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BY PHIL PROSSER Tapped Horn subwoofer This subwoofer uses just one 8-inch (200mm) driver, yet its response extends below 30Hz and it’s capable of delivering over 100dB SPL! That’s despite a modestly-sized cabinet that’s less than 30cm wide, making it relatively easy to hide. So how does it achieve this? Read on to find out. T his subwoofer is relatively inexpensive to build and not all that hard either, thanks to its clever design. If you already have most of the tools, it will probably end up costing around $200 in total (depending on where you get your hardware). You can get away with using a relatively small amplifier to drive it too, given its high efficiency, although you will need an active bandpass filter (to be described next month). Being a “Tapped horn” subwoofer means that its sole driver is placed inside a horn. This type of subwoofer was made famous by Thomas Danley of Danley Sound Labs. They are often used in sound reinforcement; visit siliconchip.com.au/link/ab9q for a few examples. If you want to see the ultimate manifestation of the tapped horn subwoofer, check out the video at https://youtu.be/ Zbf3bzpgml8 The term “tapped horn” does not sit easily with the engineer in me, as 66 Silicon Chip it is not actually horn-loaded. Instead, it would probably be more accurate to call the alignment a re-entrant resonant pipe. But let’s set semantics aside and use the common name. After reading a few articles on this approach to making a sub, I decided to see how they work. The aim was to present a tapped horn design that fits into a domestic setting, allowing readers to explore this concept in an approachable manner. So, if you have ever wondered about this sort of sub, here is your chance to spend a weekend and find out for yourself how they work! This subwoofer is more than enough for a living room, study or bedroom – it has been kept to a modest scale and cost. The design presented has been simplified to avoid odd cut angles, and I have taken out non-essential corner fillets to keep the assembly as simple as possible. I have even sized the box so that you can use standard sheets of MDF with minimal cuts. Australia’s electronics magazine In loudspeaker design, the designer needs to juggle several parameters, notably: the size of the box, how loud it will go (SPL), its low- and highfrequency extension (bandwidth), and its efficiency (how much power it takes to drive to a particular sound level). A tapped horn can push the efficiency, low frequency extension and SPL well beyond that offered by a conventional sealed or vented enclosure. It achieves this by placing the driver inside the acoustic path and folding that path around, so that the output from the back of the driver adds to the output of the front of the driver. But there ain’t no such thing as a free lunch, so you pay the price in complexity. As shown in Fig.1, one side of a loudspeaker drives the horn close to its end, and the other side of a loudspeaker drives it close to its output. If the two drivers are fed with the same signal, they deliver out-of-phase siliconchip.com.au signals into the horn since they face opposite directions. This gives the simulated response shown in Fig.2; note the extended bass response. But the same driver can’t exist in two different places, so to get the driver to fire into both the front and back ends of the horn, the enclosure is folded over on itself – see Fig.3. This single-fold design is still really long and not that convenient. It is possible to fold these up further in several ways. The configuration we have chosen is shown in Fig.4. Ideally, it would be made from conically expanding sections, but those are really fiddly to cut. You will note that we have cheated on this and made the sections straight. Our tests show that the impact is not enough to worry about. Remember that a conventional sealed enclosure is there to absorb the rearward output from a driver. By juggling the length and area of the path from the back of the driver to the mouth, we achieve constructive interference of the sound over a set bandwidth. This increases the efficiency and allows us to push the low-frequency extension further down. Of course, this comes with compromises. A tapped horn only works over a limited bandwidth, after which the output becomes a series of peaks and dips. Therefore, we need to set the crossover frequency low enough to cut out all the unwanted frequencies. Also, below the low-frequency cutoff, cone excursion becomes uncontrolled, similar to a vented enclosure. The solution is to drive the subwoofer with an active crossover that filters out high frequencies and provides a subsonic filter to remove unwanted low frequencies. Every professional sound system includes subsonic filtering for their subs. This protects the