Subwoofer Test Report - Polk Audio SR124DVC

In 1972, Matthew Polk and his friend George Klopfer, both recent graduates of Johns Hopkins University in Baltimore, MD, founded Polk Audio, one of the biggest success stories in the speaker business. The subject of this review is the latest from their Signature Reference subwoofer series, the SR124DVC 12" subwoofer.

Polk's mobile audio Signature Reference Series began last year with the introduction of the $900 SR6500 component/coax 2-way system. Reviewer Pat Turnmire loved it and confessed it was the first system he would consider putting in his own car! I expected the SR124DVC dual voice coil 12" subwoofer to be something pretty special, and I wasn't disappointed.

Like most high-performance subs I review for CA&E, the SR124DVC has its own specially designed frame - in this case, an eight-spoke cast-aluminum frame to support the substantial 16-pound magnet assembly. Incorporated into the frame are three banks of four cooling vents (12 vents total) that move air across the front plate and the voice coil as it moves outward from the plate in the area just below the dual spider-mounting shelf. Each vent has a 1/8" square opening to the voice coil area and expands to a 1.25" x 0.5" opening to the outside that prevents the vents from making unwanted noises.

The motor structure attaches to the rear of the frame and nestles between the eight extended frame spokes. The magnetic "go juice" is provided by a pair of 175 x 18mm ceramic magnets that are sandwiched between the forged, polished and chromed T-yoke and 7.5mm thick front plate. The T-yoke incorporates a large 22mm diameter pole vent with curved "venturi"-type outlets at both ends that reduce air turbulence and increase air flow (check out the cutaway of the SR124DVC). Rather than venting this straight out the back of the woofer, Polk incorporates a vent diffuser that forces the air to radiate at a 90-degree angle to the vent. Just as important, the top of the pole forms a "T" shape that increases magnetic field linearity in both directions. All of this slick engineering is at least partially made possible through the use of expensive FEA (Finite Element Analysis) magnetic design software. There are a number of good programs to use for motor design, but Polk uses one of the most popular, Ansolft's Maxwell software. Besides the FEA software, Polk also uses a Klippel Analyzer.

The SR124DVC cone assembly is as impressive as the motor structure and incorporates an injection-molded 1mm-thick ABS cone formed in a dish-like catenary shape. A catenary shape is formed when a uniform material is suspended from two fixed points (kind of like two kids with a jump rope). The famous St. Louis Gateway Arch is an inverted catenary. This shape maximizes the stiffness of the cone without resorting to using more material that would add mass. The cone is attached to the voice coil by a 4.5"-diameter vented glass-filled ABS "cone coupler," rather than the more common direct voice coil-to-cone neck method. Using a coupling device is not unique to Polk, but it is a very effective way to attach the voice coil to the cone and gives greater support and stability.

Compliance, the restoring "spring" that keeps your woofer from becoming a magnetic rail gun, is provided by a 30mm-wide and 20mm-tall injection-molded rubber (actually NBR, acrylonitrile butadiene) surround - incidentally, Polk Audio was among the first U.S. manufacturers to successfully produce injection-molded rubber surrounds. Remaining compliance is provided by two 8" diameter Conex spiders. Both spiders are progressive types (the "spring" tension increases as the spider stretches on long excursions). Distortion is minimized by mounting the twin spiders (also called "dampers") with the rolls inverted in a back-to-back orientation, a standard practice when using dual spider configurations.

Driving the cone assembly is a 2.5"-diameter voice coil that is wound with lightweight aluminum wire on a non-conductive glass epoxy voice coil former. The twin, dual-layer coils are connected to a very cool hex-key terminal block that will take up to 8-gauge speaker wire.

In the Lab
As with all the subwoofer reviews I do at CA&E, the objective measurement section is divided into two parts: the Klippel analysis followed by the LEAP 5 analysis. Using the Klippel analyzer (on loan from Klippel GmbH), Patrick Turmire, CA&E reviewer and CEO of Red Rock Acoustics, performed the large signal analysis on the Polk Audio SR124DVC and provided the Bl(x) curve shown in the graph. The Bl(x) curve shows the woofer voice coil has a fairly large 5.5mm forward displacement from its 0mm rest position, however the Bl curve itself has a nice broad shape and is rather symmetrical with nearly equal slopes in either direction. Having a broad symmetrical shape is much more important than the offset at rest in this case. The displacement at operating SPL near Xmax decreases to 2mm, which is not significant in terms of offset. Bl can decrease to approximately 70 percent of its small signal value and the driver will still function in a satisfactory manner, only with an elevated level of distortion, and since this is not really perceivable, it's not an issue. The 70 percent of maximum Bl displacement limit for the Polk 12" is greater than 11.7mm, which if the curve is extended out, would be nearly the same as the physical Xmax of 16.25mm.

