Test Report: Polk Audio LSiM Speaker System Page 3

Extended Test Bench

Frequency response

  • tower 34 Hz to 20 kHz ±4.6 dB
  • center 46 Hz to 20 kHz ±3.6 dB
  • surround 65 Hz to 20 kHz ±5.3 dB
  • subwoofer 34 to 450 Hz ±3 dB

Sensitivity (SPL at 1 meter/1 watt)

  • tower 86.5 dB
  • center 87.7 dB
  • surround 87.8 dB

Impedance (minimum/nominal)

  • tower 2.8/5 ohms
  • center 3.5/5 ohms
  • surround 4.0/5 ohms

Bass output, tower (CEA-2010 standard)

• Ultra-low bass (20-31.5 Hz) average: 99.2 dB

  • 20 Hz: 73.8 dB
  • 25 Hz: 92.2 dB
  • 31.5 Hz: 107.1 dB

• Low bass (40-63 Hz) average: 120.1 dB

  • 40 Hz: 119.5 dB
  • 50 Hz: 120.4 dB
  • 63 Hz: 120.3 dB

Bass output, subwoofer (CEA-2010 standard)

Ultra-low bass (20-31.5 Hz) average: 100.9 dB

  • 20 Hz: 85.1 dB
  • 25 Hz: 100.7 dB
  • 31.5 Hz: 106.3 dB

Low bass (40-63 Hz) average: 118.5 dB

  • 40 Hz: 118.8 dB
  • 50 Hz: 118.4 dB
  • 63 Hz: 118.2 dB

Bass limits

  • center 100.0 dB at 40 Hz
  • surround 91.9 dB at 40 Hz

I measured the Polk LSiM speakers with the microphone placed at a distance of 2 meters, using quasi-anechoic technique to remove the effects of reflections from nearby objects. The LSiM 707 tower speaker was placed directly on my measurement turntable, with 2 feet of attic insulation on the ground between the speaker and the mike to minimize the bounce from the ground. The LSiM 706c center speaker was placed atop my 2-meter-high measurement stand. Because it’s designed for on-wall use, the LSiM 702f/x surround speaker was mounted on the 2-by-4-foot swivelable ersatz wall I use to measure on-wall speakers. Because the surround is designed more like a wall-mounted minispeaker than a typical dipolar or bipolar surround speaker, I measured it just as I did the tower and center: I adjusted the microphone position for the flattest on-axis response, then averaged the measurements at 0°, ±10°, ±20°, and ±30°, smoothed to 1/12th octave. Bass response of all speakers (including the DSW 3000 MicroPRO subwoofer) was measured using ground plane technique with the microphone on the ground 2 meters from the speaker; these were smoothed to 1/3rd octave, then spliced to the quasi-anechoic measurements at 300 Hz. All frequency response measurements were made with a Clio FW audio analyzer, then imported into a LinearX LMS analyzer for post-processing.

The measurements of the LSiM 707 tower and the LSiM 706c center are fantastic. Although on-axis measurements are not shown in the accompanying chart, they’re as close to perfect between 500 Hz and 20 kHz as I can remember seeing: ±1.37 dB for the tower and ±1.49 dB for the center.

The tower speaker’s off-axis response is similarly great. Out at ±30°, there are no off-axis response artifacts except a mild rolloff above 8 kHz, which pretty much every speaker with a 1-inch tweeter has. Even at ±60°, the only response error besides the increased treble rolloff is a dip of about 2 dB between 800 Hz and 2.5 kHz. As you can see from the measurement chart, the tower’s bass is boosted by about 5 dB between 40 and 90 Hz, although the effects of this boost will be altered tremendously by the distance from the speaker to the nearby walls, and by room acoustics.

The center speaker frequency response measures almost just like the tower speaker’s, which is pretty remarkable — something made possible by its three-way, tweeter-over-midrange design. It shows the same off-axis treble rolloff, but instead of the dip between 800 and 2.5 kHz ±60°, the center shows a deeper, narrower dip: 5 dB between 500 and 650 Hz.

I measured the center speaker without its grille, but then also measured it with the grille to gauge the effect. There’s no difference in the midrange, but the grille causes a treble rolloff that begins above 9 kHz and runs -2 to -3 dB above 12 kHz. Because the tower speaker’s grille looks to be constructed the same way, its effects are probably similar.

That’s kind of a lot of rolloff for a grille, so I recommend using the grilles only when kids, dogs, or inebriated houseguests are present.

Because the surround speaker was wall-mounted for the measurements, and because of its angled design, it showed fairly random frequency response errors off-axis. However, it shares the basic design of the tower and center,and the same basically good behavior, so all the response errors caused by wall-mounting average out into a reasonably flat response.

Impedance measurements (also performed with the Clio FW) indicate that these speakers may be a little tough to drive for inexpensive receivers. All have a fairly low measured nominal impedance of 5 ohms. The tower is the toughest load, hitting a minimum impedance of 2.8 ohms at a low frequency of 66 Hz with a fairly reactive phase angle of -32°. That’s enough to cause some inexpensive receivers to shut down if you crank up the volume, but I think a typical mid-to-high-end receiver (say, $1,500+) or any good stereo amp should handle it fine. The center and surround present a somewhat easier load. The center hits minimum impedance of 3.5 ohms at 9.5 kHz/+4° phase angle, while the surround’s minimum is 4.0 ohms at 7.2 kHz/+1° phase angle. Fortunately, all of the speakers deliver roughly average sensitivity, so they usually won’t be pulling more than a few watts from your receiver or amp.

The DSW 3000 MicroPRO subwoofer’s frequency response measurement that you see here was taken through the LFE input, after I hit the reset button to clear out any previous auto-EQ settings. The response shown here is in the Cabinet mode, which gave the flattest response. The other modes boost bass to varying degrees below 50 Hz, with the strongest effects at about 40 Hz: +1.2 dB for Corner mode, +2.8 dB for Mid-Wall mode, and +4.0 dB for Mid-Room mode. The low-pass function of the internal crossover set to 80 Hz was about -22 dB per octave, but the control is not well calibrated; the rolloff was 3 dB at 52 Hz.

CEA-2010 output measurements for the subwoofer and the tower speaker were taken at 2 meters, then scaled up +6 dB per CEA-2010 requirements so that they are equivalent to 1-meter results. At press time, the CEA had instituted changes to the CEA-2010 standard but had yet to publish them. The CEA has informed me that the new standard requires averaging in pascals rather than in decibels, so that’s the procedure I followed here. The subwoofer’s output is: low bass 118.5 dB, ultra-low bass 100.9 dB. That’s pretty high for a subwoofer of this size, but not for one in this price range. With most subwoofers, second- or third-order harmonic distortion dictates the maximum output rating, but with the DSW 3000 MicroPro, higher-order distortion and mechanical noises from the driver defined its limits. I didn’t encounter a limiter threshold during my measurements, indicating that the internal limiter’s threshold is set very high or that maybe there isn’t an internal limiter.

I measured the tower’s bass output by connecting it to the Krell S-300i integrated amp. The amp was turned full up, and it was definitely running near its limits during the 63-Hz measurements; it’s possible that a more powerful amp would have delivered a better measurement at this frequency. That said, the tower’s bass output measurements were great: low bass: 120.1 dB; ultra-low bass: 99.2 dB. This is comparable to what I’ve measured from some good (and fairly large) 12-inch subwoofers. — Brent Butterworth