drivers from over-excursion and avoids the amplifiers wasting power by driving the speakers with signals they cannot generate. This article presents only the subwoofer. It should be driven with a signal that’s been through a 20Hz subsonic filter (high-pass) of 24dB/octave and a low-pass filter of -24dB/octave with a -3dB point of 80Hz. We will present an active crossover design to provide this next month. Still, you can probably drive it from the subwoofer output on many home siliconchip.com.au Fig.1: the basic concept of a tapped horn subwoofer. The two drivers are supplied with the same signal. As they are mounted rotated 180° compared to each other, the signals they generate within the horn are out-of-phase. But it takes time for sound to travel down the horn, so over a certain range of frequencies, the sound reaching the outer driver is in-phase, resulting in constructive interference and reinforcement. Fig.2: the simulated response of a folded horn. It gives a nice broad plateau over the range from just below 30Hz up to about 100Hz plus a series of peaks and troughs at higher frequencies, as the sound waves constructively or destructively interfere depending on the specific frequency. So we need a lowpass filter to eliminate signals above 100Hz for it to sound good. Fig.3: this rearrangement of the tapped horn shown in Fig.1 is more practical to build since it is both shorter and uses just one driver instead of two, but it achieves the same result. Fig.4: more folding of the horn (and a bit of creativity regarding how it tapers) allows us to create an even more compact enclosure without sacrificing much in the way of performance. Australia’s electronics magazine September 2021  67 9 00 TOP 884 EXIT OF HORN THIS SOUND PATH IS ABOUT 2.54m LONG 153 762 41 6 PANEL C – STEP #3 315 649 FRONT - STEP #1 PANEL B – STEP #2 20 2 635 PANEL A – STEP #6 500 468 200 BACK - STEP #8 START OF HORN PANEL D – STEP #5 PANEL E – STEP #4 18 4 72 346 Fig.5: this diagram shows the order in which we suggest you attach the internal panels to the side and show the two acoustic paths as dashed lines. It also includes most of the important dimensions, so you can check that you’re building it right, but as you’re unlikely to cut the panels to exactly the right sizes, don’t expect a perfect match. Also note that the top and bottom panels sit above and below the side panel, not on it. 868 BOTTOM - STEP #7 theatre systems, noting that these rarely include a subsonic filter. Design This subwoofer was designed using a program called “Hornresp”, written by David McBean. This is freely available from www.hornresp.net and supported on several DIY Audio forums. It would be fair to say that this program is not super-easy to use, but it does allow us to model what various lengths and diameters of horn sections will do. If you try this program out, we recommend using the “Loudspeaker wizard” via the Tools menu. This lets you change the lengths and diameters of each horn section while watching the power response of the horn. The horn we present juggles the following requirements: • A -3dB point below 30Hz. • A passband ripple of no more than 4dB; in the real world, rooms have all sorts of resonances. • Using a readily available, lowcost driver. • Material able to be transported in a small car; say, a VW Golf. • Only small sheets of material required to make the enclosure, ideally with minimum cuts. • An enclosure that can be hidden under a desk or behind a couch. For the driver, the Altronics C3088 is a good balance of size, power handling and cost while providing pretty decent cone excursion compared to its peers. Cone excursion is really important in subwoofers and is often overlooked. At a given SPL, the lower you want to go in frequency demands rapidly increasing cone excursion. 68 Silicon Chip Consideration of this is essential in designing a sub. Ultimately, a driver with a ‘good Xmax’ is essential. The C3088 has a 4.5mm voice coil overhang, and in our tests, more than 5mm effective Xmax, which is pretty good. Folding the horn as shown in Fig.4, to achieve the above, we need the following: • 200mm from the start of the horn to the back of the driver. • 2.54m from the back of the driver to the front. • ~420mm (416mm actual) from the front of the driver to the exit of the horn. This defines our overall enclosure as having the following dimensions: • Internal width (z-axis in Fig.5): 250mm, external 282mm • Internal depth: 868mm, external 900mm • External height: 500mm Performance The resulting tapped horn subwoofer is shown in Fig.5. Measuring the performance of subwoofers is much harder than full-range speakers due to reflections and resonances in the room. I made the measurements shown in Fig.6 at one metre, but not in the corner of a room. Placing the subwoofer facing the corner of a room, with about 20cm