- Written by Tyler Stripko
- Published on 28 May 2013
Design of the Polk LSiM Home Theater Speaker System
The media room in my home measures approximately 23' X 26' with a 7.5' ceiling height. This puts the approximately 4500 cubic foot space firmly into the "large room" category. As such, I built this setup around the largest speaker in the LSiM line, the LSiM 707 tower. Weighing in at 99.2 pounds and standing 50-13/16" tall, the 707 is not for the feint hearted. This speaker will definitely draw attention to itself due to its commanding presence. For center channel duty, I chose the LSiM 706c. This is the larger of two available center channels and its drivers match up well with the 707 tower. For surround duty, I requested both the LSiM 703 bookshelf speaker as well as the LSiM 702F/X dedicated surround-channel speaker so that I could compare a direct-radiating speaker to something more diffuse for the surround channels.
Befitting their all-new design, there are a ton of new features common to all models in the LSiM range. First is the "Dynamic Sonic Engine," Polk's term for the molded enclosure that isolates the tweeter and midrange driver in their own separate molded enclosures. Each of these enclosures is tuned for their resident driver, resulting in increased dispersion, better imaging, improved detail retrieval, and better off-axis response. Next, Polk took the ring-radiator tweeter from the LSi series and enhanced it, resulting in increased power handling, more extension (to a claimed 40kHz), and better dispersion. The midrange/woofer drivers are now made of "Super Cell Aerated Polypropylene," which is said to offer a nearly ideal combination of weight versus stiffness. The subwoofer drivers in the LSiM 707 and LSiM 705 tower are a completely new "Cassini Oval" design. This is Polk's take on the oval "racetrack" shape that has been used by other companies over the years. The oval shape increases the overall surface area of the driver (key to moving enough air for deep bass response) without dramatically increasing the overall width of the speaker. Used in combination with Polk's trademarked PowerPort® bass venting, the new bass drivers generate more powerful bass without additional distortion or port noise. Extended Linear Motion (ELM) voice coils combined with cast aluminum baskets further contribute to improved power handling and output levels. Polk's proprietary "Orth" crossovers (named for one of Polk's Systems Engineers) mate the drivers to one another as well as level out the impedance curve.
Cabinet construction has also taken a huge step forward in the LSiM series. The sides are now 1" MDF and are curved to reduce standing waves and internal resonance. Back and bottom panels are ¾" MDF while the front baffle is 1-1/4" MDF. To further improve efficiency and reduce resonance, each driver is housed in its own chamber within the main cabinet. The standard "knuckle test" produced a very solid "thunk" on all of the LSiM speakers with one exception. Knocking on the lower portion of the LSiM 707 towers where the bass drivers reside did result in a bit of perceived resonance, with a slight hollow ringing that was detectable to my ears. Other than that, the overall construction quality of all of the LSiM speakers really impressed me. From the hefty feeling dual 5-way binding posts to the excellent "hex-head floor spikes" on the 707s, I was really impressed with the build quality on these speakers. I want to go back to "hex-head floor spikes" for a moment. Instead of having to lift the big speaker and fiddle with spikes that adjust from underneath, you simply insert the included hex-head wrench and turn. I know that other manufactures have used similar systems in the past, but why this hasn't become the industry standard is beyond me as they are just so easy to use. Out of the box, these spikes are covered with rubber feet to protect wood floors but you just pull off the feet to access the spikes.
All of the speakers except the F/X surrounds can be covered in one of two real-wood veneers: Mt. Vernon Cherry or Midnight Mahogany. The F/X surrounds come only in a black gloss finish. The two wood veneers have a satin varnish, which helps cut down on reflections in a darkened room. As I watch all of my movies in a blacked out environment I really appreciate non-glossy speakers, particularly the center channel as it sits directly beneath my display. Regardless of color, the overall fit and finish of the veneer is excellent. I couldn't detect a single seam on any of my speakers and many visitors to my home couldn't believe that they were looking at a Polk speaker. They all thought it was something far more exotic. It's pretty amazing how far Chinese manufacturing has come. I also liked the contrast offered by the gloss black front baffle against the wood veneer. Invisible magnets are used to attach the "zero diffraction" speaker grilles to each speaker. The grilles felt a bit flimsy to me, but attached securely and were very easy to remove.