Originally Posted by Bill Fitzmaurice
Ported designs go back just as far as sealed, and while all acoustic suspension cabs are sealed, not all sealed cabs are acoustic suspension. For that matter today you can hardly find a true acoustic suspension cab.
Quite interesting reading: (from wikipedia - loudspeaker enclosure)
"Before the 1950s many manufacturers did not fully enclose their loudspeaker cabinets; the back of the cabinet was typically left open. This was done for several reasons, not least because electronics (at that time tube equipment) could be placed inside and cooled by convection in the open enclosure.
Most of the enclosure types discussed in this article were invented either to wall off the out of phase sound from one side of the driver, or to modify it so that it could be used to enhance the sound produced from the other side. However, a few designs have ventured in a different direction, attempting to incorporate the natural acoustic properties of the cabinet material rather than deaden it, and shape the cabinet so that the rear can remain open and still provide good bass response with limited comb filtering.
The loudspeaker driver's moving mass and compliance (slackness or reciprocal stiffness of the suspension) determines the driver's resonant frequency. In combination with the damping properties of the system (both mechanical and electrical) all these factors affect the low-frequency response of sealed-box systems. Output falls below the system's resonant frequency (Fs), defined as the frequency of peak impedance. In a closed-box, the air inside the box acts as a spring, returning the cone to the 'zero' position in the absence of a signal. A significant increase in the effective volume of a sealed-box loudspeaker can be achieved by a filling of fibrous material, typically fiberglass, bonded acetate fiber (BAF) or long-fiber wool. The effective volume increase can be as much as 40% and is due primarily to a reduction in the speed of sound, and not to the popular misconception of a change in operating conditions from adiabatic to isothermal. The enclosure or driver must have a small leak so internal and external pressures can equalise over time, to compensate for barometric pressure or altitude; the porous nature of paper cones, or an imperfectly sealed enclosure, is normally sufficient to provide this slow pressure equalisation.
Acoustic suspension or air suspension is a variation of the closed-box enclosure, using a smaller box to exploit the almost linear air spring which results. The "spring" suspension that restores the cone to a neutral position is a combination of an exceptionally compliant (soft) woofer suspension, and the air inside the enclosure. At frequencies below system resonance, the air pressure caused by the cone motion is the dominant force. Developed by Edgar Villchur in 1954, this technique was used in the very successful Acoustic Research line of "bookshelf" speakers in the 1960s-70s. Although no longer popular in commercial designs, the acoustic suspension principle takes advantage of this relatively linear spring. The enhanced suspension linearity of this type of system is off-set by rather low efficiency. Drivers for these designs rely more upon the enclosure characteristics than typical drivers, and most modern woofers are not well suited to acoustic suspension use.
Also known as vented (or ported) systems, these enclosures improve low-frequency output, increase efficiency, or reduce the size of an enclosure, using cabinet openings or passive radiating elements to transform and transmit low-frequency energy from the rear of the speaker to the listener. They deliberately and successfully exploit the principles of the Helmholtz resonator. As with sealed enclosures, they may be empty, lined, filled or (rarely) stuffed with damping materials. Port tuning frequency is a function of cross-section and length. This enclosure type is very common, and provides the maximum deep-bass output for a given enclosure volume. Malcolm Hill pioneered the use of these designs in a live event context in the early 1970s. Vented system design using computer modeling has been practiced since about 1985, when researchers Thiele and Small first systematically applied electrical filter theory to the acoustic behavior of loudspeakers in enclosures. While ported loudspeakers had been produced for many years before computer modeling, achieving optimum performance was challenging, as it is a complex sum of the properties of the specific driver, the enclosure and port, because of imperfect understanding of the assorted interactions. These enclosures are sensitive to small variations in driver characteristics and require special quality control concern for uniform performance across a production run."
I don't remember open back speakers, clearly before my time (pre 1950s). I do remember sealed speakers, and the AR "acoustic suspension" reference. I was quite young but my brother in law had a pair of the early bookshelf ARs. The reference dates above also supports my assertion that mass produced bass reflex or ported cabs came later after sealed cabs.