between the sub and the walls, will give better performance (ie, more bass!). The sound level is shown by the black line (axis in dB on the left), while the fainter line is the phase (axis in degrees on the right). Note the peak in Fig.6: the measured response of the prototype subwoofer without the bandpass filter in place. The dark line is the amplitude, while the lighter dashed grey line is the phase. This agrees pretty well with the simulation, although the response actually extends to over 200Hz before the severe peaks and dips start to appear. Australia’s electronics magazine siliconchip.com.au The subwoofer was tested in my workshop setting as shown here, and in a spacious church hall shown adjacent. the response at 200Hz. You really need a crossover that provides a minimum of 18dB attenuation by this point, or you will be able to hear the resonance of the tapped horn. The response is somewhat smoother than predicted but does present the predicted ripple above 100Hz, the peak at 200Hz and a deep dip at about 250Hz. There is no question that this subwoofer needs a steep crossover. I carried out further tests in my workshop, a 60m2 converted shed, where this sub generated very solid bass and rattled the tin exterior (see above). It integrated very neatly with some small monitor speakers using five-inch Vifa bass-mid drivers. I set the tapped horn to main speaker crossover at 80Hz, and I applied no attenuation to either the sub or midrange. The next test was to challenge the sub. After painting, I took it to a rather large church hall and integrated it with some old but extremely efficient 10-inch bass-mid driver based speakers. These have an efficiency well above 90dB at 1W & 1m. I kept the crossover at 80Hz but turned up the sub quite a lot to match the level of the bass-mids. In this 110m2 metre hall (shown at upper right), which is 10 metres tall, the tapped horn made a good showing of itself. While you would not run a disco with it, it handled pop and blues music to ‘enthusiastic’, but short of ‘extreme’, levels. The author does, however, have a fairly high tolerance for noise. Being in a church, I tried some very sub laden Gregorian chant music, and Parts List – Tapped Horn Subwoofer 1 Altronics C3088 8-inch 70W woofer [or Wagner SB20PFC30-8] 3 1200 x 900mm 16mm MDF sheets 134 50mm-long 8-10G countersunk wood screws (get a box of 250) 8 16mm 8G screws (for mounting the driver) 1 1m length of 10mm-wide adhesive-backed foam tape (can be cut from a wider strip). 1 pair of speaker terminals (we used a Speakon connector, but you can use any type) 1 1m length of speaker cable (twin-lead, 17AWG) [Altronics W1936] 1 bottle of PVA glue, at least 200mL 1 tub of “builder’s bog”, at least 200mL 1 can of primer paint suitable for timber 1 square of 120 grit sandpaper (buy more than one so you have spares) 1 square of 240 grit sandpaper (buy more than one so you have spares) 1 litre of DuraTex textured paint (bed liner paint would probably work too) [www.cannonsound.com] 1 tube of acrylic gap filler (in case you have unexpected gaps in your joints) 4 feet (we used four 38mm Surface Gard Round Side Glide feet from Bunnings) siliconchip.com.au Australia’s electronics magazine was quite impressed at being able to feel the bass. Construction See the parts list to see what material you need to buy. You will also need the following tools: • A simple hand-held circular saw; you do not need a fancy table saw. Alternatively, get your local hardware store person to make the long cuts and use a hand saw for the remaining, shorter cuts. • A hand-held drill with 3mm and 4mm drill bits, a countersinking bit and a Philips-head screwdriver bit. • A long metal ruler or straight edge. • Either G-clamps or sash clamps, to hold the MDF while cutting. • A router with a 12mm radius bit, for finishing the edges. • A 10mm diameter, 100mm-long nap roller. • A spatula and scraper, for mixing and applying filler over the screw holes. • A tub of water and a dishcloth, to clean up glue spills and the excess squeezed from joints. Cutting the sheets We have laid the panels out on three sheets of timber that you can transport in a VW Golf or larger, per the earlier requirements. Review the drawings (Figs.7-9) before you start cutting. The majority of pieces needed are either 250mm or 282mm wide. After making these main cuts, you can cut the sides from the offcuts, plus a series of lengths from these 250mm and 282mm wide panels. September 2021  69 Figs.7-9: here are the panels that need to be cut from the three 1200 x 900mm sheets. You might be able to cut them all from a single 1200 x 2400mm sheet if you have a way to transport it (or get it delivered), although we haven’t verified that. It’s also a bit easier to work with smaller sheets. Even better, get the hardware store to make the initial cuts for you, yielding