TDL RTL2 Floor standing speakers
In general, bass speaker enclosures can be classified as closed (fully sealed) or vented through one or more holes (ports). Although each type has its advantages and drawbacks, practically all speakers fall clearly into one or the other category.
There is, however, another type, known generically as transmission-line, that might be viewed as something of a hybrid of the other two. The enclosure is ported through a long, tapered duct that contains damping material to absorb most of the back wave from the woofer while still providing reinforcement of the lowest frequencies. The English company of TDL
Electronics has been known for its transmission-line speakers for some twenty-five years. Although recognized for their excellent acoustic properties, their internal complexity has limited transmission-line speakers mostly to relatively high price brackets (TDL's larger speakers cost several thousand dollars a pair).
TDL has recently introduced a line of smaller and more affordable speakers, using what it describes as a "reflex transmission line" design, although specific internal construction details are not indicated in the company's literature. The middle model of the group, the RTL2, is a compact, floor-standing columnar speaker, not much larger than a typical system intended for bookshelf mounting. It is a two-way system, whose single 6-3/4-inch, long-throw, polypropylene-cone woofer has a rubber surround, a two-layer voice coil, and a hard plastic phasing plug at its center. There is a second-order (12 dB per octave) crossover at 3 kHz to a 3/4-inch soft-dome tweeter with magnetic-fluid cooling.
The cabinet is finished on all visible surfaces as well as on the front panel, which is normally protected by a removable black cloth grille, retained by plastic snaps. The woofer and tweeter are on the upper portion of the front panel, and the bass port is at the bottom, its relatively large area filled with a plastic foam material. Transmission-line enclosures normally contain a relatively long, labyrinthian sound path, terminating in an opening to the exterior, and presumably the RTL2 has a comparable internal structure. Four insulated binding-post connectors (separate pairs for the two drivers) are recessed into the back panel and are normally joined by wire links. Removing the links enables bi-wired or biamplified operation of the system. As with most British speakers, the terminal spacing is intentionally too large for double banana plugs, but they are compatible with single banana plugs as well as bare wires or spade lugs.
The RTL2's rated frequency range is 40 Hz to 20 kHz (no tolerance given), with a sensitivity of 87 dB sound pressure level (SPL) at 1 meter for a 1 -watt input to its nominal 8-ohm impedance. It is recommended for use with amplifiers delivering between 20 and 80 watts per channel.
In our close-miked woofer measurement, the RTL2 produced an exceptionally flat and extended response, varying only +2.5 dB from 60 Hz to 2.5 kHz. The effective crossover to the port occurred between 50 and 60 Hz, and the output fell off gradually below that point.
The averaged room response of the two speakers showed moderate variations of perhaps +/-3 dB between 70 Hz and 2 kHz, followed by a flat output between 2 and 10 kHz. The woofer curve spliced unambiguously to the room curve, resulting in one of the flattest composite response measurements we have seen from a speaker: +2 dB from 60 Hz to 20 kHz, and falling off at only 6 dB per octave at lower frequencies.
A quasi-anechoic MLS measurement at 2 meters (on axis) indicated response within +3 dB from 600 Hz to 20 kHz. The system's horizontal treble dispersion was typical of many dome tweeters, with little change in output below 8 kHz over a +45-degree range relative to the tweeter axis, a drop of about 5 dB at 15 kHz, and a drop of about 12 dB at 20 kHz.
The system impedance plot showed a peak to 19 ohms at the bass resonance frequency of 70 Hz and another rise to 15 ohms at 20 Hz (the lower limit of our measurement). The 6.5-ohm reading at 200 Hz could be considered the system's nominal impedance, although it reached 10 ohms at 1.1 kHz and fell to a broad minimum of 4.5 ohms between 6 and 15 kHz.
Sensitivity was better than specified: 90 dB SPL at 1 meter with a 2.83-volt input of pink noise. At that input level, the woofer distortion averaged about 1 percent (with overall variations from 0.6 to 1.5 percent) from 55 Hz to 2 kHz and reached only 3 percent at 40 Hz.
Single-cycle, 100-Hz tone bursts drove the woofer cone to its limits at an input of 270 watts into its 9-ohm impedance. At higher frequencies, our amplifier clipped (at power levels from 600 to 1,200 watts) before the speaker produced any serious sounds of distress.
In listening tests, the RTL2 sounded pretty much as it measured, with a smooth, uncolored output that was notably free from emphasis (or a deficiency) in any particular part of the frequency spectrum. Such neutrality is probably the goal of many speaker designers, but it is rarely achieved to this degree. In A/Â comparisons with other speakers of comparable price or size, the TDL RTL2 easily held its own, although it should be realized that all speakers (even those with very similar design and construction) sound different from each other when compared in a side-by-side listening test, and that the differences are often highly program-sensitive.
On the stereo-imaging tests of the Chesky JD37 test/demo CD, the performance of the RTL2's was well-nigh perfect - at least, we have never heard a more unambiguous demonstration of lateral and vertical source positioning from this disc. Our tests also confirmed what is probably a major advantage of the transmission-line enclosure (and apparently the "reflex transmission line" as well). Most speakers, whether in a sealed box or vented enclosure, have a resonant rise or peak in the upper bass or lower midrange, which can introduce coloration on vocal programs. A flat upper-bass response is, unfortunately, a rarity. The RTL2 shares with its larger and costlier relatives their freedom from bass heaviness or boom, without sacrificing whatever low-bass capability has been designed into them.
The TDL RTL2 is a very attractive and listenable speaker at a most attractive price. It is well worth hearing if you are in the market for a compact, topnotch speaker system that won't bankrupt you or dominate your home decor.
