Alpine SBR-S8D4
There’s an idiomatic proverb that states one cannot have their cake and eat it too. In much the same vein intuition dictates that you cannot have impressive levels of sub-bass in your car unless you’re prepared to part with significant money and space. 
That’s pretty much the point we’ve sat idly at for years, and although numerous companies have attempted to overcome these barriers most have enjoyed but a limited degree of success. 
It’d take one tenacious company to tackle this prickly topic again given so many others failed to put paid to the issue previously. Not surprisingly, Japanese born and bred electro-wizard Alpine is just one such company. Never one to shy away from a tough challenge, Alpine recently decided that it would take the prebuilt enclosure genre back to the drawing board and give this challenge a red hot go.
It’s just physics
Genius though it may be, Alpine is no deity. Meaning consequently, it also had to comply with those niggling laws of physics that have brought so many others unglued, as physical space is perhaps the prime hurdle to overcome when designing an enclosure of this ilk. 
As imagined, it’s anything but easy and often becomes an unpleasant case of either/or, with some of the smaller prebuilds proving loud bass that suffers poor control due to erratic suspension and porting, while others remain fairly accurate however not offering much punch nor extension due to their intrinsically esoteric motor structures. 
Ergo do let’s take a look at what Alpine has actually come up with and why it’s proving to be head and shoulders above much of its competition.
Alpine SBR-S8D4
Fundamentally speaking, the SBR-S8D4 is an 8-inch subwoofer loaded into a ported enclosure and, although it’s fair to say it’s not the result of ‘endless amounts’ of research, it’s no slap together design either. Alpine had to reign in the price tag on this design, giving causation to pull considerable technological ideologies from products within its stable boasting more outlandish price tags. Couple that with over half a century of experience and the result is conducive to the superbly designed product we have before us.
The sturdy timber enclosure is smartly finished in durable grey carpet front and back while around the perimeter edge is black laminate. The subwoofer driver resides on the front face and it’s protected by a mesh grille that’s also finished in gloss black. Removing it to inspect the internal build of the enclosure affords you opportunity to view all the distinctive attention to detail such as internal bracing, radii upon all the port mouths, foam covered fly leads and fibrefill lining etc. All these are proof positive that Alpine is a master craftsman. The slot port is quite a lengthy affair too due to its tune being quite low at 30Hz, thus dictating it must snake around the internal perimeter before exiting upon one end.
The driver itself is actually Alpine’s legacy Type-R unit, thus has nothing to prove. It’s quite substantial, with its 83.5dB efficiency dictating it’s no lightweight. Alpine chose this driver in order to harness its considerable strength and control, as opposed to developing some new lightweight unit which would simply flap about all over the place with diminished composure. The design starts with a two piece diaphragm structure, the lower cone being made from Kevlar reinforced paper fibre chosen for its combination of strength and natural sound. The upper dust cover portion is constructed from a poly-mica material and is shaped with a particular parabolic profile in order to project the sound forward of the driver. Encircling this is a surround made from injection moulded Santoprene which is shaped with a multi-role profile that allows for superior resistance to radial float whether playing softly with only a portion of it moving, or flat-out with the whole shebang playing a supporting role. 
Although impressive the surround still has to work in conjunction with the flat rolled progressive Nomex spider below, which couples to the cone structure via a reinforced neck and spider joint. The suspension pairing is significant yet despite this still quite flexible, returning a resonant frequency of 38Hz while also possessing the ability to handle extreme power bursts without fear of bottoming out; to the tune of the maximum power rating being just shy of 1000 watts!
Alpine SBR-S8D4
Deep Hertz 
Delving deeper reveals a serious motor residing within the 115mm high cast aluminium frame, a portion of that height courtesy of an integral shorting sleeve and radial vented heat sink that are incorporated into the lower confines. The motor is comprised of a 38mm spiral-cut aluminium former upon which is wrapped a dual 4-ohms, quad layer 180-degrees high temperature voice coil. The bottom end of the former has a specially curved compound radius which aids both thermal and physical performance. 
