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Features: me ss in boot volu • Very little lo en installation • Largely hidd an an addition to • Suitable as al equipment system existing origin ots sedans with bo • Suitable for kers of suitable spea • Wide range Installing a Free-Air Subwoofer in Your Car This design removes many of the space and weight compromises of car subwoofers. It is ideal for those wishing to upgrade the standard sound system in a late-model sedan. By Julian Edgar O ne of the major problems with car subwoofers is that they take up a lot of space. A small, well-designed subwoofer enclosure fills about forty or fifty litres but the situation is in fact even worse than that. When you locate a large, square box in the boot, you find that lots of the nooks and crannies that were previously available to be filled by soft bags are now impossible to access. The other major drama with a conventional car subwoofer is that it’s heavy. By the time you incorporate thick MDF panels and the driver itself, you can easily have added 40-50kg – weight whose fare you pay for at the petrol pump every time you fill up. So how have we overcome these problems? OOctober CTOBER2000  53 2000  53  Like many modern cars, the Audi has a central rear armrest that folds down into position. However, no opening through to the boot is then created – a steel panel remains in place behind the leather trim. A hole for the subwoofer grille could have been cut in the plate using a jigsaw equipped with a metal-cutting blade, however…. Simple – we’ve used the boot as the speaker enclosure! This type of free air (infinite baffle) design makes use of the hundreds of litres of boot volume that’s available the majority of time that the boot is not full to the brim. And when you want to place some luggage in the boot, there’s no problem at all – the speaker enclosure just gets a little smaller! However, it should be noted that using a free-air subwoofer does not give the same sound quality as a subwoofer mounted in a smaller, carefully-designed (eg isobaric) enclosure. Also, the power handling of the Because a large speaker cone area will allow the movement of a lot of air for minimal cone dis-placement a ten-inch speaker was selected. However, the ski-port opening width would not permit a ten-inch diameter grille, with a 6.5-inch grille being most appropriate. As a result, a spacer ring was cut from MDF, allowing the speaker to be mounted to fire through the smaller hole without its rubber-roll suspension coming into contact with the baffle. You could of course use a subwoofer driver whose diameter matches the available grille diameter. 54  Silicon Chip 54  Silicon Chip  … when the interior boot trim was pulled away, a factory pre-cut opening was revealed. In some markets this car is equipped with a “ski port”, but in this particular model the plate was not fully cut out. Some judicious use of an angle grinder soon removed the short pieces of metal that held the blanking panel in place, allowing the easy creation of a rectangular opening. driver will be reduced over that which would have been possible in a smaller enclosure. But taking the approach outlined here is ideal for people owning a late model sedan that’s equipped with a decent sound system. It just lacks the required bass punch. Selecting the Driver There are two major criteria to examine when selecting the most appropriate driver. First, how much space is available to mount the speaker? In the example discussed here, the speaker was positioned so that it fires through a grille located behind the rear seat armrest. Other methods of mounting include positioning the speaker so that it works through a hole cut in the rear deck, or so that it fires through one side of a 60:40 split rear seat. In many cases (especially in those cars that do not have a metal panel behind the rear seat) the speaker can be located so that it works through the back seat itself – a large speaker driven with adequate power will still be quite effective, even with no direct air connection with the cabin.  The Soundstream Rubicon 10 can be seen mounted on the baffle with the spacer ring. Only four bolts were used to mount the baffle board against the metal panel across the rear of the seat. High tensile nuts and bolts (available from auto accessory suppliers) were used so that they could be adequately torqued without failing in the way all too familiar to those who use common hardware store nuts and bolts!    Note that if the grille area of the new speaker is limited in diameter – but the rear mounting space is plentiful – the selected speaker can be larger than the grille. The Audi S4 installation shown here used a ten-inch speaker and a 6.5-inch grille. The second criterion is that the speaker must have appropriate specifications that allow it to work effectively in a free-air environment. In simple terms, the speaker’s suspension must be stiff enough that the speaker cone will not bottom-out when working hard, even without the Once the opening through which the speaker is going to fire has been created, an MDF mounting board should be cut to size. Don’t be tempted to use chipboard for this piece – it will easily crumble at the edges and also have poor strength and vibration dampening abilities. The piece of 18mm MDF was sized so that the selected driver and amplifier could be mounted on it, with room left for another amplifier, for later upgrading of the system. restoring force normally generated by the springiness of the relatively small volume of air trapped behind the cone. Manufacturers and retailers selling automotive subwoofers have available recommendations for appropriate enclosures for each of their speakers. If these include infinite baffle or free-air types of enclosures, then the speaker can be used in the type of application being discussed here. (If no specific recommendations are made, look for those that drivers that have a high Qts value – eg, above 0.6). The power rating of the driver  The mounted driver, viewed from the ‘seat back’ side. The eight screws and nuts that were used to hold the driver in place were countersunk so that the baffle board could fit snugly up against the panel behind the seats, without the heads of the screws coming into contact with the panel. The five recesses (cut with a large drill bit) around the edge of the speaker opening are there to take the five prongs present on the grille that was used. (More on the grille later.) Note the offset of the speaker relative to the opening.   