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A Clever Subwoofer Solution for Classic 900's

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You don't have to sacrifice the versatility of your trunk and folding rear seat in order to get deep, tight and clean bass. At least, not as long as you're driving a Classic 900 and not unless you want to wake up the neighborhood with 'thump, thump'... By removing the storage bin from your center console, you can create about 10 litres (0.35 cu.ft) of internal enclosure volume, which is enough for some high end 8" subwoofers on the market to give astounding bass response, extending as low as 25 Hz! You can even continue to use the upper DIN slot of the center console for accessory gauges.

But it gets even better. Getting your subwoofer up front, where your main stereo speakers are, offers some great advantages for you car audio configuration. Firstly, you won't need that pair of 6.5" kick bass speakers in your doors anymore making for much simpler and more elegant system design. Secondly, the subwoofer is guaranteed not to distract the imageing properties of your main stereo spekers anymore. Conventional wisdom says you can put the sub(s) in your trunk becuase the human ear can't locate where sounds below about 100Hz come for. Whilst this is absolutely true, one look at the frequency response graphs of subwoofer systems is that all sealed and vented enclusures, and even so-called band pass boxes do emit a considerable amount of sonic energy above 100Hz, unless you use them with a 4th order crossover network which causes some quite serious problems itself. As the center console mounted subwoofer is located exaclty in the middle between left and right front speakers its output is completely in phase, and you can use a crossover with a very gentle slope to 'blend' the bass with the main speakers output. Depending on cut off frequency and crossover properties, it even works as a center speaker to some extent... Furthermore, there is nothing between the speaker and the listener's ear. The amount of 'direct' sound is the reason why sealed or simple ported bass enclosures are preferred to band pass subwoofers for audiophile purposes. When you put a bass speaker in the trunk of a car, the trunk walls, parcel shelf, back seat, etc. that are between it and the car's passenger cell, will act as a band pass to some extent. On top of that, there's far less large surfaces inside the car's interior that are prone to adding unwanted resonances. Another advantage is that having all your speakers in the front of the car gives you the possibility for short cable runs when you mount your amplifier(s) somewhere in the dashboard area or under the front passenger seat. If you incorporate some kind of rear fill speakers in your sytem, they will more likely than not run from the head unit speaker outputs.

Building a subwoofer enclosure like described here saves weight. If you weren't concerned about your Saab's performance other than the sonic properties of it's sound system, you wouldn't have found this site in the first place, right?Adding a sub box weighing in excess of 20 kgs might thus not sound all that alluring to you. Building a sub enclosure likes this will only add the weight of two small MDF board panels, a layer of fibreglass, and the subwoofer itself to the center console and will allow the rest of the system to be designed in a very simple and elegant way. Home audio buffs have found out that 'less is more' decades ago, and there's no reason the basic principle would not apply to car stereo systems. The speaker set up in Saabine consists of just a pair of 4" component systems, and the subwoofer. It is capable of competition-grade sound quality (but don't take my word for it - I'm actually going to compete this season, and will report back here about results and subsequent developments).

How is it done? Quite simple. Remove the center console assembly from the car and separate the plastic frame from the carpeted part. The latter will form the base of our sub box. Laminate its walls with glassfibre matting from the inside. This requires some experience with glassfibre, so I decided to have the enclosure built by a car stereo installer, which cost about 2 -2.5 hours of labour. The top side and front baffle are made from 3/4" (18mm) MDF board. The top will have to be shaped with a file at one point (see pics) in order to make it fit under the heater ducts. For the front baffle, the hole for the 8" speaker will largely determine the shap which will 'bulge' the side walls a bit to the outside. Do't worry, the plastic of the center console frame (to which the storage bin and -where applicable- the bracket for extra gauges are attached) is flexible enough to cope, and it does not look strange or even 'non-stock' after everything is put back together. After the top and front baffles have been fit to make an enclosure, great care has to be taken in making sure it is air tight. This is done with fibreglass reinforced body filler which is used for repairing rust holes etc. in car body panels. For the insides of the enclosure walls, Bofoam is being used as dampening material. In this case, using a Rockford Fosgate woofer, the best results were achieve with inserting a 5cm diameter port of about 18 cm length. The port is located at the passenger side footwell. The front baffle is finished with black cloth (padded black vinyl would be nice, too) before the subwoofer is installed with a suitable length of cable. A small hole is drilled for the cable at the back of the enclosure, and made airtight with sealant compound after the cable is run through it. Now you can place the enclosure in the car and put the frame with the ash tray, but with the storage bin removed, back. You can cover the hole in front of the speaker-where the storage bin was before- with acoustic cloth over a frame made of thin MDF board. This will make the speaker completely invisible for the car's occupants. They might not notice it altogether, until you crank up your sound system...

centersub.JPG (60613 bytes)And this is what the end product looks like.The cloth grille is not in place here to show the position of the woofer in the center console assembly. The plastic center console -including the ash tray and the part between the seats, where the ignition switch and window lifter switches are- is sprayed in Opel Rembrandt Silver (137) to match the colour of the Kemwood PS909 CD head unit face plate. Looks pretty cool with a tan interior IMO, but in order to avoid the tacky effect of Japanese/Korean 'spray painted plastic' a lot of preparation is needed. Start with two layers of plastic primer, then apply two or three coats of spray paint, with the application of 1200 grade waterproof sanding paper inbetween. The final stage is a clear coat lacquer. The custom gauge panel (with original Saab accessory gauges to match the dashboard instrumentation) is made out of MDF board (2 layers of 4 mm thickness), and finished with tan vinyl that matches the tops of the 4/5dr door panels.