Encapsulating this is a 52oz six piece, double stacked, radially segmented strontium ferrite magnet, and if that’s too much of a mouthful then rest assured it exerts an extremely dense flux over the entire 14mm one way Xmax. Yes folks linearly she can move quite some distance, maintaining superb control during its entire 28Hz – 200Hz allotted frequency range, although in reality you’ll only reproduce the lower half of that. 
Cooling is handled via air induction through five pairs of twin vertical intake ports located about half way down the web fingers. From here the zephyr is fed around the inner pole and outer coil air gaps before being expelled through the spider as the motor reciprocates. This cooling system allows for the motor to handle over 300 watts continuously without fear or trepidation.
Having tested a spate of rather noisy subwoofers of late I was quite keen to get the SBR-S8D4 into the test car, especially given its impressive specifications. It was time to see if in reality it could deliver what it promised on paper. As a musician who likes sound quality, I tend to look for particular specifications or skillset if you like, which dictates how a subwoofer should sound. On paper SBR-S8D4 presents as not only accurate but also its combination of ancillary specifications in Xmax, Vas, F3 and frequency response dictate that it ought to play extremely low. Hell, its response starts at 28Hz; there are 15-inch subwoofers out there that aren’t enthusiastic about playing that frequency! Hence the case in point, in that for years those of us playing instruments the likes of organ, bassoon, double bass, tuba, sousaphone, cello, etc., the only way to appreciate the full gamut of these instruments hitherto was to have a larger subwoofer usually housed within an even larger enclosure. 
Could the SBR-S8D4 now spell the end to this conundrum? The answer is best put as: ‘to an extent’. Now, despite all your sighing, let me explain why I put it like that as opposed to gushing unequivocally about how wonderful it is. In the interests of honesty we need to be forthright about what the unit is designed for, and I’m of the belief Alpine would agree given its professional integrity. Now don’t misunderstand me; as aforesaid I’ve tested enclosed subwoofers aplenty recently and the SBR-S8D4 is easily the single best enclosed 8-inch subwoofer I’ve heard, period. It’s a wonderful sounding unit, astutely accurate and most authoritarian when coupled with quality amplification… and yes, that proficient port and motor combination do make for stunningly controlled bottom end extension.
Nonetheless that important word ‘context’ must enter into the fray at some point. See, the SBR-S8D4 is designed to fill out the bottom end of many a system while consuming minimal space, and that means the actual bottom three 1/3 octave steps in 20Hz, 25Hz and 31.5Hz. You know, those regions that many smaller subwoofers refuse to even acknowledge exist let alone reproduce faithfully. However, in fairness please don’t go stacking this unit up against full sized 12-, 15-or even 18-inch subwoofers valued at well into four or five figure territory, as that’s just a travesty to common sense. Grab Alpine’s Type-X 12-inch subwoofer if such misadventures are more your thing.
Although designed for sound quality buffs I couldn’t let it get away without giving it just a little tickle up, for poops and giggles, as it’s often colloquially put, if nothing else. However, rather than just hammer it with electronic music and square waveforms I instead decided to blow my head off with some harder technical metal genres featuring thunderous blast-beat double bass drumming. Again when paired with a competent amplifier the SBR-S8D4 is quite the impressive little beast; scarily so at times as it outputs an astounding level of racket given its size.
Put simply, the SBR-S8D4 will prove more than capable of keeping the vast majority of consumers happy, and I’m not just referring to AM radio listeners. It’s a serious little powerhouse which blends superb subsonic qualities with considerable force. Ergo if you’re in the market for a subsonic solution and have overlooked 8-inch units because you think they won’t pass muster perhaps slow your roll and revisit this one. You might just be pleasantly surprised at the heights it can reach.  
Alpine SBR-S8D4
Alpine SBR-S8D4 8-inch Enclosure Subwoofer
Cost: $299
+ Superb 8-inch driver tech
+ Punches much harder than its size
+ Great attention to detail

- In its intended use, nothing
Type: 8-inch dual voice coil subwoofer in ported enclosure
Power Handling: 150 - 300 watts continuous, up to 1000 watts maximum
Frequency Response: 28Hz – 200Hz
Impedance: dual 4-ohms on the driver, single 2-ohms at the terminal plug