The acostic-suspension speaker, introduced about 25 years ago, was the first to provide ample bass response from what was then regarded as an impossibly small enclosure (about two cubic feet). Prior to the a/s system, full bass response was available only from larger, floor-based systems. The a/s format, fitting comfortably into most living rooms, caught on quickly in pre-stereo days and really took off with the rise of stereo since even two such speakers did not take up an impossible amount of installation space. And they did sound good.
There was, of course, a catch. The kind of sound the a/s speaker produced demanded relatively "high" amplifier power, or at least about 3 dB higher than was customarily regarded as "adequate" for some hi-fi (that is, a 20-watt amplifier instead of a 10-watt amplifier).
As amplifier design goes, neither 20 watts nor 10 watts seems like a lot of power these days, but if you translate these wattage numbers into listening terms (descibels of sound-pressure-level, or dB/SPL), look what happens when the listening volume is doubled. The 10-watt system suddenly has to produce 100 watts; the 20-watt system, 200 watts (each doubling of apparent loudness involves tenfold increase in wattage). Hold onto this idea for a moment.
All other things being equal, it would be desirable to increase the effeciency of any speaker system (i.e., get it to produce more sound for the same amount of amplifier power, or comparable sound for less amplifier power, or comparable sound for less amplifier power). But efficiency is tied in with the system's size and with its ultimate bass response. The original "two-cubic-footer" was regarded for many years as an optimum balance of trade-offs among the three factors of size, efficiency and bass response. There were many examples of smaller a/s systems with less bass, and there were several of the same size with higher efficiency -- and less bass.
Today, the taboo of staying within the two-cubic-foot size has all but vanished. AR, the company that launched the a/s idea, is now producing larger systems that in no way can be called, even by over-eager ad copy writers, "bookshelf size." An example is the AR-19 which stands 31 1/2 inches high. Efficiency as such is about the same as in older AR systems but the bass reaches down a little deeper. The midrange also sounds fuller, more "opened up."
Not quite as "large" but still almost a cubic foot bigger overall than the traditional a/s speaker is the Dynaco A-250 (25-by-14 1/4-by-14 1/4 inches). Its efficiency is a little higher than that of the AR-91 -- for instance, it takes about 13 watts for the Dynaco to produce an amount of sound that requires 20 watts for the AR-91. In return, however, the Dynaco's bass outputs rolls off a little more than the other's -- its is strong down to about 40 Hz as compared to 35 Hz in the AR.
One of the highest-efficiency a/s systems is the Infinity RSa which needs only 8 watts to produce loudness levels comparable to the AR at 20 watts, or to the Dynaco at 13 watts. Its overall size is closer to the old two-cubic-foot formula. But again, the deepest bass rolls off a little more than the others -- this time, it is strong down to about 45 Hz.
For a really small size, but high-quality a/s system, there's the Allison: Four (11 inches high; 19.6 inches wide; 10 inches deep). This system manages to respond cleanly to below 50 Hz, but the price you pay for this kind of response from such a petite speaker is its really low efficiency; it needs about 30 watts to produce sound levels comparable to the others.
Efficiency, size and ultimate bass aside, all of these speakers sound generally better than older a/s systems. For this we can thank the improvements in diaphragm materials, cabinet construction, crossover network design and so on. None of them has broken the laws of physics, although they all seem to have bent them somewhat in favor of the stereo listener.