three 292mm wide strips, three 250mm wide strips and two 468mm wide strips. You will then just need to make a few extra cuts to get all the pieces you need. Measure carefully and double-check that the side panels are not too tall or deep, as an error in this dimension will result in an overhang of the top or rear panels. Some hints: • Check that all parts are within ±2mm, although you will need to do better than this for ‘living room furniture’. • Mark the hole locations (see photo below). Use a pencil to mark the panels on the inside. Do not be afraid to measure and mark liberally, as once the box is assembled, these will be hidden. • Take your time and check that all the markings are in the right place. Once you are assembling this speaker, it will be a real nuisance if you need to move things! • There are many screw holes through the side panels. Make sure these are marked within 2mm or so. These measurements are essential to the screws going into the internal panels. • Panel C has the speaker driver cut-out, which you should make after the panel has been cut but before the cabinet is assembled. Use a compass to mark the hole in pencil, then use a jigsaw to cut it out. If you’re lucky enough to have a suitable hole saw, that’s even better. You can use a small handsaw if you don’t have either, although it does take a little perseverance! Now make any assembly markings you feel will help you get the panels aligned. Refer to the photos; placing “V” marks will assist you in getting the panels in the proper alignment. The screw holes define the centres along which of the 16mm-thick panels will be attached, so the edges of these To figure out where the panels are going to lie and where to drill holes, you will need to temporarily arrange the cut panels as shown in Fig.5, then use a pencil or other marker to trace their outlines. You can then use a ruler to draw lines down the centre of each panel location and the locations to drill holes will be along these lines. You can see from my photos how I did this (although I didn't mark the panel edges, only the centres, as I have the experience to do that). 70 Silicon Chip Australia’s electronics magazine siliconchip.com.au panels will be 8mm on either side of the row of holes. Once the screw holes have been marked, mark the panel edge locations and add Vs on either side of the panel lines so that you can see how well centred the panel is along the screw hole line when you are installing the panels. Once everything checks out, drill 4mm diameter holes for the screws. Drill from the inside. There will be some chipping of the MDF where the drill exits, but this will be dealt with in the next step. Then countersink all holes from the outside so that the panels are neat and tidy. Countersink the holes deep enough that the screw heads will sit flush with the panels (as shown below). Assembly Refer now to Fig.5 for the order in which you should attach the panels. We’ll go through these steps one at a time. Step 1 is to attach the front panel that sits in the cut-out in the corner of the side panel. If necessary, file the cut-out on the side panel so that the front panel is well-aligned at the top edge of the side panel. Take time to get this right, as all the following panels align to this. Check that the markups on the insides of the front and side panels line up well, then put a modest amount of glue on the joint. The 3mm drill bit is for pre-drilling holes into the sides of the MDF panels where the screws will enter. This is important to keep the panels from splitting. When you have everything aligned, pre-drill one hole (3mm) to a minimum depth of 50mm into the side of the panel, then put that screw in. siliconchip.com.au Australia’s electronics magazine September 2021  71 1 2 When gluing the panels together, you will want to make sure to use a set of clamps and/or weights while it sets. Titebond wood glue is quite good for these types of jobs. Note that these panels are also kept together via screws and not just glue. Take the opportunity now to nudge the panel so that it is straight and well-aligned. Do this before you predrill the remaining holes. Be sure you are happy, as everything that follows hangs off this panel! Once you are satisfied everything is good, pre-drill the remaining holes and then screw the panels together. Steps 2 & 3 are internal baffles B & C. Run glue along the bottom and front edges of panels 2 and 3, but do so one at a time. Push the panel into alignment and use the marks you made to get everything aligned. The V-marks will help you get each panel square over the drill holes. While pushing the panel in place, pre-drill then screw the bottom hole in the front panel (from step 1). Note that by starting with a screw in the bottom hole first, you will pull Panels B and C tight into the front panel with a minimum of error. 