Once the fill panel had been removed, tough rubber edging was placed around the exposed metalwork. When performing any work on a car, the potential for injury from sharp edges (especially in a vehicle accident) should always be considered. The type of edging used here is quite strong, as it uses internal spring metal clips to really grab the edge over which it is placed. This edging was purchased from a rubber store. should also be appropriate for the amplifier that is going to be used to drive the sub, noting that the efficiency of a free-air design is lower than that achieved with the same speaker mounted in a ported enclosure. We used a Soundstream Rubicon 10 subwoofer, with a program power handling of 250 watts. This is the same driver that we used in the Bass Cube ported subwoofer, featured in the April 1999 issue of SILICON CHIP. A very versatile speaker, it can be used in infinite baffle, sealed, ported and bandpass enclosures. It costs The panel across the back of the Audi’s rear seats has multiple ribs and other shapes pressed into it to provide structural stiffness and prevent vibrations. So that the MDF baffle would adequately seal against the metalwork (and also to prevent rattles), self-adhesive high density foam rubber strip was attached just inside the periphery of the baffle. It attached the foam rubber to the clean board surprisingly well. Note the mitred join at the corner – a dob of contact adhesive was used to join these faces firmly together.  October 2000  55 OCTOBER 2000  55  The baffle was then bolted into position but not before some thought had been given to how the trim panel was to be mounted. A panel was used to hide both the amplifier and the subwoofer driver. This must be mounted securely if vibration isn’t to be a problem. To allow for a firm mounting and to provide clearance to the amplifier, the baffle was extended by four pieces of 65 x 19mm timber, glued and screwed to the MDF and visible each side of the subwoofer in this picture. about $200 from Strathfield Car Radio stores and dealers. Selecting the amplifier The vast majority of modern cars use a four (or six) speaker system. Two speakers are mounted in the front doors or dashboard, often with an additional pair of tweeters. In the rear deck or rear doors another two drivers are positioned. Generally, line-level outputs (ie  The Audi has its battery under the back seat, so the positive supply cable for the amplifier didn’t have to travel very far! A heavy-duty in-line blade-type fuseholder was placed close to the battery and the heavy cable soldered to one of the main original power supply cables coming from the positive terminal. Note that the fuse should be placed as close to the battery as possible. Where the amplifier power supply cable came in contact with the bodywork edges, it was protected by having a piece of snugly-fitting fuel hose slid over the cable. RCA-type connections) are not available from the stereo systems (or “front ends”) installed by the vehicle manufacturers. These factors mean that an amplifier that will accept speaker-level inputs is far easier to integrate into a current system than one requiring line-level signals. In addition to having speaker-level inputs, a suitable amplifier should also incorporate a low-pass filter (preferably with switchable centre frequencies) and a bass boost switch (preferably with switchable boost levels). As the amplifier will be driving only the one subwoofer, it is also helpful if it can be wired in bridge mode, substantially boosting the output power. There are many car audio power amplifiers that satisfy all of these criteria – you just need to have a clear idea of what you want before venturing into a shop! In the Audi we used an older Coustic  With the trim board cut to approximate shape and a small hole cut to allow the rear of the subwoofer driver to project through it, the board was placed into position and markings made with a texta to show where further trimming was needed. Note that in the Audi, the trim board also needed to fit in front of (ie behind, in this view) the factory-mounted CD changer. And yes, I did get sick of getting in and out of the boot! 56  Silicon Chip As can be seen in this view – showing the trim panel temporarily in place to check for fit – the magnet and part of the speaker basket protrude through the panel. Incidentally, it was a deliberate decision that the trim panel be mounted not sufficiently rearwards to cover the rear of the speaker – doing so would also unnecessarily remove boot volume either side of the speaker. The hole for the speaker is at this stage a little small – but it’s easier to enlarge it than reduce its diameter!   amplifier, rated at 50W per channel and incorporating all of the points mentioned above. Secondhand, this type of amplifier can be picked up for about $150. Step-by-step photos While the photo sequence shows how the amplifier and subwoofer were installed in an Audi S4, with variations on the theme, this will provide a good SC guide for most sedans. The system can be tested, with attention paid to locating buzzes or other vibrations. Once happy with it, the trim panel can be made. If the car has as standard a trim panel that fits against the front face of the boot, this can be checked for fit in its new (more rearwards) position. If still a good fit, it can be used as a template to mark out the board that will now support it. Our trim panel used 5mm thick Masonite for the trim panel, pre-punched with 7mm holes, which allow airflow through the trim panel and reduces mass. Finally, a grille is gled into place. This grille is actually from a Volvo, obtained from a crash repair business specialising in prestige cars. It looks far more ‘original equipment’ than a typical aftermarket grille. The grille is completely hidden when the armrest is raised to its vertical position and in fact in this position, very little reduction in the subwoofer output occurs.  Car audio amplifiers use a remote switch-on input, triggered by 12V. This is most easily accessed at the electric aerial wiring – use a multimeter to probe the wiring to the aerial motor until a wire is found that has 12Von it when the radio is on, and 0V when the radio is off. Check this thoroughly – not all electric aerial wiring is as self-explanatory as it might first appear! If an electric aerial is not available, the remote switch-on power can be accessed from an ignition ‘accessories’ source.     To provide strong but attractive speaker protection, a cheap stainless steel mixing bowl was glued to the back of the trim panel using “water clean-up” Liquid Nails with a thick layer of quilt wadding glued into its recess to avoid hollow ‘ringing’ noises. The bowl was mounted with its flange against the back of the trim board but if less speaker clearance was needed, it could have been inserted from the other side. When the glue hardened, the bowl and back of the trim board were painted black with a spray can. A matching sized hole was cut out of the standard trim which was then glued to the new masonite trim backing. While contact adhesive can be used, small dobs of Liquid Nails is much cheaper and just as effective – and without the smell, too! The trim panel was held in place with black-head self-tapping screws, screwing into the four wooden pieces previously attached to the baffle. Standard black fasteners were inserted to fill the holes that would otherwise have existed in the trim (seen either side of the speaker). O October CTOBER2000  57 2000  57