Fullscreen HTML5Med Res (??MB)HTML4

By JULIAN EDGAR Little Dynamite Subwoofer Build your own compact subwoofer – ideal for use in a car or home unit S UBWOOFERS ARE NOW almost universal in any car system that attempts to produce quality music. They’re also prevalent in home theatre systems and also make a great upgrade for a compact sound system that needs a bit more punch. The subwoofer described here is dead-easy to make and gives excellent perfor­mance, considering its compact size and low cost. And if you think that it’s really a 68  Silicon Chip bit too limited in power handling and low-down frequency response, stay tuned – we’ve got another more complex, higher-power design coming soon. But honestly, unless you want to make a social statement to sidewalk pedestrians or your neighbours, this subwoofer will be more than adequate for most music! The design We’d been considering doing a sub- woofer project for some time – and in fact went as far as designing a bandpass model using a 10-inch driver. However, by the time we’d priced the particle board and added in carpet, glue, terminals and a grille, the cost was around $130 for the enclosure alone. What’s more, it would have taken quite some effort to build. It was about this time that Jaycar Electronics released a range of sub-woofer enclosures, all pre-built www.siliconchip.com.au Main Features • • • • • Small enclosure 125 watts power handling Excellent in-car response Competent in-room response Easy to build & low cost and finished with carpet and loudspeaker terminals. Their boxes start at just $59.50, so we made a radical change to our plans. The bandpass design was scrapped and we set about designing a traditional ported design instead. A ported enclosure is one of the oldest box types around (see “Ports Enclosure Size & Other Design Considerations”). However, the design of this type of enclosure has been revolutionised in recent times with the devel­opment of loudspeaker design computer software. Rather than building a box and then laboriously testing it in many different configurations, computer software now allows the virtual con­struction of dozens of different enclosures. In addition, good subwoofer design software (like the Bass­ Box package that we used) makes various suggestions during the design process. However, if you want to aim for an optimal trade-off in terms of size, response, sensitivity and power handling, such a design process can still take many hours to complete. The first step in the design can be either the selection of the driver or the enclosure. In our case, we did a The driver used in the Little Dynamite subwoofer is 10 inches (25.4cm) in diameter, is rated at 125W RMS and uses a voice coil 50mm in diameter. It costs $99 and is available from Jaycar. bit of both. First, the enclosure – the new pre-built subwoofer boxes come in 25, 35 and 45-litre nominal interior volumes (we’ve written “nominal” for good reasons which we’ll come back to later) and the hole for the driver is already cut. This means that if you want to use a 10-inch driver, then the 25-litre box is the only one to go for (the 35-litre box is precut for a 12-inch driver, while the 45-litre box is cut for a 15-inch driver). Note that the boxes are available in both sealed and ported designs. We leaned towards a 10-inch driver as in general terms they’re cheaper than larger sizes and can usually be fitted into a smaller box, which again has a cost advantage. Choosing the subwoofer So with the 25-litre box the chosen BassBox: Loudspeaker Design Software The BassBox software package is available in two forms – BassBox Lite and the BassBox Pro professional version. The Lite version is quite sufficient for any speaker design work that most people will want to carry out and is the package that was used to design this subwoofer. In addition to being able to model the performance of bass-reflex (ported) speaker enclosure designs, this software can also model sealed and passive radiator boxes and a range of bandpass designs. The Lite version comprises just a single CD www.siliconchip.com.au and the Help and background explanations are all available on-screen. In fact, this information is very good indeed – with some study, there’s enough information for even a beginner to start designing sophisticated speaker enclosures. And if you have a background in audio, the flexibility and detail of the program will be even more useful. BassBox Lite is available in Australia for $165 from ME Technologies at www.me-au.com and also from Harris Technologies at http://www. ht-audio.com March 2003  69 The “step” in the inner surface of the port that would otherwise be formed between the flared ports and the plastic pipe is smoothed away using a half-round file and sandpaper. The port is formed