Product Review - Polk Audio RT800
Floorstanding Speakers - December, 1997
By Karl Suager
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Polk Audio RT-800 Floorstanding Loudspeakers; Two-way vented design;
One 1" tri-laminate metal dome tweeter, two 6 ½" mid-bass drivers; Rated
Frequency response, 42 Hz - 25 kHz (± 3 dB); Nominal impedance 8 Ohms; Rated sensitivity,
90 dB/w/m; Cabinet dimensions 40 ¼" H x 8" W x 10 ½" D; Base dimensions,
9 ¼" W x 14" D; Detachable rubber feet; Weight 38 pounds each; Black woodgrain
vinyl; $900/pair; Polk Audio, 5601 Metro Drive, Baltimore, Maryland 21215, USA; http://www.polkaudio.com
Polk Audio is one of those companies that knows what they're doing. After all, longevity in the speaker business doesn't come easily. Competition is pretty fierce, and almost everybody has an opinion about the best way to design a device that necessarily pits an engineer against a flood of compromises due to cost restraints, marketing priorities, and the inflexible laws of physics. Whether or not Polk Audio makes the best products on the planet, the company undeniably sits in a line of survivors.
But Polklore regarding who is the best of the best doesn't interest us at the moment. Besides being a very subjective matter, Polk's stated design goal, in the RT Series and in their speakers in general, has been a combination of performance and value, a combination which, in addition to clever marketing and design methods, probably accounts for the company's success.
Polk holds over 20 patents in loudspeaker design. I don't know what they are, but a registered trademark they like to associate with their speakers in a very scientific fashion is "Dynamic Balance" ® Technology. If you took this term literally, it would mean the balance of the dynamics, a dynamic adjustment of some kind of balance between whatever, or however you chose to interpret it. By itself, it's really pretty vague. In Polk Audio's case, they engaged in a joint project with Johns Hopkins University, Baltimore, Maryland, and developed an imaging technique based on lasers that enabled them to see patterns of vibrations in driver diaphragms that manifest distortion. This allowed them to more easily develop diaphragms with better structural and damping characteristics. It also provided a great marketing tool, which is why they called Dynamic Balance Technology.
Even though I've enjoyed Polking fun at their creative marketing, minimizing vibrations on the speaker diaphragm, or membrane if you wish, does lower distortion and minimize, if not eliminate, annoying artifacts like"ringing." Many metal dome tweeters are notorious for this (as in ringing like a bell), despite the subjective extension and clarity of such drivers that many users love. The tweeters of the RT800s, as well as the mid-bass units, take advantage of this research methodology. Membrane vibrations aren't everything, though, as suspension, motor linearity, and damping also come into play. Then there's cabinet resonance, cabinet tuning (in the case of ported designs), diffraction effects, and a whole swarm of crossover decisions to be made. Polk Audio made these decisions quite wisely, I think.
After tipping each of the RT800s over on their poor little heads to get these things out of the box, I raised an eyebrow at the cabinet construction. Very solid, very dense, well-braced, and the front baffle, covered with of some kind of semi-soft polymer, rang dead with a knuckle rap. Considering the efforts taken in the cosmetics of the speaker, with its fancy schmancy front baffle, I did not expect build quality of this caliber from a full-range speaker at this price, especially when so much of the competition from main-stream manufacturers has chosen to build pretty, but cheap. Bravo Polk Audio!
The stylized front baffle also serves a functional purpose. The 1" thick MDF is covered with a soft damping material which resembles the appearance of rock. The tweeter sits inside a mounting plate that, when screwed into the baffle, sandwiches the mid-bass drivers between itself and the baffle. This does two things. With the supplied grille, all drivers, inset slightly, have a smooth path from the diaphragms, along the face of the cabinet, to the curved edges, reducing diffraction artifacts, (sound waves reflecting off the baffle after turning sharp edges.) Diffraction problems can cause frequency response variations that may skew tonality in addition to causing delayed arrivals that result in smearing and clumping of the stereo image. In addition, the rubber surround of the mid-bass drivers extends to the edge of their steel baskets. In turn, the metal baskets themselves only touch the acoustically dead surface of the polymer-treated baffle and their own acoustically dead suspension material, ensuring minimal vibration transfer to the cabinet itself.
The cabinet utilizes a downward firing port with a somewhat unique flair. Polk calls the design a "Power Port". The cabinet itself is elevated above a stand. That stand places a cone with a concave curvature directly below the port, minimizing turbulence while more effectively loading the room in the lower frequencies which benefit most from this arrangement. Clever, I'd say. There is also a smaller port immediately beneath the lower woofer.
Not able to resist my own curiosity, I pulled out the crossover module attached to the dual sets of binding posts (allowing passive bi-amping and bi-wiring.) It consisted of a simple 2nd order high-pass filter for the tweeter in line with a pair of resistors for attenuation. The woofers, wired in series, met up with a single inductor, resulting in a 1st order electrical slope. I stress electrical slope, as opposed to actual roll-off, because I think there's a bit of smart planning in this. The RT800s, like most speakers from Polk Audio, intentionally use a relatively high crossover point for a 2-way system, between 3 and 4 kHz, in order to move that crossover point farther away from the critical mid-range between 500 Hz and 2 kHz. If the mid-bass drivers have sufficient bandwidth to accomplish this, problems can still arise if not addressed. Larger diaphragms beam higher frequencies. The larger the diaphragm, and the higher the frequency, the more it beams. Speakers begin to beam as the wavelength approaches the diaphragm size. A 2-way speaker with 6" drivers crossed over at 2 kHz, wouldn't have much problem with beaming, as the wavelength of 2 kHz is about 6 inches (about), and so beaming above 2 kHz isn't an issue, as the mid-bass doesn't receive the information. A mid-bass with response approaching 4 kHz, however, will beam. If the mid-bass does extend this far, it means a rising response, and a peak before final roll-off, which may translate into an overly forward, even harsh sound. I'm speculating that the shallow crossover compensates for the natural rising response of the driver until the crossover point, at which the combined natural roll-off of the driver and the 1st order electrical crossover provides an actually sharper acoustical roll-off. The combined slopes for the tweeter and woofer are probably somewhere around a 3rd order (18dB/octave.) Like I said, clever.
