Klipsch RB-15 loudspeaker
Klipsch is one of the longest-surviving names in American audio. The company was founded by Paul W. Klipsch in Hope, Arkansas, in 1946 to manufacture the corner-loaded horn speaker that was to become known as the "Klipschorn." Paul passed away at the age of 98 on May 5, 2002, after an illustrious career, but his Klipschorn is still in production, relatively unchanged, more than half a century after its introduction.
The $299/pair Klipsch RB-15 is the smallest model in the now-Indianapolis-based company's current Reference series. Its plastic front plate is almost entirely taken up by the 5.25" woofer's copper-anodized aluminum cone and rubber roll surround, and the square opening of the horn that loads the 1" titanium-dome tweeter.
Horn theory is one of the more arcane subjects in audiophiledom. A horn acts as an impedance converter, allowing a small and therefore high-impedance diaphragm to efficiently transfer acoustic power to the low-impedance atmosphere. How it does this depends, among other things, on the horn's flare, with proponents of the various kinds of flares that can be used coming to blows at regular intervals about which offers the best transfer of acoustic energy, the widest bandwidth and dispersion, and the lowest coloration and distortion.
The RB-15's tweeter uses what is called a Tractrix flare, first developed by another of the great names of early speaker design, P.G.A.H. Voigt, in the late 1920s, and rediscovered in modern times by horn aficionado Bruce Edgar. This is a complicated curve that combines some of the elements of a conical flare with hyperbolic and exponential profiles to give what Klipsch's designers regard as the best of all worlds: controlled, even dispersion, a wide bandwidth, and optimal loading for the drive-unit diaphragm.
The RB-15 tweeter's Tractrix horn is part of the plastic molding that covers the front baffle and trims the woofer's mounting flange. The horn profile differs in the horizontal and vertical planes to give coverage angles of 90 degrees and 60 degrees, respectively. The direct-radiating woofer is reflex-loaded by a deep rectangular port with rounded sides on the cabinet's bottom rear. Electrical connection is via two pairs of binding posts on a plastic panel inset into the cabinet rear. The crossover is mounted on a printed circuit board attached to the inside of this panel; it uses two inductors, three plastic-film capacitors, and a resistor. The internal wiring is quite substantial, a sticker proclaiming it to be sourced from Monster Cable.
The RB-15's foam-lined cabinet is made of ¾" MDF and covered in woodgrain vinyl. A single horizontal brace provides some control of sidewall resonances. Overall, the RB-15 seems to be well-made and is quite attractive-looking, despite its low price.
A reviewer is always faced by a conflict between system stability and component churn: To do a responsible review means keeping everything the same, but the review system might well then be unrepresentative. For example, these $299/pair Klipsch speakers will never be used with the expensive components with which I auditioned them. All I can do, therefore, is describe the speaker's character as accurately as I can.
The Klipsch RB-15s were placed on 24" Celestion Si stands. Their tonal balance seemed relatively unaffected by their placement in the room, but I did find that what there was of the low frequencies became too hooty when the speakers were close to the wall behind them. "What there was"? The 1/3-octave warble tones on my Editor's Choice CD (Stereophile STPH016-2) were reproduced in full measure down to 100Hz, with some output audible in the 80Hz and 63Hz bands, but nothing below that. The organ tones on the Dream of Gerontius excerpt on Test CD 2 (Stereophile STPH004-2) were missing in action, though the orchestra was reproduced with a pleasingly warm tonal balance.
The RB-15 is definitely a miniature, but it did sound a little larger than it looks. The bass-guitar channel-identification tracks on Editor's Choice were decidedly lightweight, but the instrument sounded quite "boppy," with an emphasis on its harmonics, even with the iron-fisted control exerted by the relatively lean-sounding Musical Fidelity amplifier. This was not unpleasant—the bass voices on Cantus's new CD of spirituals, Deep River (CTS-1203, available from this website), were reproduced high enough in level for musical satisfaction—but it might become too warm and indistinct with, for example, a tube amplifier.
At the other end of the spectrum, there was some mid-treble emphasis, in that flutes were pushed forward in the soundstage and violins occasionally sounded a little more steely than usual. However, the RB-15's treble sounded very smooth overall, which is commendable at this price point. There was no emphasis of tape hiss, no thrusting forward of sibilance on female voices, and cymbals sounded more like bronze platters than shaped and textured white noise. I did find that I needed to sit exactly on the axes of the horn-loaded tweeters to get a sufficient degree of top-octave air; otherwise, the speakers lacked sparkle.
However, at $300/pair there has to be some compromise, and the Klipsch's weak point was its midrange. The solo clarinet on Mosaic (Stereophile STPH015-2) seemed to excite some midrange resonances, some notes jumping forward and sounding blurry. Piano, too, sounded uneven, with some notes sounding louder, less well-defined than others. Listening to the cabinet walls with a stethoscope while the speaker reproduced the half-step-spaced toneburst scales on Editor's Choice revealed all the panels—particularly the rear—to be very lively between 200 and 300Hz, and again an octave higher.
When I stood behind the RB-15, I heard a rather hooty coloration emanating from the port. Fortunately, this was very much suppressed at a normal listening position, but it did lend fast piano passages a rather confused character by emphasizing the noises of the action, giving the instrument a bit of a "clattery" character. In this respect, rock and electronic music fared better than acoustic classical or jazz.
Dynamics were necessarily limited by the small size of the woofers, though the speaker did play reasonably loud with only a few watts of power. However, massed voices took on a hard quality at levels much above 90dB at the listening position. But while hitting the speaker hard with high levels of low-bass information caused it to work very hard, with some wind noises coming from the small port on the cabinet's rear, there was not as much audible distortion as I was expecting.
At the end of the review period, I tried the RB-15s in my bedroom system, again on Celestion stands but driven by a Linn Classik receiver and a Panasonic DVD player, where it fared very well. More important, my opinions of the speaker's strengths and weaknesses were not changed by its performance in this real-world setting.
One I'd grown accustomed to its lack of top-octave energy, its lack of low frequencies, and its rather "boppy" upper bass, the Klipsch RB-15 offered a basically well-balanced sound at a very competitive price. The horn-loaded tweeter is particularly fine, I feel, though the coloration from the RB-15's rather lively cabinet is going to bother some listeners more than others, especially if they are lovers of small-scale classical music.
The RB-15 comes up against stiff competition in its price range from the Epos ELS-3 (reviewed by Bob Reina in January 2004), the Alón Li'l Rascal Mk.II (reviewed by Bob in December 2003), the Paradigm Atom 3 (reviewed in September 2002), and the PSB Alpha B (May 2002). None of these speakers is perfect, each offering a different balance of virtues and drawbacks. You should audition all of them to see which best matches your tastes and needs, but the little Klipsch can hold up its head in this distinguished company. It is definitely a contender.