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Want to build a big subwoofer but don’t want to get involved with high quality cabinet work? Then have a look at this design. Based on a readily available TV/VCR cabinet, it has the looks and it has the grunt - using a 10-inch Soundstream woofer rated at 250 watts. Build the BASS CUBE 38  Silicon Chip By JULIAN EDGAR U NLIKE OUR compact Bass Barrel featured in the August 1997 issue of SILICON CHIP, the Bass Cube is designed to deliver high power. It will really liven up the low bass in even the biggest lounge rooms. You won’t have to apologise for its appearance either because it is based on a readily available commercial product. Best of all, you won’t need to go far to buy the enclosure – it’s available for under $50 from all Big W stores, where it masquerades as the Economy Video/TV Cabinet! OK , you still have to do some woodworking to convert it to the Bass Cube but this is quite straightforward. And because it’s based on a commercial cabinet, you should have no problems at all when it comes to obtaining a high-quality finish. The Bass Cube uses a Soundstream Rubicon 10-inch (250mm) subwoofer with a maximum continuous program power rating of 250W. That’s a serious amount of power in anybody’s lan- guage so it will really deliver the grunt. This driver uses a 4-layer voice coil with a Kapton-epoxy former which is 50mm (2 inches) in diameter. The woofer has a vented pole-piece to provide forced-air cooling, while high emissivity coatings are used on the steel plates to improve power handling. The cone is made from reinforced fibre pulp with a synthetic rubber roll surround. Installed in the Bass Cube enclosure, the driver reproduces frequencies down to about 30Hz, tapering off below that. In the lounge room it’s a real window rattler. We also tested the Bass Cube in a car and while it’s really too large for most vehicles, the results were pretty impressive. By the way, the total cost of materials used to make your Bass Cube should be well under $300. Most of that is spent on the driver. Design When designing a subwoofer, there are numerous conflicting criteria to be taken into account. The first thing that needs to be considered is size. A software program like BassBox makes it easy to come up with a subwoofer that will provide thunderous bass – if the enclosure is as big as a fridge, that is! Getting good bass response from a smaller enclosure is much more difficult and in fact, can be Loudspeaker Parameters General Information Company: Soundstream Model: Rubicon 10 Mechanical Parameters Fs Qms Vas Cms Mms Rms Xmax Sd Dia = = = = = = = = = 34 Hertz 11.5 45.4 litres 0.209mm/N 104 grams 1.947 kg/sec 21mm 391 sq.cm 22.3cm Electrical Parameters Qes Re Le Z BL Pe Fig.1: the predicted frequency response of the Bass Cube shows that its -3dB point is at 33.6Hz and that is has good efficiency. = = = = = = 0.450 3.6W 2.9mH 4W 13.4 N/A 250W Combination Parameters Qts = no = Sens = 0.430 0.341% 91dB (2.83V) Vented Box Parameters Fig.2: the predicted impedance plot of the design shows that the minimum impedance seen by the amplifier is 4.8Ω. Vb Fb F3 QL Fill Ports Dv Lv = = = = = = = = 40.00 litres 34.6 Hertz 33.6 Hertz 7.0 normal 1 (round) 8.6cm 28cm April 1999  39 amplifier is 4.8Ω. Fig.1 shows the predicted frequency response, while Fig.2 plots the impedance curve. Before its response starts to taper off, the subwoofer should produce a sound pressure level (SPL) of 91dB at 1W (at 1 metre with a 2.83V input). That is a relatively high efficiency as far as subwoofers go. It is suitable for use with amplifiers rated from 50 to 250 watts. Modifying the TV cabinet At this stage the side pieces have been shortened, the black woodwork assembled ‘dry’, and the bottom part of the internal frame loosely placed into position. Resting the speaker and the port into position will show you where you need to cut these holes in the baseplate. impossible with some drivers. Fortunately, car subwoofers have boomed (pun intended) over the last five years, so there are now lots of high-power drivers available that are suited to compact enclosures. The Soundstream driver specified here is rugged, comes with full specifications and is widely available from Strathfield Car Radio stores and other suppliers. At $199, it is also relatively cheap for a driver of this quality. Having selected the driver, the next problem involved choosing a suitable enclosure. There are three commonly used enclosures: (1) sealed, (2) bass reflex (sometimes called ported) and (3) bandpass. Bandpass enclosures allow very high efficiency (big acoustic power out for not much input) but they only cover a narrow frequency range; eg, from 30-90Hz, or less than two octaves. In this type of design, the driver radiates via two ports rather than directly from the enclosure. Unfortunately, it requires very long tuning ports if we want good response at low frequencies. In fact, if the ports are made large enough in diameter so that “chuffing” noises don’t occur, they may need to be metres long to correctly tune the enclosure! The Bose Cannon is a classic exam40  Silicon Chip ple of this approach and it is 4 