Review: Polk Audio Blackstone TL350 Page 3

Extended Test Bench

Frequency response (at 1 meter)

• satellite 130 Hz to 20 kHz ±6.7 dB
• center 100 Hz to 20 kHz ±5.0 dB
• subwoofer 29 to 111 kHz ±3 dB

Sensitivity (SPL at 1 meter with 2.8 volts of pink-noise input)
• satellite 83 dB
• center 87 dB

Impedance (minimum/nominal)
• satellite 3.4/10 ohms
• center 3.5/7 ohms

Bass output, subwoofer (CEA-2010 standard)
• Ultra-low bass (20-31.5 Hz): 85.5 dB
• Low bass (40-63 Hz): 107.4 dB

Bass limits
• satellite 80 Hz at 71 dB
• center 80 Hz at 79 dB

I measured the TL3 satellite and the TL3 center channel at a distance of 1 meter, far away enough to incorporate the contributions of all drivers and the effects of cabinet diffraction. Both sat atop a 6-foot stand to give quasi-anechoic results down to 240 Hz.

The curves you see in the graph show an averaged response from 0° to 30°, smoothed to 1/12th of an octave. To get the speakers’ bass response, I close-miked the woofers and ports, then scaled and summed the results. I then spliced the bass responses to the averaged quasi-anechoic responses to produce the curves shown here. I used ground-plane technique — a microphone on the ground at 1 meter — to measure the subwoofer’s frequency response, with the expectation that it would deliver the most accurate portrayal of the blend between the long slot port and the down-firing woofer. Satellite and center speaker responses are normalized to 0 dB at 1 kHz, the subwoofer normalized so that its peak response shows as +3 dB.

The TL3 satellite measures quite well. It loses a little bit of zip at 20 kHz, which is the cause of its not-impressive-at-first-glance ±6.7-dB response variation, but below 13.3 kHz it measured ±3.4 dB — a good result for any speaker and fantastic at the TL3’s price. Off-axis response is ideal, with no increased response errors at 45° and 60° except the usual gradual treble rolloff. It produces a little bit of response all the way down at 80 Hz, where noise from the woofer and/or port becomes intrusive.

The TL3 center channel has a smoother averaged response than the satellite, but at all angles from 0° and 60° it has a broad dip between 1.4 kHz and 4 kHz that mars its response somewhat. Off-axis response out to 30° is pretty good for a mid-tweet-mid center speaker design, but a big dip develops between 1 kHz and 2 kHz due to interference between the two woofers; at 45° it’s –20 dB at 1.3 kHz. The PowerPort seems to be working, because the center channel puts out a substantial (for its size) 79 dB at 80 Hz. Give the center’s good bass response most of the credit for this system’s robust sound.

Both the TL3 satellite and center channel should be fairly easy to drive, although the satellite is a bit power-hungry. The satellite presents its toughest load of 3.4 ohms at 3 kHz, with an insignificant impedance phase shift of +6°. For the center, it’s 3.5 ohms at 360 Hz, with a –11° phase shift. The only issue is the satellite’s low 83-dB sensitivity, which means you’ll need 64 watts to get it up to 101 dB at 1 meter. Almost all A/V receivers can muster that, but I wouldn’t recommend using this system with one of those feeble little receivers included with some home-theater-in-a-box systems unless you’re content to play it at modest volumes.

I was pleased with the performance of the DSWpro550wi. Like most good subs in its price range, it delivers serious output in the second octave of bass (40-63 Hz), putting out an average of 107.4 dB before the limiter steps in. It has okay output in the bottom octave (20-31.5 Hz), too, averaging 85.5 dB and even mustering 76.3 dB at 20 Hz.

I measured the effect of the Polk Room Optimzer in-room, using TrueRTA and a calibrated mike. It seems to be effective mainly below 45 Hz, with the Corner setting attenuating the most, the Cabinet setting delivering maximum output in this band, and the Mid-Wall and Mid-Room settings falling in between. However, the difference between the two most extreme modes maxed out at about 3.5 dB, so PRO probably won’t have a dramatic effect.

Not that I needed to use the subwoofer’s internal subwoofer crossover, but I was surprised that its highest setting is 120 Hz, just barely adequate to blend with the TL3 satellites. Low-pass function of the crossover is roughly –14 dB/octave. — B.B.