from two flared speaker vents (available from Jaycar) joined by a length of 65mm-diameter plastic pipe. we’d like is sensitivity – with a sound pressure level (SPL) of 88.2dB at 1 Watt 1 metre, it needs to have both an enclosure design which is efficient and a power amplifier with at least 100 watts RMS behind it. Port size The completed port – the two flared ends reduce turbulence so that there are no problems with port noise. Make sure that you don’t glue the ends on at this stage, though – otherwise you’ll never get the assembly into the box! enclosure, which speak­er would be suitable for it? Keeping it in the Jaycar family, we opted for the CS-2274, a 10-inch “titanium” finish driver with 125W RMS power handling, an X-max (maximum cone movement) of 9mm, a voice coil diameter of 50mm and a 70  Silicon Chip resonant frequency of 33Hz. What these specs add up to is a competent driver that should be able to produce good bass in the right enclosure with­out breaking the bank (the driver retails for just $99). The only spec that isn’t quite as good as While free speaker design software is available on the web, we’ve not seen any program that comes close to BassBox – so it makes sense to pay the necessary dollars if you want this kind of package. One area that we were very conscious of when software modelling the design was the behaviour of the air within the port. Typically, ports are made too small in diameter – it’s easy that way because then they can be made shorter for the same box tuned frequency. However, small ports give rise to high air velocities, which in turn causes port noise or “chuffing”. Bass-Box can pre­dict maximum port velocities and recommend minimum port diameters to eliminate this problem. However, after looking at the modelled behaviour of a system using the prebuilt ported enclosures (the 25-litre box comes with one 50mm ID port), we decided to buy a sealed box and then cut a hole for a port of our own making. So what does the final design look like? First, the Jaycar “25-litre” box is actually 23 litres – an important difference when it comes to the design. Second, the volume taken up by both the driver and the port need to be subtracted from the available internal volume (BassBox does both of these things automatically if the right data is entered). In addition, the effect of www.siliconchip.com.au Ports, Enclosure Size & Other Design Considerations A speaker’s role in life is to create sound waves and it does this by moving its cone back and forth in response to elec­trical signals. When it pushes forward, higher air pressure is created in front of the cone; conversely, when it moves back, lower air pressure is the result. If a bare woofer is sat on the bench and driven by a bass signal, the pressure waves find their way around the edge of the frame and partially cancel each other out. A fundamental task of a loudspeaker enclosure is to prevent this wave cancellation and its adverse effect on bass response. A sealed enclosure, for example, dissipates the energy from the back of the cone into the box (and/or the box filling). However, because these pressure waves are being wasted, the efficiency of a sealed enclosure is not as high as for a ported enclosure. In a ported enclosure, radiation from the back of the cone is used to reinforce the pressure waves being generated by the front of the cone. This is achieved by using a tunedlength port or vent, which has the effect of altering the phase of the waves emanating from the rear of the speaker. Put your hand near the port used in this type of design and you’ll notice that as the woofer cone moves forward, so does the air in the port – the pressure waves from the back of the cone are now reinforcing those coming from the front. However, if the port isn’t just the right length and dia­ meter, the response of the speaker can be all “wrong”. For exam­ple, in this design, we used a port 63mm in diameter and 360mm long. But what if the port had been only 150mm long? the acrylic speaker fill also needs to be taken into account (confus­ ingly, the fill expands the volume that the driver sees). In the end, we had 19.5 litres available and chose to use a port 63mm in diameter and 360mm long. This gives a box tuned frequency of 33Hz. Note that the tuned frequency in a ported design is often close to the speaker’s resonant frequency (in this case, it is identical). www.siliconchip.com.au Fig.1: the red curve show the in-car response of the subwoofer using a 360mm-long port, while the yellow curve plots the predicted response if the port is reduced to just 150mm long. Fig.2: the green curve here shows the effect of dropping the enclosure volume to 12 litres and using a 75mm-diameter port that’s just 100mm long. Fig.1 shows the changed response – the red line