Polk RTA 11t loudspeaker
As Polk declined to supply review samples of the RTA 11t, we bought a pair from a local retailer.
Polk RTA 11t: $950/pair
Polk Audio may have the appearance of a marketing-led company, but according to surveys in the trade press, they appear to have loyal dealers who have a good regard both for the performance and saleability of Polk loudspeakers and for the level of service backup offered by the company. So much for the conventional wisdom! In fact, since its founding in the early '70s, Polk has become one of the most commercially successful specialist loudspeaker companies in the US.
Most of the publicity generated by Polk products in recent years has concerned the "SDA True Stereo" principle, by which one or more extra midrange units in each loudspeaker reproduce an anti-phase version of the opposite channel's feed. As with Bob Carver's "Sonic Hologram" circuit, this is intended to provide a degree of cancellation of the interaural crosstalk signaleach ear normally hears a time-delayed version of the signal intended for the other earthe result being a soundstage that can transcend the speaker positions. My listening to Polk's SDA models has been limited to CE Shows, but while feeling that SDA does indeed widen the stereo stage at some frequencies, I remain unimpressed overall. I found it unable to reproduce what on good conventional speakers should be stable, precisely positioned images. If such soundstage expansion is what you want, I feel the full-range Carver "Sonic Hologram" generator does a much better job (though in my experience, you do need to hold your head in a clamp). When we decided to review a Polk product, therefore, I felt it more appropriate to choose one of their non-SDA loudspeakers.
The RTA 11t is the top model in Polk's conventional line of speakers. An attractively proportioned, deeper-than-it-is-wide, floorstanding tower design, its most obvious feature is the large number of drive-units on the speaker's front baffle. In fact, the lower two 8" units lack magnets and voice-coils and are passively drivenPolk is one of the most successful users of passive-radiator technologywith different mass-loadings applied to each to result in what Polk terms a "high-resonance fluid-coupled subwoofer" and a "low-resonance fluid-coupled subwoofer." (The fluid in question is, of course, the air within the cabinet.)
As with the slug of air in a reflex port, the passive radiator works in conjunction with the woofer to extend the low-frequency extension and reduce cone excursion and distortion levels, the tradeoffs being a sharper (24dB/octave) final roll-out compared with a sealed box's 12dB/octave and a lack of control of woofer motion below resonance. The passive radiator also blocks internal cabinet colorations from reaching the outside world in a more efficient manner than a plain reflex vent.
The RTA 11t features what is generally known in the US as the "D'Appolito" drive-unit configuration (footnote 1), where the tweeter is placed vertically midway between two woofers. Polk's literature states that by having the tweeter positioned coincident with the virtual image of the two woofers, the result is "coincident radiation" from a point source, resulting in "perfect blending at the critical crossover point."
The tweeter is a version of a unit used throughout Polk's extensive range, a soft-plasticdome type with a faceplate that gently slopes away from the dome, this helping with diffractive problems. Called by Polk a "silver-coil" tweeter, I initially thought that this refers to the coil material. Reading Polk's literature, however, suggests that as the tweeter is said to use "high-conductivity coated wire," the "silver" probably refers to the coating. The high-blown terminology also applies to the twin 6.5" woofers, where the "Trilaminate Polymer" appellation refers to a relatively conventional pulp cone that is doped with two different polymers, one to stiffen the material, the second to damp breakup resonances. Butyl rubber surrounds are used, and the woofers are constructed on pressed-steel chassis.
Footnote 1: After a paper presented at the October 1983 AES Convention by Joe D'Appolito, "A Geometric Approach to Eliminating Lobing Errors in Multiway Loudspeakers," AES Preprint 2000. I first saw this arrangement of drivers in 1980, on the Meridian M2 active loudspeaker, but according to Vance Dickason in The Loudspeaker Design Cookbook, it is also essential to use an odd-order Butterworth network (typically third-order, 18dB/octave) if the benefits of an even and symmetrical vertical dispersion are to accrue.