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Car Sound, Pt.3: Building A Tube Sub-Woofer Car sub-woofers are now very widely used by those who wish to listen to music containing sub-100Hz frequencies. In this article, we show you how to build a car sub-woofer capable of reproducing frequencies down to 25Hz. By JULIAN EDGAR The vented tube-type sub-woofer design shown here has several advantages over a conventional, built-in box design. These advantages are as follows: (1). compact size compared to many sub-woofer installations; (2). portability – the sub-woofer can be easily removed when more boot space is required; (3). can be used in cars with folddown rear seats, hatchbacks, station wagons, and mid-engine cars; (4). low cost (around $200 each); (5). effectiveness, with response down to 25Hz; and 16  Silicon Chip (6). good power handling capability, allowing the use of high-pow­ ered bridged amplifiers. Subwoofer design A large diameter, suitably enclosed woofer will generate good bass. However the problem is in fitting the required large enclosure into a car. As an example, the recommended vented box size for a typical 15-inch sub-woofer is 222 litres. That’s equivalent to a box size of 100 x 50 x 44.4cm, which means good­bye to the back seat or boot! On the other hand, a 10-inch driver with suitable specifi­ cations can be accommodated in a vented enclosure of just 50 li­tres. That’s less than one quarter the size required for a 15-inch driver! If two such 10-inch sub-woofers are used, their en­closures occupy less than half the volume of the 15-inch unit with only a slight reduction in effective cone area. Consequently, I decided to use two 10-inch sub-woofers, each with an enclosure volume of about 50 litres in my own car. However, the in-car results were so good that only one unit is really required – unless you want to shake the rear vision mirror so much that it is impossible to use! But how do you build-in two enclosures of about 50 litres each and still leave room to carry luggage if required? The answer is to build the sub-woofers into two 12-inch diameter tubes, one running down each side of the boot. The accompanying photos show how the tubes were fitted into the boot of my Subaru Liberty. The tubes are held in place by aluminium straps and, when bulky loads need to be carried, can be easily removed in just a few minutes. Plastic storm-water pipe (78cm long) is used for the main body of the enclosure. The end-pieces are cut from 16mm MDF & the speaker end-piece is shown here being trialfitted before being finally attached. Make sure that the end pieces are not undersized – it’s better to have to sand them back to ensure a tight fit. A sealant/glue such as “Liquid Nails” should be used to form an airtight seal & at least 16 countersunk screws (see text) should be installed around the periphery of the tube. The drivers are mounted at the front of the tubes and nor­mally pump bass through the back of the rear seat. Alternatively, if the rear seat is folded, the sub-woofers pump bass straight into the cabin. Building the unit The bass tubes are made from heavywalled (5mm thick) 32cm diameter plastic stormwater pipe. The cost from a plumbing supply house was $55 a metre. Note that this material The vents were made from 50mm-diameter plastic pipe fitted with an outside ring cut from a plumbing adapter fitting. Each vent was cut to a length of 54mm & sprayed with black paint before being fitted to the end-piece opposite the driver. is also available in sewer pipe form with a slightly thicker wall but the cost escalates to almost $80 a metre! The bass tubes were each cut to 78cm long, giving an inter­nal volume of about 60 litres with the end pieces in place. Once these pieces had been cut, the two 32cm-diameter end pieces for each tube were cut from 16mm-thick medium-density fibreboard (MDF). One end-piece has a hole cut into it to accommodate the driver, while the other has This view shows the completed enclosure, before the installation of the driver. The inside of the enclosure was lined with acous­tic material to damp out reflections. This material is actually dressmaker’s quilt wadding, which is about a quarter of the price of Innerbond. In addition, a thin layer of car carpet has been used to cover the outside of the tube & this was glued into place using contact adhesive. Parts List & Costs 1 10-inch sub-woofer, Jaycar Electronics Cat. CW-2166, $109 1 1-metre length of 32cm-dia. stormwater pipe, $55 1 10-inch speaker grille, Jaycar Electronics Cat. AX-3522, $14 1 1-metre length of 150cm-wide quilt wadding, $5 1 1-metre length of car carpet, $5 Miscellaneous: screws, “Liquid Nails” glue, scrap 16mm-thick medium-density fibreboard (MDF) for tube end pieces, loudspeaker terminal, speaker cable, $20 Total Cost: $208 