Fullscreen HTML5Med Res (??MB)HTML4

A high-performance subwoofer that’s compact, cheap & easy to build By JULIAN EDGAR Photos: GEORGINA COBBIN The 300mm stormwater pipe which forms the main body of the subwoofer enclosure can be cut with an electric jigsaw. 12  Silicon Chip I F YOU’RE TIRED of subwoofer designs that have internal volumes the size of a road tanker or stand as tall as a refrig­erator, this one’s for you. The Bass Barrel has an effective volume of only 20 litres and uses two 6.5-inch woofers. But it can still produce room-shaking bass, especially in smaller houses. In fact, the Bass Barrel would be ideal for use in a flat or unit in company with two small satellite speakers. The design is also well-suited to in-car duties where the small air volume inside the car’s cabin improves the bass re­sponse even further. It can easily be accommodated in the boot or the rear hatch area of a compact car. While we aren’t going to claim that the Bass Barrel will transport you to an actual rock concert, it delivers impressive performance for such a small package. The enclosure was designed using the brilliant Bass Box computer program, with literally hundreds of designs modelled on the screen before this one was selected. This approach bears no comparison with past design methods which relied on tables, graphs, rule-of-thumb and build-it-and-listen techniques. Now you can adopt an intelligent approach to subwoofer design and to designing other types of loudspeaker systems as well. The subwoofer was designed with several aims in mind. It had to: (1) use a compact, easy-to-build enclosure; (2) have reasonably high efficiency; Fig.1: the internal layout of the Bass Barrel. It is built inside a short length of 300mm plastic stormwater pipe. Note that the 63mm ID vent exits through one end panel, while the 55mm ID vent exits through the other. and (3) have a good bass response. The design is based on two Altronics C3086 6.5-inch Redback woofers. These are mounted face-to-face in what is called a compound isobaric configuration – see Fig.1. Mounting the drivers in this manner reduces non-linear distortion and, more im­ portant­ly, reduces the required enclosure volume to just half that required for a single driver! The down­side is that the sensitivi­ty of the compound The three internal baffles (left) are cut from 20mm Medium Density Fibreboard. This is the central baffle, which has holes for the two vent tubes and the speakers. The tuned-length ports (above) are made from 55mm ID and 63mm ID plastic pipe. Both ports are 270mm long. August 1997  13 The baffles, ports and speakers are arranged inside the main 300mm tube as shown here. The smaller 55mm ID vent tube is in the foreground. The three baffles are glued one-by-one inside such as Liquid Nails. pair is reduced by 3dB compared to a single speaker design (with a 1W input) but this can be compensated for by careful box design. Note that the two 8Ω drivers are mounted in parallel, which means that the design has a nominal impedance of 4Ω. So what sort of box design have we used? It’s called a 6th Order (A) Bandpass Double Vent design or more precisely, with the speakers mounted tune the chambers. While there are numerous types of bandpass boxes, the 6th order (A) design de­scribed here vents both chambers to the outside but has no con­necting port between the chambers. So why use a bandpass design rather than a conventional bass reflex or sealed enclosure? The answer is that a bandpass design is especially suited to subwoofer applications because both as in Fig.1, a Compound Isobaric 6th Order (A) Bandpass Double Vent design. Now even if you never make it, you can still impress others with your new-found knowledge! A bandpass enclosure basically has two separate chambers, so that each side of the driver works into a separate air volume. The bass frequencies produced result from air movement in the vent or vents that are used to Fig.2 (left): the central baffle requires holes for the paired drivers, the small vent and the large vent. The exact positions of the holes is not critical; just arrange them as shown here. Fig.3: the drivers are wired out of phase so that their cones travel in the same direction. When one cone move forwards, the other moves backwards and vice versa. 14  Silicon Chip the main cylinder using a building adhesive the upper and lower frequency rolloff points can be con­trolled, meaning that it can be used without a cross­over (although we don’t recommend this). The enclosure can also be designed to give a very good low frequency response and the distortion is low. On the list of negatives, a band­pass design has a narrow bandwidth and is generally more complex to build than a conventional enclosure but we reckon we’ve solved that last problem. What’s more, a sub­woofer should only have a narrow response, so lack of bandwidth is not really a problem at all. Fig.1 shows the layout of the Bass Barrel enclosure. A 15-litre volume is used on one side of the drivers and a 5-litre volume on the other side. The larger of the two volumes is tuned to 38Hz via a 55mm-diameter vent which is 270mm long, while the smaller volume is tuned to 75Hz by a 63mm diameter vent, also 270mm long. The ports are cunningly arranged so that they are entire­ly within the enclosure. This is harder to arrange than it sounds, given that each port is longer than the longest dimension of the volume it is venting! The trick is