One of the major problems with car subwoofers is that they take up a lot of space. Or, to put it another way, they remove from normal use lots of boot volume. A small, well-designed subwoofer enclosure still takes up about forty or fifty litres - but oftentimes it's even worse than that. When you plonk a large, square box in the boot, you find that lots of nooks and crannies that were previously available to be filled by soft bags are now impossible to get at. 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? 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 doesn't give the same sound quality as a subwoofer mounted in a smaller, carefully-designed (eg isobaric) enclosure, and the power handling of the driver will also 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 as standard 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. Firstly, 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. However, other methods of mounting the speaker include positioning it so that it works through a hole cut in the rear deck, or mounting it so that it fires through one side of a 60:40 split rear seat. In many cases (especially in those cars that don't have a metal panel behind the rear seat) the speaker can be located so that it works through the back seat itself. 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 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. The power rating of the driver should also be appropriate for the amplifier that is going to be used to drive the sub.
We used a Soundstream Rubicon 10 subwoofer, with a program power handling of 250 watts. A very versatile speaker, it can be used in infinite baffle, sealed, ported and bandpass enclosures.
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 (ie RCA-type connections) are not available from Original Equipment front-ends. 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 per-channel output power figure. There are lots of 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 amplifier, rated at 50W per channel and incorporating all of the points mentioned above.
Step By Step
While the following sequence shows how the amplifier and subwoofer were installed in an Audi S4, with variations on the theme, the following pictures will provide a good guide for most sedan installations.
Like many 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....
... 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.
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. It was purchased from a rubber store.
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. Here, the piece of MDF was sized so that the selected driver and amplifier could be mounted on it, with room left for another amplifier, if later upgrading of the system was thought necessary. The selected MDF was 18mm thick.
A large speaker cone area will allow the movement of a lot of air for minimal cone displacement. It was for this reason that the 10-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 mounted to fire through the smaller hole without its rubber-roll suspension coming into contact with the baffle. If you don't want to do it this way, simply use a subwoofer driver whose diameter matches the available grille diameter.
Here, 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. However, high tensile nuts and bolts were selected so that they could be adequately torqued without failing in the way all too familiar to those using common-grade hardware store nuts and bolts! Automotive accessory supply shops normally have a selection of high tensile hardware available.
The mounted driver, viewed from the 'seat back' side. The eight screws and nuts that were used to hold the driver in place have been countersunk. This was done 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.
In the Audi, the panel across the back of the rear seats had multiple ribs and other shapes pressed into it to provide structural stiffness and prevent vibrations. In order that the MDF baffle would adequately seal against the metalwork (and also to prevent rattles), high density foam rubber strip was attached just inside the periphery of the baffle. The material that was used possessed a self-adhesive side, and this 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.
With the speaker mounted, it was time to mount and the amplifier and do some preliminary wiring. The amplifier will require a positive power supply from the battery, a good earth (any chassis bolt), speaker level input wires connecting the left and right channel OE speakers to the amp, and the subwoofer cables. In addition, a 12 volt ignition switched source will be needed to turn the amp on and off.
The baffle was then bolted into position, but not before some thought had been given to how the trim panel was to be mounted. The panel that hides both the amplifier and the subwoofer driver 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. (The other two pieces are to the left, out of view.)
Before the rear trim panel is made and fitted, the subwoofer should be tested - but the wiring needs to be finished first! On the Audi, the speaker level connections were easily made with light gauge hook-up wire to the rear deck OE speakers. The only trick here is to make sure that the appropriate polarity is maintained - positive from the original left-hand speaker to positive of the left-hand amplifier input, and ditto for the right-hand channel. Both the earth and power supply wires for the amplifier should be in heavy duty cable - Jaycar 8 gauge cable was used here. With a convenient bolt located nearby (one of the bolts holding the baffle in place was used) the earth cable didn't have far to travel, while the positive power supply cable was led to the battery.
The Audi has its battery located under the back seat, so the positive supply cable 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 edges of bodywork, it was protected it by having a piece of snugly-fitting fuel hose slid over the cable.
Car audio amplifiers use a remote switch-on input, triggered by the application of 12 volts. 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 12 volts on it when the radio is on - and zero volts 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, such as the power supply for the standard radio.
The subwoofer system should now be able to tested, with special attention paid to locating buzzes or other vibrations. Once you are happy with how it sounds, 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 it is still a good fit, it can be used as a template to mark out the board that will now support it. (If it needs to be trimmed to a slightly different shape, do so at this point. And if it doesn't exist, you will need to buy some automotive carpet to make a new trim.) For the trim support board we used 5mm thick masonite, pre-punched with 7mm holes. This material was selected for two reasons - to allow airflow through the trim panel, and to reduce mass.
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. Oh yes, I did get sick of getting in and out of the boot! 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.
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 not be moved back far enough to cover the rear of the speaker - doing so would also unnecessarily remove boot volume either side of the speaker.
To provide strong but attractive speaker protection, a stainless steel mixing bowl was sourced from a supermarket. This was glued to the back of the trim panel using "water clean-up" Liquid Nails. In this case 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 had hardened, the bowl and back of the trim board were painted black with a spray can. If more durability of the coating is required, have the bowl powder-coated black before it is glued into place. To avoid hollow 'ringing' noises reverberating out of the bowl, a layer of quilt wadding was glued into its recess.
A matching sized hole was cut out of the standard trim which was then glued to the new trim backing. Note that while a contact adhesive can be used to do this, using small dobs of Liquid Nails is much cheaper and just as effective - and without the smells, 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. The standard black fasteners were also inserted (they can be seen either side of the speaker) to fill the holes that would otherwise have existed in the trim.
On the inside of the car, the leather trim piece over the now-unearthed ski port was cut away and a grille glued into place. This grille is actually Original Equipment in a Volvo, and was obtained from a crash repair business specialising in prestige cars. It looks far more 'original equipment' than a typical aftermarket grille. Note that the armrest can still be raised to its vertical position, and in fact when it is in the upright position, very little reduction in the subwoofer output occurs.
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