each a 50mm-diameter hole to accept the vent tube. An electric jig-saw was used to make these cutouts. The end pieces are held in place by countersunk wood screws inserted from the periphery of the pipe, with 16 screws and a sealant/adhesive (eg, “Liquid Nails”) used to ensure an airtight seal at each end. Note that screws with parallel sides (not tradition­ al woodscrews) should be used to get maximum purchase when screw­ ing March 1995  17 The two bass tubes are held in position inside the boot using brushed aluminium straps. On the left is the amplifier which drives both the bass tubes & also the rear deck-mounted 3-way loudspeakers. into MDF. The large number of screws (16) proved necessary to ensure an airtight seal. Even tiny leaks can allow whistles and buzzes. Each tube was lined with a soft acrylic filling. This prevents reflections within the tube, which colour the sound and give the bass a hollow timbre. Innerbond speaker-box filling is available for about $9 a metre but very similar material can be bought from dressmaking shops under the name of quilt wadding. In this form, it’s about a quarter of the price. After testing various drivers, it was decided that the Jaycar CW-2166 had the right mix of low cost ($109) and perfor­mance. This woofer uses a rigid cast frame and has a polypropy­lene cone. Its free air resonance is 31Hz, while its power han­dling capability is quoted as 120 watts RMS and the sensitivity as 91dB. In addition, its Qts is a low 0.33, allowing it to be used in compact enclosures. In fact, the enclosure design recommended by Jaycar is perfect for use as a bass tube – a volume of 45 litres (with the acrylic filling, the tube I used would be very close to this figure), coupled with a vent 50mm in diameter and 54mm long. The speaker’s impedance is 6 ohms. Testing Testing was initially done using an Akai 45W RMS/channel home stereo amplifier and a standard graphic equalizer. Also used was a frequency generator. By feeding the frequency generator output to the sub-woofer via the amplifier, the frequency re­ Fig.1: this graph plots the performance of the author’s system. With the +12dB at 45Hz amplifier equalization switch activated, there is a peak in the frequency response of about 12dB at 31.5Hz. 18  Silicon Chip This is the view from the inside of the cabin when the rear seat is folded down. The sub-woofers are reasily removed when the full volume of the boot is needed by undoing eight wing-nuts & un­plugging two cables. Even with the seat back in its normal posi­tion, the bass is sufficient for most people! sponse (and any peaks or troughs) could be roughly determined by ear. In a domestic situation, the response sounded smooth but with plenty of punch. The next step was to mount one of the tubes in a car. My car sound system uses a mix of original and aftermarket equipment. The original Subaru front-end comprising a radio-cassette player and single CD player has been retained. The two front channels of this original system are used to drive the original dual-cone 6-inch speakers mounted in the front doors, along with a pair of Jaycar Super Tweeters which have been added to the front door sail areas (the triangular areas where the rear vision mirrors are). The two rear channels are used to drive the speaker-level inputs of a Coustic AMP-268 4 x 45W car amplifier and two of its outputs in turn drive rear deck-mounted Jaycar 6 x 9 3-way speak­ers. The other two channels of this amplifier were used to drive the twin sub-woofers via a built-in variable low-pass crossover network. With just one of the bass tubes connected, the bass was superb. The driver A steel mesh grille costing $14 was used to protect the driver. It is held in place using brackets & roundhead screws. showed no signs of being overloaded, even when driven by the amplifier in high-power bridged mono mode. In fact, my reaction was that if the bass was this good with just one bass tube, what would it be like with two? The answer is even better. Frequency response plot A visit to Adelaide car sound dealer Cartronics was made so that a Coustic Real Time Analyser could be used to check the system. This is effectively a spectrum analyser which records and prints the system’s in-car response. With the system being driven by a CD-recorded pink noise signal, and with the +12dB at 45Hz amplifier equal­ ization switch activated, the graph showed a peak in the frequency response of about 12dB at 31.5Hz. Being the sort of person who likes lots of bass, I am happy to leave the system with this boosted low-frequency response. On the other hand, purists could simply reduce the switched amplifi­ er bass boost to achieve a flatter response. Either way, the sub-woofers showed that they were capable of reproducing low SC frequen­cies with ease. March 1995  19