The Kms(x) or stiffness of suspension curve for the SR124 12" also shows a good degree of symmetry in both directions of travel. The compliance offset is a minimal 2mm forward of the rest position and decreases slightly at the limits of the operating range of this woofer. The compliance limit for the suspension when it drops to 50 percent of its rest value is greater than 11.7mm. Both "limit" numbers, Bl and compliance, are the level at which distortion climbs to 20 percent, which is an acceptable criteria for subwoofers (10 percent is used for fullrange drivers). Using this 20 percent distortion criteria, both Bl and compliance contribute equally in determining the distortion level.

Following Patrick's Klippel measurements, I measured the T/S (Thiele/Small) parameters for the SR124DVC using the standard procedures established for CA&E sub reviews that incorporate the LinearX LMS (Loudspeaker Measurement System) analyzer and VIBox for measuring dynamic impedance (impedance at different voltages). Testing consists of doing a series of voltage and current sweeps that are later converted to multiple voltage impedance curves. With the SR124DVC clamped to a rigid test stand, measurements were made at 1 volt, 3 volts, 6 volts, 10 volts, 15 volts, 20 volts and 30 volts. Because Polk supplied the actual measured weight of the cone assembly (cone, dust cap, voice coil and 50 percent of the surround and 50 percent of the spider put on an accurate digital scale), the added mass measurements were not required. The SR124 measured reasonably linear out to 30 volts in free-air (no box), so all the collected data was used for the T/S calculation. This group of multi-voltage impedance curves was pasted from LMS into LEAP 5 and the Transducer Model Derivation utility used to produce the T/S parameters, with the results listed in the Data Chart.

The SR124DVC parameters listed in the Data Chart were used to create computer box simulations using the Leap 5 Enclosure Shop software. The software was configured to simulate the woofer's low-frequency performance in the same size box recommended in the SR124DVC manual, a 0.88ft3 sealed boxes with no fill material, plus I added a somewhat larger 1.25ft3 sealed box with 50 percent fill material (fiberglass). Loaded in the virtual 0.88ft3 sealed box, the Polk 12" had a -3dB frequency of 47.5Hz with a box "Q" (Qtc) of 1.00 using the series jumper connection. A box Qtc of 1 describes a damping level that is less damped than the "Butterworth" 0.7 damping number, which describes near optimal transient response. However, a "Q" of 1 does coincide with minimum excursion and maximum power handling.

The larger 1.25ft3 box, as you would expect, resulted in both a lower roll-off of 40.5Hz and a better-damped and more controlled response that had lower "Q" of 0.87. The LEAP 5 graph curves in Figure 4 show the SPL at 2.83 volts (black curves) in half-space, 2.83 volts in a 154ft3 car compartment (blue curves), and the SPL at a power level required to get maximum linear excursion (red curves, also half-space). The smaller sealed box curves are solid lines and the larger sealed enclosure curves are the dashed curves.

Increasing the simulated input voltage for the 0.88ft3 computer simulation to 116 volts increased excursion to the Xmax +15 percent level and pushed the SPL to a substantial 118dB. It took only 92 simulated volts to drive the Polk 12" to just beyond Xmax (Xmax + 15% = 18.7mm) and resulted in somewhat lower SPL of 115dB. It's pretty obvious that the new Polk SR124DVC is capable of very high undistorted SPL.

All of this taken together, the Polk engineers have done a great job with this new Signature Reference Series subwoofer - but for a company with its years of experience, I wouldn't expect anything less. As good as this sub measures, I would suspect the sound quality is good as well, but that's not my job. How about it, Eric? -VD

Subjective
Back from a very rare weekend off and there it was in my office - a large, heavy, sexy-looking black box from Polk Audio.

Opening up the box revealed the all-new, well-packaged, beefy Polk Audio Signature Reference SR124DVC 12" subwoofer in a dual 4-ohm configuration. It's a very nice-looking sub, too, and weighty at 26 1/2 pounds. The hulking cast aluminum basket is powder coated in a medium gray and it has a very solid feel. The cone is a one-piece ABS dish with a killer looking "SR" logo embossed in silver. There are a number of very cool details in the construction, such as the upside-down writing on the input terminal block and the ring around the basket, just in case you wanted to mount the SR with the magnet out.

Installation
Paul Di Como, Polk Audio's marketing manager gave us his recommendations for the proper size and type of enclosure to achieve the very best in sound quality when using the SR124DVC. Paul recommended a sealed enclosure that was 1.25ft3, which was quite a bit larger than the 0.88ft3 called out in the installation manual, and I think the SR124DVC may have performed better in an even larger enclosure. I just wish these companies would recommend the correct size of enclosure in their owner's manuals and not try to "win" the smallest enclosure award. A larger enclosure will typically flatten the frequency response of the speaker - this is a good thing for sound quality. Doing this will generally allow the subwoofer to reproduce lower frequencies in the playback. The trade-off can be lower power handling and sometimes a looser impact response, but in the case of the Polk, I would go with a larger enclosure.