is the in-car performance of the subwoofer as described in this article, while the yellow line shows the predicted response with the shorter port. As shown, with the shorter port, the bass falls away more quickly and is also “peakier”. (Actually, the selected driver is quite tolerant of design changes – with some speakers, the results of such a port mismatch would be much worse!) The speaker enclosure volume is Modelled in a car environment, the response is effectively strong down to 15Hz, while in a room the -3dB point is at 38Hz. The modelled maximum port velocity is only 23 metres/second at 125W input power and 22Hz which – not coincidentally – are also the exact conditions where the cone reaches its maximum excur­sion. To avoid any possibility of port noise in this long vent, both ends of the vent are flared. This is achieved by using also a vital part of the design. In the case of the response curve indicated by the green line in Fig.2, we’ve dropped the enclosure volume to 12 litres and used a short, fat port – 75mm in diameter and 100mm long. The re­sulting in-car response is very peaky at 100Hz. As a result, this type of enclosure design that would give “one-note” bass, a problem often found in ported loudspeakers enclosures that are not well de­signed. two Jaycar CX-2688 flared speaker vents, joined with 65mm-dia­ meter plastic pipe. The two vents slip tightly into the pipe, allowing the easy construction of a double-ended flared port. Building It If you have available an electric jigsaw, some hand tools and a tube of general purpose building adhesive (eg, water clean-up Liquid Nails), building the complete subwoofer will March 2003  71 Parts List 1 10-inch Response Subwoofer; Jaycar Cat. CS-2274 1 25-litre sealed subwoofer enclosure; Jaycar Cat. CS-2520 1 acrylic speaker damping material; Jaycar Cat. AX-3690 1 10-inch protective grille; Jaycar Cat.AX-3522 2 flared speaker ports; Jaycar Cat. CX-2688 1 500mm (approx.) length 65mm-dia. plastic pipe 1 short length heavy-duty speaker wire 1 tube building adhesive (eg, Liquid Nails) 8 speaker attachment screws The hole for the port is marked on one side of the enclosure, to­wards the back of the woofer. This 85mm-diameter food tin was an ideal size for marking out the hole which was the cut out using a jigsaw. take you nearly no time at all – in fact an hour, tops! The first step is to remove the sharp inside edge of the flared speaker vents; ie, at the end opposite the flare. This is done to eliminate any sharp steps between the flared vents and the plastic pipe when they are joined together. You can use a round or half-round file for this job, finishing off with some fine sandpaper. If you don’t have any sandpaper handy, a stainless steel kitchen scourer works quite well on the soft plastic. With these edges smoothed, cut the plastic pipe to the right length so that when both flared ends are pushed firmly into it, the total port length is 360mm. Don’t be tempted to glue both flared ports to the plastic pipe at this stage – you still need to fit the port into the enclosure! Once the port has been assembled, spray some black paint inside it to cov- Drill a hole just inside the marked area for the port cutout to allow the insertion of the electric jigsaw blade. 72  Silicon Chip er any scratches that you have made and to hide the white plastic. The next step is to cut the hole for it in the side of the speaker box. An 85mm hole is ideal – we drew the cutting line with the help of a can of food that conveniently had the right diameter. The port must be placed at the magnet end of the speak­er to give sufficient clearance, with the hole cut in an end wall. Before marking the hole, consider the placement of the terminal strip and how you intend positioning the box in its final home, as these might have a bearing on where you want the port to be. The hole should be cut so that the edge of the flare ends up about 20mm in from the edges of the box. This causes the other end of the port to sit with its flared outer edge against the inner walls of the box, allowing it to be further held in place with some dobs of adhesive. For environments where the subwoofer will be subjected to lots of shaking, use an additional internal fastening to hold the port tube in place – for example a bracket made from aluminium strip wrapped around the port and then at­tached to the inner panel of the box with short woodscrews. The flared port can now be assembled within the box. If you push the ends on after you’ve applied a smear of adhesive to the pipe, they will be held in place firmly. The flared end that sits flush on the surface of the box needs to be thoroughly sealed from behind using Liquid Nails or a www.siliconchip.com.au silicone sealant. Make sure that you give the sealant time to