metres long! We want something a bit smaller than that, please! A small, high-efficiency bandpass enclosure therefore has major drawbacks when it comes to port design. In case you’re wondering, the previous Bass Barrel bandpass design overcame some of these problems by using two speakers mounted in an isobaric configuration. However, using two Soundstream drivers would make this project just too expensive. Sealed enclosures are the easiest to make and they have a predictable response. However, the bass roll-off (gentle as it is) starts very early and so extended low frequency response from small sealed enclosures requires a substantial bass boost; ie, lots of power. The efficiency of sealed enclosures is also lower than ported designs. Finally, there are the bass reflex enclosures. These augment the bass response by coupling the output from the rear of the speaker cone via a tuned port. Fortunately, the tuned port can be made reasonably short, even with a relatively small enclosure. The Bass Cube design uses the bass reflex approach. The final design uses a 40-litre enclosure. Its -3dB point is 33.6Hz and the minimum impedance seen by the Now that we had the basic design, we started looking at suitable enclosures. To make construction as simple (and economical) as possible, we wanted a commercial enclosure that could be modified to suit. We found nothing really suitable until we started looking “outside the box”. If there was nothing available designed for the purpose, what about something designed for another purpose? That's when we spotted a Video/ TV cabinet in the local Big W store. Eureka! This cabinet is very suitable for this application. It is made from relatively hefty 16mm black plastic veneer chipboard, screws together tightly and needs only a few minor modifications to turn it into a subwoofer enclosure. The big advantage for the home constructor is that all exposed edges are finished, the panels are all cut square and the design is modern. What more could you want? Oh, you do have to put the cabinet together. Did we mention that before? It comes as flat-pack kit. As well as the materials provided with the cabinet, you will need all the materials listed in the accompanying panel. You will also need a drill, an electric jigsaw and preferably a circular saw. If you don’t have the latter, a hardware store can probably do the very few cuts required or you can make the straight cuts by running the jigsaw against a clamped straight edge. The first step is to unpack the cabinet. Incidentally, if you can’t find it at Big W, contact the Victorian makers direct (they are listed at the end of this article). With the pieces of the cabinet laid out, you will see that it comprises a large base, two sides, a smaller top and a shelf. Fig.3, reproduced from the manufac- Fig.3: the assembly instructions for the TV/Video cabinet include this diagram. In the Bass Cube application, shelf (D) becomes the new front panel, with the side panels (C) shortened to match. turer’s assembly instructions, shows the general layout. In the Bass Cube configuration, shelf (D) becomes the front panel, while the side panels (C) are reduced in height to suit. It’s a good idea to first loosely assemble the cabinet (ie, don’t fully tighten up the screws), so you can be sure how it all goes together. The action of the “cam lock” fasteners, for example, may not be clear until you do this. Cutting the panels The two side panels (C) should be cut to 337mm (high) by trimming their bottom edges. You will need to take special care to avoid chipping the black plastic veneer along the cutting edge. There are several things you can do to minimise this problem. First, use a sharp, fine-toothed saw. We’re assuming that you’ll be using a circular saw, by the way, and that you will probably use it in conjunction with a straight edge guide to ensure a nice straight cut. Of course, there is nothing to stop you from using a hand saw. Second, use a sharp Stanley knife to deeply score along both sides of the cut; ie, the width of the saw blade. In this way, the plastic is cut before the saw blade touches the material. Also note that the blade will always tend to chip the material more on the underside of the cut. So it is a good idea to make sure that the underside is the side that won’t be seen when the unit is ultimately assembled. On the other hand, you need to take care that the baseplate of the saw does not mar the material as it slides along. If the saw does happen to leave minor blemishes in the plastic veneer, fill these with black paint. Once you have shortened the side panels, redrill the pilot holes in the base of each piece. You can see where these need to go by checking the locations of the holes in the pieces that April 1999  41 driver is improved and (3) the cabinet looks neater. Bracing the enclosure With the holes cut for the speaker and port (and the mounting holes drilled for the speaker), the black woodwork can be assembled using PVA glue. have just been cut off. If you look at the inside faces of the side panels you’ll see more holes drilled to take the shelf-locating cam pins. These holes remain unused – instead, new pilot