Polk Audio LS-70 Speaker System Review

The LS-70 is one of a new series of floor-standing tower speakers from Polk Audio. Like the Polk S4 bookshelf speaker, the LS-70 features newly designed drivers and construction techniques intended to improve response smoothness and minimize diffraction from the speaker’s enclosure boundaries.

Like most other Polk speakers, the LS-70 is a two-way system. It uses two 7-1/2-inch woofers located about halfway up the front panel. Above them is a 1-inch dome tweeter, and the port of the vented enclosure is near the bottom of the panel. Although this is not an unusual driver configuration, the drivers were designed specifically for these speakers and differ in several respects from earlier drivers made by Polk or anyone else.

The woofers are larger than those in most other Polk speakers. Their injection-molded cones contain Aramid fibers, said to result in lower mass with no reduction in stiffness. The rubber surrounds extend beyond the cone edges in order to make smooth transitions to the wooden speaker board. The central dust caps are also made of soft, flexible rubber. Finally, the woofers feature low-resonance baskets, vented voice coils, and newly designed high-performance magnet structures.

Stacking the woofers one above the other controls their vertical dispersion, minimizing floor and ceiling reflections in the critical midrange. Polk found, however, that this configuration also produced comb-filtering effects (response irregularities) in the critical listening range between 1,500 Hz and the crossover to the tweeter at 3,200 Hz. The problem was solved by using an additional crossover filter that reduces the output of the bottom woofer by 6 dB at 1,500 Hz, so that the system’s behavior at higher frequencies is that of a conventional two-way system, while retaining the advantages of two woofers at lower frequencies.

The tweeter features a trilaminate dome made of polyamide, aluminum, and stainless steel. According to Polk, this layered construction produces the smooth response of a soft-dome tweeter along with the clarity and precision of a metal dome. The tweeter’s faceplate is also shaped for optimum efficiency and dispersion to provide a more accurate stereo image.

The LS-70’s enclosure is slightly tapered, and the black cloth grille is removable. Each speaker weighs about 50 pounds. Two pairs of gold-plated input terminals are inset into the bottom of the rear panel. They are normally joined by jumpers, which can be removed for bi-wiring or bi-amplifying the system. The connectors accept single or dual banana plugs, lugs, or stripped wire ends.

In our lab tests, the averaged room response of the two Polk LS-70 speakers was relatively smooth and free from the boundary-reflection effects that appear in most such measurements, varying only ±4 dB from 100 to 20,000 Hz. The close-miked response of the woofers from 20 to 500 Hz combined with the room curve to produce a composite frequency response of ± 5 dB from 42 to 20,000 Hz. Including the port radiation, the bass extended down to 20 Hz within the same limits, but under actual listening conditions the effective bass extension will depend on the speakers’ environment. In our room, the audible response was clean and usable down to about 35 to 40 Hz.

Measurements with a swept one-third-octave pink-noise signal showed a response variation of ±2.5 dB from 40 to 20,000 Hz. A quasi-anechoic (MLS) frequency-response measurement was within ±3.5 dB over its valid range of 300 to 20,000 Hz. Polk rates the LS-70’s response as 3 dB down at 37 and 25,000 Hz. Although we did not test beyond the audible range, these figures are consistent with our results.

Impedance measured a minimum of 4 ohms at about 150 Hz, rising to 15 or 16 ohms at 60 and 1,500 Hz. Polk says the speaker is “compatible with 8-ohm outputs,” and it is, but we would rate its nominal impedance at 4 to 6 ohms. Sensitivity with a 2.83-volt input measured 91 dB sound-pressure level (SPL) at 1 meter (the rating is 90 dB). With 2.52 volts input, equivalent to a 90-dB SPL, the woofer distortion varied between 0.5 and 2 percent between 42 and 2,000 Hz, reaching 9 to 10 percent between 20 and 30 Hz. The group delay in the tweeter’s operating range was constant within ±150 microseconds.

At 1,000 and 10,000 Hz, the LS-70 easily absorbed the maximum output of our driving amplifier with a single-cycle tone burst, reaching 600 watts at 1,000 Hz and 1,065 watts at 10,000 Hz. At 100 Hz, the woofer sound gradually took on a sharp, rapping quality as the power was increased beyond a few hundred watts. We stopped at 555 watts, though we hadn’t yet reached the physical limits of the drivers.

The tweeter’s horizontal dispersion was typical of a good 1-inch dome radiator, with the output 45 degrees off-axis down 2 dB from the on-axis readings at 5,000 Hz, - 6 dB at 10,000 Hz, and -9 dB at 20,000 Hz.

Quite apart from the LS-70’s measurements, which indicate that it is a very fine speaker, how did it sound? In a word, superb. Its spatial imaging, both lateral and vertical, was outstanding. Imaging tests using the Chesky JD-37 disc produced what was probably the most accurate localization we have yet experienced from a pair of speakers playing that CD. There was a completely unambiguous placement of the sound in accordance with its announced position. Many speakers do a reasonably good job in this test, but the LS-70’s put the sound exactly where it belonged.

Another interesting property of the LS-70 was that it sounded natural and listenable even at a distance of only a foot or so. Although stereo imaging is lost when you listen that close, the sound quality remained as unified and balanced at 1 foot from the speaker as at 10 feet (this quality is not typical of most loudspeakers we have used).

And that sound quality, under more normal listening conditions, was notable for its smoothness and balance. There was occasionally a slight tendency toward warmth, but the flatness of the overall response generally prevented the bass from dominating the system’s sound. Nevertheless, when the program material called for it, the LS-70 put out a quantity (and quality) of bass that belied its modest price.

In its price range, where a number of good speakers are available, the Polk LS-70 is an exceptional value. You would have to pay a lot more to gain (maybe) a small improvement in overall quality. Like its smaller relative, the S4, the LS-70 dramatically demonstrates the success of Polk’s engineering effort to create a line of speakers embodying the latest technology in materials and acoustics.

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