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One of the subtitles employed by American company Polk Audio is ‘The speaker specialists’. And while in this modern marketplace boastful by-lines are not uncommon, I don’t need to tell you that this one wasn’t one acquired by chance. Native to Baltimore Maryland and conceived back in 1972 as a humble yet specialised audio company, Polk Audio soon gathered momentum as astute customers began to take notice of a company whose products performed within the higher end of the market yet somehow managed to avoid the inherently higher-end price tags that so often accompanied other brands. It’s a phenomenon that Polk Audio (commonly referred to simply as Polk) has mastered over the years, and today the company still produces components aplenty which not only outdo much of the competition, they often freight with half the fiscal baggage too.
The new MM components are a perfect case in point. They form part of the latest incarnation of the acclaimed Mobile Monitor range, a range that itself speaks volumes when it comes to marrying top-shelf sound reproduction with affordability. The new MM range includes a full range from coaxials and component sets through to some very neat power amplifiers. However it’s the subwoofers we’re concentrating on today. The MM subs are available in sizes ranging between 8”, 10” and 12” and with either single or dual voice coil configurations — thus allowing the perfect match to your system’s impedance requirements, while aiming to provide exceptional bottom octave performance without sending you into the red with the accountant.
And I was particularly interested to review this MM1242 subwoofer because it’s receiving rave reviews overseas. Especially where its ‘output for mounting depth’ ratio is concerned, so to speak.
Before we go too far with the intricacies, though, let’s just canvass the fundamentals of its physical design, starting with a closer look at the topside technology. Given that subwoofers are ultimately air pumps, whereby the forward motion of the cone generates a high pressure compression wave and the reverse motion generates a low pressure rarefaction cell, one would imagine the diaphragm and surround are arguably the most crucial components — after all together they form the sole membrane that’s giving causation to air molecules’ kinetic journey.
Polk has invested considerable time during the development phase getting these components just right, and although many will ignorantly just state a cone and surround are easy to bang out, there’s a surprising sting in the prerequisite tail of this particular design set — the MM range is IP56 marine-certified. This means the subwoofers need to sport a cone material that’s not only strong and light, it must also be water, UV and salt resistant. So just like that, out go most of your standard cone material options, and along with them those simplistic materialist theories. Yet running with an exotic material and saddling it with a five-figure price tag wasn’t going to provide the endgame Polk was looking for — the company needs to sell subwoofers, after all.
So drawing on much of its near-half-century of design experience, the engineers started with a base of polypropylene, then working their magic in order to develop a composite material which is held in position via a Santoprene surround. Together these combat the elements, providing exceptional resistance to the aforementioned undesirables in water, UV and salt. The cone nevertheless offers a superb weight-to-strength coefficient, assisted greatly by numerous strengthening ridges built into the face design.
With the front end spoken for, let’s now turn our attention to the structure that lies beneath, because the wizardry doesn’t cease up top. One of the most fascinating aspects of the MM1242’s design is the subsonic output it can provide despite its shallow 118mm physical mounting depth.
Again a pause for consideration — when talking subwoofer motors, you’ll notice one thing common to many a high-end subwoofer is that their mounting depth is often almost as deep as they are wide. This is because of the intrinsic relationship between the motor’s position and the magnet’s flux. The densest regions of a magnet’s flux tend to be centred on the corners, so where the voice coil lives in relation to these is paramount to its influence. How far the voice coil can move and remain completely controlled is what we call Xmax, and is critical for accurately reproducing those lower subsonic frequencies. Low frequency harmonics often lend themselves to big strokes and long voice coils, thus conducive to a sizable mounting depth.
Polk engineers circumvented this issue by designing a 25mm-high 54.4oz ferrite ceramic magnet that possesses an extremely potent flux despite its limited height. The force
generated is thenceforth exerted over the multi-layered voice coil that resides upon a
judiciously positioned 50mm thermally efficient former. The end result is a motor that can play very low without requiring the customary titanic mounting depth. Dynamic regulation is assisted via a Conex spider which serves to work with the surround in keeping everything right where it should. Together these return a sensitivity of 87dB and frequency range of 26Hz–200Hz.
All this hardware is contained within a non-corrosive fibreglass-reinforced ABS basket with a weight of only 4.8kg. This basket is also home to the main portion of the cooling system, which begins with eight large perimeter intakes that are mesh covered and strategically positioned around the basket. These allow copious amounts of air to come into the motor, flowing a predicable trajectory around the outer edge of the voice coil and through the spider before returning through the inside of the voice coil gap along the inner walls for expulsion via the 20mm bell-mouthed pole vent.
This cooling system is quite efficient, allowing for a continuous power handling ability of 420 watts. Maximum power is stated at 1260 watts and while there is an extended back plate allowing for overruns in addition to that that suspension being most robust, I still don’t suggest you start testing this figure for extended periods of time. Just in case you’re tempted though, Polk has seen fit to incorporate a pair of 5A fuses into the motor design as a safety buffer.
Timber & timbre
When it comes to designing its enclosure, the shallow mounting depth allows for an abundance of designs for a variety of possible locations. My particular test vehicle was a wagon, so space was not an overbearing concern; however these subwoofers will suit perfectly utility vehicles, just to cite one example where depth is often critical. I did face one small hurdle, though, when designing my enclosure — Polk doesn’t supply any Thiele-Small specs in the manual. This tends to make optimal enclosure design somewhat challenging. Polk doesn’t leave you completely in the dark, providing a ballpark figure for enclosure volumes, both sealed and ported. However the stark reality is that this ballpark resembles Fenway Park in size, and is not too helpful if you’re like me and like to achieve a perfect Q for your enclosure. The result of a little experimenting I settled on a sealed enclosure just shy of 50 litres in order to return my desired Qtc.
Armed with my gorgeous-smelling new timber enclosure, I mounted my subwoofer and began the wear-in process playing sine waves centred upon the sub’s resonant frequency, which is quite a low Fs, given its motor depth. With enclosure loaded, subwoofer run in and all the specs pointing to an enjoyable experience, did all the stars align to provide for an out-of-world subsonic experience?
In fairness I might be overselling it a little there… its output is not quite the life-altering experience. But putting hyperbole aside, it’s certainly one exceptional performer. The output it provides is superbly controlled and accurate, and it delivers with a smoothness not often heard these days. This pertains primarily to its motor design, of course; nonetheless we ought not to discount Polk’s expertise in finessing this equation. Any subwoofer can be designed to output enormous dollops of kinetic force. The MM1242 is deliberately designed not to rupture ear drums. Because of this it’s able to provide articulate deep details in your music you may not have noticed previously. From being able to handle the ultralow requirements of genres such as classical and jazz, through to the chest kicking demands of rock and techno, it handles each and every one of them without any hint of struggle or complaint.
Overall the MM1242 is one very impressive subwoofer — perhaps not a great surprise given it’s effectively the evolution of an already battle-proven and highly awarded range.
However there’s something about its performance — its subsonic pizzazz as it were — that’s nothing short of astounding.
POLK AUDIO MM1242 SUBWOOFER
+ Exceptional pizzazz for the money
+ Usefully shallow mounting depth
+ Also marine certified!
– No ‘buts’ at this price!
Type: 12” single voice coil subwoofer
Continuous power handling: 420 watts continuous, 1260 watts maximum
Frequency Response: 26Hz – 200Hz
Impedance: single or dual four ohm
Contact: Directed Electronics Australia
on (03) 8331 4800