to vent the front chamber through the rear panel and the rear chamber through the front panel. Comparing the predicted performance with both bass reflex and sealed enclosure designs shows the This photo shows the partially completed unit with the first end baffle and the smaller vent tube in place. Quilt wad­ding is used on all exposed surfac­es. The middle baffle must be positioned exactly 250mm down from the end of the main tube. advantage of using a bandpass approach. The predicted -3dB point is 36Hz for the Bass Barrel design, 60Hz for the best ported box design, and 100Hz for the best sealed box. And importantly, that’s with the same input signal level – a point sometimes overlooked when comparing dif­ferent box designs that vary in sensitivity. Incidentally, in a car the Bass Barrel has a predicted -3dB point of 29Hz. Fig.4 shows the normal and in-car predicted response curves. Subsequent testing has shown that the completed subwoofer lives up to its modelling predictions. Gathering the parts The main body of the enclosure is made from 300mm plastic stormwater pipe, while the baffle and end pieces are cut from 20mm medium density August 1997  15 specified ac­cording to its inside dia­ meter (ID), because that’s what’s im­ portant to the design. The 63mm ID pipe is used for waste­water plumbing under sinks, etc, while the 55mm ID variety is pressure water pipe. The 20mm-thick medium-density fibreboard (MDF) is also commonly available from hardware stores, while the quilt wadding is available from cloth supply shops like Spotlight. We covered the Bass Barrel in a stretch automotive carpet called Meltrim® which is available from auto trimmers. Cutting the materials Seal around the edge of the speaker connector before turning over the last baffle and pushing it into place. fibreboard. The vents are made from PVC pipe. Plastic stormwater pipe was chosen for the main housing instead of an all-MDF design because of the difficulty that many home con­structors have cutting straight lines through sheets of fibreboard. However, if you would prefer to build an all-MDF enclosure, see the breakout box. The enclosure was lined with acoustic material in the form of quilt wadding. The 300mm PVC stormwater pipe used is a thick-walled design. It is available from major plumbing supply businesses but although less than half a metre is needed for the enclosure, the minimum that will be available is almost certain to be a metre. Be warned though that some businesses will ask that you buy a full length of six metres and that’s not really a viable proposi­tion! If you do find it impossible to buy the pipe in short lengths, talk to plumbers at major industrial building sites and see if you can scrounge an off-cut. The plastic pipe used for the two vents is commonly avail­ able from hardware stores. Note that it has been Begin the construction by cutting the main 300mm-diameter tube to a length of 370mm, making sure that the ends are square. This done, carefully measure the inside diameter of the tube and use this measurement to mark out the three MDF discs. The three MDF discs can then be cut out using an electric jigsaw. The next step is to cut a hole for the speakers plus holes for each of the two vents in one of the discs – see Fig.2. This disc becomes the central baffle. You will also have to drill a small hole for the speaker leads to pass through. Next, using the centre disc as a template, mark the loca­ tion of the large port on one of the end discs and the location of the small port on the other (ie, each end disc should have only one hole). Cut these holes using a jigsaw and carefully sand them so that the port tubes are a good fit. The two port tubes can then each be cut to a length of 270mm. Final assembly Eight countersunk MDF screws are used to hold each of the internal baffles securely in place. These should be evenly spaced around the circumference of the main tube. 16  Silicon Chip The first step in the assembly is to fit the end baffle with the small hole to one end of the main cylinder. Use Liquid Nails® (or some other similar building adhesive) to bond the disc into place, then fit eight evenly-spaced MDF screws around the outside of the cylinder to further secure the baffle. Countersink the holes so that the screw heads sit flush with the surface of the tube. Next, fit the 55mm ID vent tube into place and seal the gap between the tube and the baffle using Silastic® or a similar sealant material. Note that this sealant should also be applied when ever a vent tube goes through a baffle. You should also glue quilt wadding (or Innerbond material) to all exposed interior surfaces as you The Bass Barrel takes up almost no space at all. A CD is resting on top of the unit to give an idea of the scale. assemble each stage of the Bass Barrel. The next step is to mount the speakers on the centre disc. Unlike conventional speakers which are slipped through the mount­ing hole from the front, the Bass Barrel speakers are mounted face against the baffle. Mark out the mounting holes on one side of the baffle, then drill the holes and bolt the speakers togeth­er from either side of the baffle. Wire the speakers together as shown in Fig.3. Note that the positive terminal on each speaker joins to the negative terminal of the other speaker. When you have finished the wiring, connect a 1.5V battery across the main speaker lead and check that the Fig.4: the predicated free-air and