acoustic suspension and the moderate 87dB efficiency that some amplifiers will scream "I can't do this !" and some won't mind at all.
The other three drivers are placed vertically, all surrounded by a big (and patented) "acoustic blanket" to keep enclosure reflections to a minimum.
The high-mid and high drivers are liquid-cooled and three 3-position switches (resistor networks) allow to fine-tune response of the front drivers. In-house made drivers, of course.
The AR9's stringent engineering made it and still makes it somewhat difficult to drive, a perfect loudspeaker to some and nothing special to others...
We all have different ears. And different amplifiers. And different rooms.
The 1978 AR9 engineering in Tim Holl's own writing here, complete with complete measurements and graphs.