Room response will vary from room to room, and location to location. I tried many positions, and found the best match several feet from the rear wall for bass response, toed in considerably for the most articulate and defined imaging capability. So, did it sound clever? Clever is probably the most appropriate term. These speakers may be one of the most ear-friendly to the most people you'll ever find. The RT800s didn't manage the "peer into the window" depth of my Infinity Renaissance 90s but, to be fair, those speakers aren't available anymore, and if they were, would cost four times as much.
|Room Response - Polk Audio RT800 Loudspeakers|
|1 meter, left channel, grille on||13 feet, left channel, grille on|
|20 Hz||63.2 dB||20 Hz||74.1 dB|
|25 Hz||71.9 dB||25 Hz||82.4 dB|
|31.5 Hz||78.4 dB||31.5 Hz||87.3 dB|
|40 Hz||77.9 dB||40 Hz||81.4 dB|
|50 Hz||82.0 dB||50 Hz||63.2 dB|
|63 Hz||89.4 dB||63 Hz||88.7 dB|
|80 Hz||91.0 dB||80 Hz||97.5 dB|
|100 Hz||97.7 dB||100 Hz||94.6 dB|
|125 Hz||84.2 dB||125 Hz||95.4 dB|
|160 Hz||87.2 dB||160 Hz||85.5 dB|
|200 Hz||83.1 dB||200 Hz||91.9 dB|
|500 Hz||83.1 dB||500 Hz||85.8 dB|
|800 Hz||83.4 dB||800 Hz||83.6 dB|
|1 kHz||78.5 dB||1 kHz||80.1 dB|
|2.5 kHz||79.3 dB||2.5 kHz||76.5 dB|
|5 kHz||80.0 dB||5 kHz||79.7 dB|
|8 kHz||78.2 dB||8 kHz||72.4 dB|
|10 kHz||78.4 dB||10 kHz||68.7 dB|
|12.5 kHz||78.2 dB||12.5 kHz||76.2 dB|
|15 kHz||77.6 dB||15 kHz||72.9 dB|
|18 kHz||75.0 dB||18 kHz||71.3 dB|
The RT800s are also four times as efficient as the Renaissance 90s.
At 90 dB/2.83 volts/meter with an 8 Ohm nominal impedance, the sensitivity is double, and
the power draw half compared to 87dB/2.83volts/meter at a 4 Ohm nominal impedance. This
means that otherwise exceptional sounding but lower powered receivers or integrated amps
will work well, unhindered by a demanding load. In fact, the RT800s sang along
effortlessly not only with my beefy Aragon 8008BB, but perfectly in stride with the
beautiful, but less powerful, Myryad MI-120, rendering superb lateral image definition
rivaling, if not surpassing, my personal reference speakers.
When I first saw the "overall frequency response" bottom limit of 28 Hz, I did crack a chuckle. At -10 dB, maybe. Without active intervention, extension of that kind within the accepted -3 dB limit simply can't happen with a speaker as small and efficient as this. The -3 dB rated low-end limit is 42 Hz, which sounds correct. The RT800's bass extension did catch the body and weight of most material, but nevertheless, could significantly benefit from a good subwoofer. Then again, how many speakers can't benefit from a good subwoofer, or two for that matter? Aside from that, the treble delivers immense detail for the asked rate of exchange. Slightly understated, certainly refined, it has no obvious flaws. The upper mid-range does hold back a little, letting the upper end accept the kudos. The lower mid-range, without significant coloration, remains neutral for the most part until the lower bass (between 40 Hz and 80 Hz,) where the response peaks slightly, contributing a somewhat thumpy character. Clever again.
Huh? Think about it though. Although the upper mid-range and lowest of the speaker's bass region depart from what I deem ultimately more accurate, this does not equate necessarily to a weakness. The overall detail excels. A slight rise in the lower bass adds a sense of body without making vocals overly corpulent. A slight depreciation in the upper mid-range tilts the spectrum so that most recordings and soundtracks will inherently sound more pleasant. Punchy, rich, and smooth, even if not completely accurate, ensures that the opposite, exacerbated by some recordings and/or listening environments, most likely won't occur. Thin and harsh are the audiophile's nightmare. Unless the recordings and components are extremely messed up, it simply won't happen with these speakers.
They're fun for both theater and musical performances. They provide refined detail through most of the spectrum, image well, are built well, and will most likely mate favorably with a wide variety of associated equipment. Plus, for a full-range speaker, they're not all that expensive. If you're looking for a pair of new speakers with detailed, amiable sound, a load cooperative with any quality receiver, integrated, or power amp, and plan on dropping about a grand, check these cuties out. Personal preference will vary, but they do what they were designed to do, and they do it well.
Associated components used:
Myryad MC-100 CD player
Myryad MI-120 Integrated Amplifier
Aragon 8008BB power amplifier
Passive controller w/50 k Nobel Pot
DH Labs Silver Sonic interconnects & speaker cable
Bybee/Curl prototype power purifiers
API Power Pack V AC line conditioner
Infinity Renaissance 90 loudspeakers
© Copyright 1997 Secrets of Home Theater & High Fidelity
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