in-car response curves for the Bass Barrel. The blue line shows the in-car response. The design was produced using the Bass Box loudspeaker program. August 1997  17 PARTS LIST 2 Altronics Redback 6.5-inch woofers, Cat. C3086 1 370mm length of 30mm-dia. plastic stormwater pipe 1 600 x 700mm piece of 20mmthick Medium Density Fibreboard (MDF) 1 270mm length of 55mm ID plastic pipe 1 270mm length of 63mm ID plastic pipe 1 930 x 300mm piece of thin quilt wadding 1 1-metre length of heavy-duty figure-8 hook-up wire 32 MDF screws, 20mm long 4 3mm x 40mm-long bolts plus nuts and washers 1 loudspeaker terminal block Miscellaneous Liquid Nails® or similar building adhesive; Silastic® or similar silicone sealant; Meltrim® auto­motive carpet; mesh grilles. speaker cones both move in the same direction. If they head in opposite directions, reverse the connections to one of the speakers. Don’t forget to seal the speaker wire hole through the baffle with Silastic®. Once the speakers are in position, slide the centre baffle down the main We used a cutdown speaker grille and a shortened plastic stormwater fitting to form a grille for each of the ports. Howev­er, there is some port noise with this arrangement, so you may care to leave the grilles off. cylinder and over the small vent tube. Position it so that its upper surface is exactly 250mm down from the top of the tube, then use Liquid Nails® and screws to hold the baffle in place. The large vent can now be installed. Push it down into the central baffle until its end is flush with the end of the main cylinder, then seal the baffle holes. This done, cover the inside of the cylinder with quilt wadding and do the same to the inside of the remaining end baffle. Finally, mount the terminal block on the end baffle, wire it to the speakers and then glue and screw the baffle Special Offer On Subwoofers The Redback 6.5-inch woofers used in this project are available only from Altronics. We have negotiated a special deal with Altron­ics, so that instead of paying $90 for the two speakers you’ll pay only $69 plus $5.50 freight. Phone 1800 999 007 to place your order, or fill in and post or fax the coupon below. Please send me two Redback 6.5-inch woofers (Cat. C3086) at the special price of $69.00 + $5.50 p&p. Enclosed is my cheque/money order for $­74.50 or please debit my: ❏ Bankcard   ❏  Visa Card   ❏ MasterCard Card No. Signature­­­­­­­­­­­­_______________________________  Card expiry date______/______ Name Street __________________________________________________________ PLEASE PRINT __________________________________________________________ Suburb/town_____________________________________ Postcode_________ Send coupon to Altronics, PO Box 8350, Perth Business Cen­tre, WA 6849; or fax (08) 9328 3487 18  Silicon Chip into place. The end of the large vent should sit flush with the outer surface of the end baffle when it is in place. As before, seal the gap between the vent and the baffle. Testing Let the glues and sealants dry properly before trying it out – pre­ferably overnight! To test it, connect the Bass Barrel to an amplifier and a music source but don’t connect any other speakers to the system at this stage. You should not be able to hear very high frequencies (treble), while the bass should be a tight thump, thump. If there are any loud buzzes or whistles, you’ve got air leakage problems and the gaps will have to be tracked down and sealed. As with all subwoofers, the best sound will come if the Bass Barrel is driven by a dedicated amplifier working with a crosso­ver circuit. A suitable “Subwoofer Controller” was described in the December 1995 issue of SILICON CHIP and is available from Altronics as a kit of parts. This design features adjustable cutoff frequency and level controls and even includes automatic power switching for the subwoofer power amplifier. We don’t recommend connecting the Bass Barrel across one of your existing speakers without a crossover network, as this could unduly load the output circuit of the amplifier. Our Bass Barrel was covered in Meltrim®, a stretch automotive carpet. To tidy the ends of the cylinder, Building An All-MDF Enclosure Fig.5: if you don’t want to use 300mm plastic stormwater pipe (or can’t get hold of it), the same basic design can be made entirely from MDF, with 10mm-thick material recommended. This diagram gives the dimensions. Make sure that all the joints are well sealed. we covered the carpet joins with a rubber moulding which is readily available from specialist rubber shops. Alternatively, the countersunk screw holes can be suitably filled, the enclosure sanded smooth (use only very fine sandpaper on the plastic of the main cylinder) and the unit paint­ed. Mesh grilles We used mesh grilles (cut down from a larger grille) over the ports, with the surrounds made from suitably shortened plas­tic stormwater fittings which were painted black. Note, however, that the mesh grilles cause some port noise to occur, so leave these out if very clean bass is required (the noise is only very minor). So there you have it – a compact subwoofer that delivers superb bass, costs very little and is easy to build. It sure changes perceptions that bigger is always better when it comes to a SC subwoofer. Fig.6: the impedance curve for the Bass Barrel subwoofer. The minimum impedance is just under 4Ω which means that the Bass Barrel represents a safe load at all frequencies of interest. August 1997  19