... the brands shift to acoustic suspension design exclusively. ... Boasting compact footprints of 12.5" and 14" respectively,
I applaud NHT's decision to get back to the acoustic suspension design principle. The acoustic suspension design principle is inherently superior from a musicality standpoint, because it avoids the introduction of a resonance ripple overlaying the sensitivity curve, and because it is inherently lower in distortion, owing to the fact that the centering force for the diaphragm remains more nearly linear over the range of motion of the driver.
The reasons why Kloss and his pals came up with the idea of acoustic suspension back in the late '50s were fully legitimate, and they remain as legitimate today as they were then. The shift in commercial speakers to ported enclosures and passive radiators in recent decades is a reflection only of the fact that for a speaker to be commercially successful, the quantity of bass matters a whole lot and the quality of bass matters hardly at all. When this trend first started back in the '70s and early '80s, few people would dared to have suggested that there was anything truly superior about ported enclosures, from a musical or hifi standpoint. But because it has gained so much in popularity, most people nowadays assume that there is nothing inherently wrong with ported enclosures from the standpoint of accuracy and musicality, but this is wrong.
That said, not all sealed enclosures are true acoustic suspension designs. You can seal up the enclosure and avoid the Helmholtz resonator effect and the ripple, and that solves most of the problem. These new passive radiators probably do that, and on this basis alone, and assuming that the damping is adequate and the power is adequate, they are inherently superior to almost all other subwoofers of similar size. If I were presently in the market for a subwoofer, I would certainly take a good listen and pay attention to the extension into very low frequency and the avoidance of the "one note" bass effect.
Yet, unless the enclosure is very large, these subwoofers are not true acoustic suspension designs. To qualify as such, the centering force on the diaphragm has to derive from the pressure difference, not the stiffness of the suspension. Drivers that are true drivers for acoustic suspension designs use very supple suspensions. You can tell the difference when the driver is removed from the enclosure because the slightest push on the cone causes it to move. The difference is unmistakable, once the driver has been removed from the enclosure. You can put a driver with a stiff suspension in a sealed enclosure and get results that are superior to ported enclosures and passive radiators. And that is a good thing, and is probably what these new subwoofers do. But a true acoustic suspension design, that uses the pressure difference for the centering force and that is consequently lower in distortion because the centering force remains more nearly constant with displacement of the diaphragm, requires a comparatively large enclosure. In a smaller enclosure, the pressure difference at large excursion is great such that the advantage is lost. The centering force can vary more even than is typical with a driver having a stiff suspension, in which case it makes more sense to use a driver with a stiff suspension that provides a reasonably constant centering force over an adequate range of excursion, compensating for the loss in efficiency through an increase in power. You can do this, and get a result that is musically superior to what is possible with ported enclosures, but it still is not quite the same thing as a true acoustic suspension design, which uses a very floppy mechanical suspension, a comparatively lesser amount of magnetic flux, and a very large enclosure in order that the centering force will remain nearly constant with large cone excursion. These were the not-so-secret secrets to good high-fidelity speaker design forty years ago, and it has not changed. What has changed is that commercial speaker designers stopped caring about accuracy and high fidelity and started caring more about quantity of bass, which they were forced to do because when Joe Sixpack walks into the electronic megaplex to buy speakers, what he is looking for, to the exclusion of almost everything else, is the perceived quantity of bass.