set! Next, the acrylic speaker damping material can be cut to size and stuck to the inner walls of the box. We suggest 350 grams/square metre material (Jaycar AX-3690) but any similar material is fine – eg, acrylic quilt wadding. Be careful that you don’t block the port – in fact it is wise to be quite sparing in your use of the material around the port entrance. Next solder some heavy duty speaker cable to the box termi­nals and attach the other ends to the screw terminals on the loudspeaker. Make sure that the positive terminal on the box is con­ nected to the positive terminal of the loudspeaker and similarly for the negative terminals! The woofer can then be slipped into its precut hole and the positions marked for its mounting screws. That done, remove the speaker and drill small diameter pilot holes for the screws. Clean away any shavings, then rein­stall the driver and fasten it into place using eight coarse-thread MDF screws. Here’s an important note: the carpet will compress as you tighten the screws, so go right around the speak­er at least three times, tightening them each time. You’ll note that we’ve left off the protective grille at this stage. Instead, it’s time to do some testing. A jigsaw will make quick work of the port hole. The rim of the flared port covers the edge of the hole so don’t worry if you don’t cut a perfect circle. Testing two reasons for this buzz. First, at high output levels, the interior of the car was getting excited – ie, bits of trim inside the car were resonating. And second, we were driving the 150 watts/ channel ampli­fier so hard that it was going into distortion. The latter is pretty important to avoid – while this is a rugged speaker with cone movement well controlled down to about 22Hz, start pumping up the SPL and you might find that The first step is easy – apply a 1.5V battery across the terminals and make sure that when the positive and negative battery terminals are applied to the positive and negative sub­woofer terminals respectively, the woofer cone moves forward. If it moves backwards, open up the enclosure and swap the wiring connections at the speaker! The next step is to connect the subwoofer to an amplifier. Begin by driving the unit quite gently. Moisten a finger and move it around the edge of the driver, to check for any air leaks past the frame. Now do the same around the edge of the port – there will be air movement within the port itself but there shouldn’t be any around the edge of the flare. Next, listen carefully for buzzes, rattles and whistles. During the prototype’s development, we had a buzz that we chased and chased and chased. It turned out that there were The flared speaker port inserted through the hole. The striped background is the rug on which the box is sitting while the work is being done – unlike an untrimmed box, this one needs to have its finish protected during the build process. www.siliconchip.com.au you’re working the amplifier harder than you thought. And remember: more speakers get destroyed by pushing the amplifier into distortion than ever get damaged by exceeding the speaker’s limits! If everything seems fine at low volume, wind up the wick a bit more. Naturally, during this test procedure, you should disconnect all the other speakers, so that you’re just listening to the subwoofer. It won’t sound won- March 2003  73 This is the inside view – the plastic pipe is slid over the external flared port after which the second flared port is added at the other end. derful (no subwoofer does on its own) but it will allow you to easily identify any problems. As stated earlier, because of its relatively low sensitivi­ty, this subwoofer isn’t going to deliver ear-splitting SPLs from modest amplifiers. However, if you have 100-125W available, it should be capable of delivering lots of low-end bass. If all is well, you can now reconnect the other speakers and then balance the system for the bass response that you want. Finishing Acrylic filling is glued to the walls of the enclosure, making sure that the port entrance remains clear. The port is held in place by sealant/glue applied at each end. If the subwoofer is going to be subjected to lots of movement, add an extra bracket to secure the port tube in place. 74  Silicon Chip The final step is to attach the protective speaker grille. To do this, you’ll have to take the driver out again so that you can place the “T” nuts on the back of the baffle. While the speaker is out, check that the port is still rigidly held in place by its glue and/or brackets and that none of the acrylic speaker filling has moved around. You should especially check that the port entrance has remained clear. It’s then just a matter of completing the assembly and enjoying the fruits SC of your labours! www.siliconchip.com.au