holes need to be drilled to hold the shelf in its new location as the front panel. We recessed the front panel by 4mm so that it matched the appearance of the rest of the cabinet. Be careful when drilling these holes that you don’t drill right through the panel. If you do, you will spoil the appearance of the finished job. Once all the holes have been drilled, you’re ready for a trial assembly. Don’t use glue or screw anything fully home at this stage – just make sure that it all goes together in Bass Cube form without any problems. If all goes well, you’re ready to add the internal framework before fitting the back, the speaker and the port. Internal framework As shown in the photos, the speaker is mounted face downwards inside the enclosure and “fires” through a hole cut in the baseplate. The port also vents through this bottom panel. However, because the Bass Cube sits up on the integral feet provided with the cabinet, there’s plenty of room for the sound to escape, although if you place the Cube on deep pile carpet, you should extend the feet or possibly fit furniture casters. Taking the bottom-firing approach has a number of advantages: (1) the driver is protected without the need for a grille, (2) the air-loading of the The Materials Required          PVA glue and a tube of Liquid Nails (or a similar adhesive) One MDF panel about 443 x 335 x 18mm 5 metres of 20 x 20mm DAR pine, Meranti or similar timber 28cm of 90mm plastic stormwater pipe 50 6G x 30mm self-drilling plasterboard screws 12 6g x 35mm (or 40mm) self-drilling plasterboard screws 8 nuts, bolts and washers to suit the speaker mounting 1 speaker terminal and heavy-duty speaker wire 1 square metre of quilt wadding. 42  Silicon Chip An internal framework braces each of the panel joins of the enclosure. This stiffens the box, provides added insurance against air leaks and locates the new rear panel. The framework is made from 20 x 20mm DAR (dressed all round) pine and is screwed and glued into place. The exact dimensions of the framework will be determined by the amount that you recess the front panel. Assuming that it’s recessed by 4mm, the long pieces of timber bracing will be 445mm long and the short pieces 296mm long. The accompanying photographs show the layout of this framework. We used butt joints since they’re easy to make. After cutting the four pieces, temporarily position them on the bottom panel. When you’ve done this, it will be apparent that there’s only just enough room for the speaker to fit. When you add the port dimensions, it becomes clear that the driver needs to go at one side of the bottom panel and the port at the other! Mark the holes for the speaker and port on the baseplate, then cut them out with a jigsaw. Once this has been done, you can glue the Bass Cube together. We suggest that you use white PVA woodworking glue for the initial assembly. This can easily be wiped off with a wet rag and also dries clear, so any bits that you forget to wipe away are not visible. Begin by assembling all the black panels, ie; the base, the two side panels, the top and the front panel. The front and side panels are also secured to the base using screws. Let the PVA glue dry for a few hours before installing the internal pine framework. This should be glued into place using generous applications of Liquid Nails. Pick the water-soluble type of Liquid Nails so that it’s easy to clean up and be sure to choose a well-ventilated area when applying the glue, to avoid inhaling the fumes. In addition to the glue, we used two or three 6G x 30mm self-drilling plasterboard screws (inserted from each direction) to hold every piece of the framework firmly in position. When inserting the screws, be sure to drill pilot holes to avoid splitting The internal framework is screwed and glued into position using chipboard or plasterboard screws and copious quantities of Liquid Nails. Note the use of a brace across the front panel (seen at back of picture). There is also another hidden piece across the underside of the top panel. the timber. We also stiffened the large front and top panels with additional lengths of 20 x 20mm pine, as shown in the photos. When the glue dries you should have a stiff, well-sealed enclosure – apart from the open back, of course. against the internal framework, with its outer edge flush with the rear of the Cube. The back is held in place by 12 6G x 35mm screws but don’t put them in just yet! First, you need to mount the rear terminal block (we used one from Dick Smith Electronics) and wire the speaker to it. Soundstream state that the leads should not be soldered to the speaker (and may not honour a warranty claim if they are), so we used push-on spade terminals. Make sure that the positive and negative terminals on the driver are wired to the corresponding terminals on the rear panel terminal and use heavy-gauge speaker wire for this job. Now cut some quilt wadding and glue it into place on the inside surfaces of the enclosure, not forgetting the inside of the back panel. Make sure that the wadding doesn’t block the port though, because that would seriously upset the performance of the Bass Cube. At this stage, it’s