passive radiator = port = phoney bass
I presently own a Definitive Technology SuperCube II. It is an exceptionally nice subwoofer for its size, and I fully understand that given the small size, it is a practical necessity that it have passive radiators, or else be ported.
That said, the best subwoofers are based on the acoustic suspension principle, and when I eventually get around to buying another subwoofer, it will be an acoustic suspension subwoofer. It will necessarily be large, but that is price you have to pay.
The bass you get with ported enclosures (and passive radiators are in every significant way equivalent to ported enclosures) is phony bass as far as I am concerned, and it never ceases to befuddle me that more people do not realize this. The fact that almost all commercial speakers these days use ported enclosures or passive radiators and that the inherently superior acoustic suspension approach has largely fallen by the wayside, is a sad commentary on what has happened in the hifi industry over the past 2-3 decades.
Ported designs, to include passive radiators, turn the enclosure into a Helmholtz resonator. The mere fact that the word "resonator" is the primary descriptive word should tell you something. A resonance is being superimposed on the speaker's sensitivity curve. The resonance has a pronounced fundamental peak, and it is positioned, by way of tuning or aligning the port/radiator to the enclosure volume, so that it occurs at the frequency that is deemed optimal. But in fact the process of tuning or alignment amounts to deciding where the peak in the ripple should occur and then causing it to occur at that frequency. The rolloff below that frequency is abrupt, and this is contrary to the basic notion of a subwoofer. The idea of a subwoofer is that it should extend the bass response as deeply as possible, even below 20 Hz. Vinyl records involve equalization in order to deal with the mechanical limitations of the groove and tracking, and they are inherently limited in deep bass as a result. The same is not true of CDs and digital recording in general, where bass often occurs even deeper than 20 Hz.
It is just wrong to apply a ripple to the sensitivity curve of a subwoofer, because the effect is to emphasize bass at one frequency that is deemed, subjectively, as low as is needed or desirable, while dramatically de-emphasizing the sensitivity at lower frequency. The effect of the ripple is also evident at frequencies above the tuning frequency. The only way to avoid the ripple is to not use a passive radiator or port, because the effect of the principle, that is the rationale for using ports or passive radiators, comes by way of the ripple. You can't have the beneficial effect without the ripple, because the effect IS the ripple. Even if the resonance is damped so as to avoid the typical hump at the tuning frequency and flatten out the response down to a certain frequency, the response is still going to roll off abruptly below the tuning frequency, and the ear will still perceive the corner as a resonance or peak in the response. In other words, the "one note" bass effect will still apply even if the hump is avoided and replaced with a sharp corner in the response. The only way to avoid that effect is to avoid the corner, i.e., is for the response to roll off gradually and smoothly, with no abrupt roll off below the tuning frequency. This is why older acoustic suspension speakers in general have more natural sounding bass than almost all modern speakers that use ports or passive radiators. People who are accustomed to the sound of good acoustic suspension speakers generally find it difficult to listen to ported speakers, and there is no mystery why. The bass you get with ported enclosures and passive radiators is quantity in lieu of quality. It's just that simple. It's all about the perception of greater quantity of bass for the typical consumer who walks into a showroom at an electronics megastore and listens for speakers that have exaggerated, cheap, non-musical bass. Even most of the commercial brands associated with high quality have long since been forced to jump on the ported speaker bandwagon, because it is what sells. But the fact that it sells better says nothing about the true sound quality. If you want true sound quality, one of the things that you want to look for is true acoustic suspension, and not ported enclosures or enclosures with passive radiators.