As usual, the guys at the Speaker Works assembled the test enclosure. Master installer Paul Rivero pointed out that the SR124DVC fits a standard 12" subwoofer cutout, even though it features the massive cast frame. This makes the Polk very user friendly and allows the end user the flexibility and savings of using a pre-made enclosure. Rivero liked the wire terminations that are conveniently located next to each other and the high-quality single-piece gasket that comes packed in the box. Polk even goes so far as to supply you with a pair of red and black jumpers so you can configure the voice coils in either series or parallel. Installers will be happy to see that someone is actually thinking of them for once.

To power the Polk Audio SR124DVC, I installed two Crossfire VR1000d amplifiers in a bridged configuration. These two amps running into the 2-ohm load of the SR124DVC's dual 4-ohm voice coil in parallel will produce over 2,000 watts of RMS power. The front half of my reference speaker system consists of a pair of USD Audio B-72pro WaveGuide separates, an Alpine center channel speaker and USD Audio S-51 separates for rear surround, all powered by Zapco amplifiers. The built-in crossovers and subsonic filters are bypassed on all amplifiers (the non-defeatable crossovers on the Crossfire amps turned to the highest setting). Instead, an Alpine PXA-H701 Multimedia Manager was used to set levels and to actively divide the frequency range to all of the speakers.

Listening
I was relieved when Paul from Polk told me that the SR124DVC that he had shipped to me had already been put through the required break-in period until Zach Abass, a Speaker Works employee, reminded me that I now had even less of an excuse for not hitting my deadline! So, after a bit of fiddling with position and phase, it was time to start listening in earnest.

Jazz
Diana Krall Love Scenes
Diana Krall's Love Scenes album, with its big stand-up bass totally exposed and alone, gave the SR124DVC just about the perfect musical source for its design. The SR124DVC was snappy and forceful. It handled the kick drum respectably with decent detail and integration. The Polk did nicely, having to work closely with the midbass to deliver a convincing rendition. Other than a tiny bit of fatness in the deeper, decaying notes of the bass, it succeeded. The imaging of the drums and bass was relatively forward in the vehicle.
Score: 8/10

Instrumental
Focal Demonstration Disc No.4 "Boxenkiller"
Next I cued up "Boxenkiller" from the Focal Demonstration Disc No. 4, and Zach, my listening compadre, got as far as writing "very nice - maybe the tiniest bit of bloom and resonance at very beginning of the track..." and then he got so engrossed by the sound he just forgot to write anymore. Now that is a compelling sound quality! I found that the SR124DVC gave its best performance on tracks like "Boxenkiller" and the Krall piece. The job of listening just disappears and the enjoyment becomes the experience. How sweet is that? This track begs to be played not just at "10," but all the way to "11," and the Polk was more than happy to oblige! It pumped out the larger floor toms, kick drums and the bass guitar like a thing of beauty.
Score: 9/10

Rock
Rush YYZ
On Rush's YYZ (MoFi release,) Geddy Lee's bass guitar was distinguishable from the lead guitar as it alternately followed along and ran off on its own. Some of the lowest note details were hard to discern from the kick drum, but for the most part, the Polk did a very good job of putting out a clear reproduction of the band.
Score: 6/10

Easy Listening
David Benoit Linus & Lucy
David Benoit's classic Linus & Lucy came through well in the initial 15 seconds when the subwoofer has to do the intro bass guitar licks, but then the track started in earnest with piano, bass and drums all requiring a goodly amount of attention from the SR124DVC. This is a fun piece of music with a beautifully layered bass line. The sub stayed tight and tonally solid while reproducing each instrument nicely.
Score: 7/10

Hip Hop
Eminem The Eminem Show
On the final album, Eminem's The Eminem Show, the Polk pounded out the typical 50 to 60Hz booming of hip-hop, but the really deep stuff rolled off a touch - this is an example of where I think an even larger enclosure volume would help flatten out the frequency response and improve the overall performance of the Polk SR124DVC. Musically, the Polk sounded great on most of the rap and hip-hop that we tried. The bass imaging was fair, with minimal pulling to the rear of the vehicle. With a rated power handling of 700 watts, the SR124DVC never really ran out of power handling and took the 2,000-watt pounding that we were giving it.
Score: 7/10

Spl
SPL in car measurement at 2.83 volts, 1 meter - with Bruel & Kjaer Type 2231 Level Meter set to: Un-weighted, SPL and Fast.

Subjective Score Chart

Conclusion
The Polk Audio SR124DVC is a beautiful-looking subwoofer that has all the technological makings of a powerful performer. It has a cast frame, double-stack magnet, airflow management design, stiff cone, dual-progressive spiders and lots of engineering to maximize performance. It is a touch expensive at $425, but Zach Abass and I were both impressed with the sound of the SR124DVC. The Polk was really quite spectacular on just about anything we played. If the price point fits your budget, you should look the Polk SR up and give it an ear. -EH