a good idea to connect the subwoofer to an amplifier and play some music, just to make sure that the driver and your wiring are OK. Keep the level reasonably low for this test though, as the unit is not yet sealed (the back isn’t on). If everything seems fine, disconnect the amplifier and then glue and screw the back panel into place. Again, be lavish with your use of Liquid Nails – you don’t want any air leaks at all. Once you have done Mounting the speaker The next step is to mount the 10inch woofer and the port tube. The 28cm long port is cut from plastic stormwater pipe. It has a nominal outside diameter of 90mm and an internal diameter of 86mm. To mount the driver, first mark and drill the eight mounting holes in the base. Be sure to use the gasket provided when installing the driver and tighten its mounting bolts down evenly so that the frame doesn’t distort. The port tube can be glued into place using Liquid nails, making sure that there are no air leaks around its edge. The tube should be located so that its end is flush with the bottom surface of the base panel. The rear panel is made from a 443 x 335mm piece of 18mm thick MDF (medium density fibreboard). Note that these were the dimensions used on the prototype; it would be wise to measure your own enclosure just in case it is slightly different. The thicker rear panel does not require internal bracing. It should fit neatly into the enclosure and nestle The final steps before sealing the box are to glue quilt wadding on the inner surfaces and wire up the terminal. Make sure that you use heavy-gauge cable for this purpose. April 1999  43 MAKING YOUR OWN BASS CUBE BOX If you don’t want to buy the TV/ Video cabinet we modified for this article, there’s nothing to stop you constructing your own Bass Cube in the conventional manner. The diagram below shows how this can be done using 18mm MDF. With this thickness of timber, internal bracing should not be required 44  Silicon Chip but all joins must be completely airtight. The speaker baffle should be the last panel fitted, following a similar test procedure as outlined in the text. Note that this enclosure is shown upside down – like the enclosure featured in this article, it is designed to have the speaker and port aimed at the floor. Similarly, the enclosure will need to be supported clear of the carpet – and note the comments about shag pile carpet in the text. Incidentally, at 480 x 340 x 370mm, this subwoofer is getting close to the size seen in many large cars. Yes, it’s big but the bass is amazing! The completed subwoofer lying on its front face. This photo gives a good idea of how all the pieces go together and the mounting positions for the speaker and port. Note the large ‘feet’ – this size lifts the speaker baffle off the floor enough for normal carpet but these would need to be even higher if the Bass Cube was sitting on thick, shag-pile carpet. that, let the adhesive harden before launching into action. Note that the rear panel should not be glued, although some sort of sealant should be used to avoid leaks. Testing The Bass Cube should be tested alone first, without other speakers playing. At this stage you want to be able to hear just what the Cube is doing – not have its sounds partially drowned out by the rest of the system. Play some music relatively quietly through the sub and listen for buzzing noises – they can be evidence of air leaks. Moistening your fingers and moving them along all of the joins will also help you locate any leaks. These leaks must be sealed if the subwoofer is to perform well. Perhaps the easiest way to seal leaks is by smearing some PVA glue into the join at the point where there is a leak and say 50mm each side, then wiping the excess off with a damp rag. When the subwoofer is working without buzzes, turn up the volume – again with just the Bass Cube connected. Listen for distortion, clacking, buzzes, whistles and the like. If there aren’t any, turn up the volume a little more. If you have a powerful amplifier (or more frequently, a small amplifier that’s driven into distortion!) you will clearly hear when the limits are reached. Except for a very brief time during this testing, don’t ever drive the Bass Cube into distortion. Note that this may occur without being noticed when the other speakers are connected and playing – you have been warned! Like all speakers, the frequency response of the Bass Cube will be affected by its location within the room. If it is placed against a wall or in a corner, its bass will be augmented – but will tend to be boomy or “muddy” as well. If your listening situation allows it, move the Cube around during testing until you find the most pleasing location. Remember also that if you wade through the carpet at your place, you’ll need to extend the Bass Cube’s feet to lift the base panel above the shag pile. Given its size and cost, we were pretty pleased with the performance of the Bass Cube. We are sure that you will be too. SC WHERE TO BUY THE CABINET The Economy Video/TV Cabinet is available at any Big W store, or failing that is manufactured by: Koala Furniture International Pty Ltd (03) 9878 3688 April 1999  45