3b Continue pre-drilling and screwing all the remaining holes. Clean up any glue that has seeped out of the joints. Step 4 is to fit internal baffle E. Push it down between Panels B and C. This will be tight. Try to get some glue in there, but assuming you have a good fit, this should not be critical. If you have a gap here and there, run a bead of acrylic filler over the gap(s). Pre-drill and screw this in place from both panels B and C, and through the side panel. Step 5 is to fit internal baffle D. Line up panel D with panel C. Again, use those V marks on the panel to get the panel A end of panel D in the right spot. The trick here is to get a good alignment at the corner of panels C and D. Start again with the screw at the bottom of the junction of Panels C and D. Once it is in place, pre-drill and screw in all screws, checking alignments as you go. 5 For step 6, fit panel A similarly to panel D. Steps 7 & 8 are to fit the top and rear panels. Start with the top panel, ensuring a clean edge is presented at the juncture of the front and the top panel. Get this clean and screw along the front and side panel. Pre-drill and screw all screws for this panel. Then screw the rear panel on with two screws only – don’t glue it yet. Check the fit of the bottom panel, trying to get good alignment with the rear panel and front edge of the side panel. Jiggle this around to get the best fit you can. If there is a slight misalignment, it’s much better for it to turn up now. Remember that before painting, you will be filling and sanding – so minor indiscretions will disappear. If you need to slightly shift the rear panel, remove the two screws and predrill new holes to fix this panel where you want it. Once you are sure it is OK, 6 This is a close-up view of the panel in Step 3 showing the gap between Panel B & C. 72 Silicon Chip Australia’s electronics magazine siliconchip.com.au 3a 4 Once the glue has been applied and the joints clamped, they should be left clamped for at least one hour, then left to cure for approximately a day. pre-drill, glue and screw the remaining holes in the rear panel. Do not drill, glue or screw the bottom panel yet! Step 9 is to fit the other side panel. I used acrylic filler rather than PVA to glue the side panel on, but this is not essential, especially if you cut your panels accurately. After applying the adhesive, slide the side panel into place, then drop it onto the internal baffles. Push the side panel in place so that there is a flush fit along the top panel, then pre-drill and screw along this edge. Next, push the front and rear edges of the side panel to get good alignment with the front and rear panels, and again, pre-drill and screw. Now pre-drill and screw all the holes on the side panel. If your measurements were good, all the screws will go into the internal baffles. If the drill falls through the holes and misses the internal baffle, drill at an angle 7 siliconchip.com.au that does catch the internal baffle (this should not be necessary!). Mounting the driver The driver needs to be mounted before the bottom panel is installed. With everything in place, jiggle the C3088 speaker to ensure that it sits neatly in the hole you have cut. If the hole is a touch undersized, the speaker will not sit snugly. If that is the case, now is the time to fix it! Carpenters may shake their finger at us, but you can use a rasp to enlarge the hole slightly, given this is hidden inside. Then stick foam tape around the edge of the driver hole. This will ensure that a good seal is achieved between the driver and Panel C. Then install speaker wire as shown in the photo overleaf, ensuring there is sufficient length to pull through the driver hole and solder to the driver. Make sure you can identify the “+” wire to the driver as this needs to connect to the “+” pin of the speaker connector. Run the speaker wire through to the speaker connector. We used a Speakon connector as many of our speakers use these, although you might prefer to use banana sockets and/or binding posts on your sub. We drilled a 25mm hole on the rear panel to mount the connectors we used. We haven’t shown a location or size for this hole on the cutting diagrams because its size and shape will depend on your connector, and it can go pretty much anywhere you like on the rear panel. It will probably look best if it’s somewhere along the vertical centreline, though. Now seat the speaker in the hole and mount it using 16mm 8G screws that do not pull through the hole in the speaker frame (ie, with large enough heads, or washers if necessary). Predrill the holes to 2mm, then insert the 8 Australia’s electronics magazine September 2021  73 9 Make sure to seal the speaker wires when finished. eight screws. Progressively tighten screws on opposite sides of the driver until all are tight. Do not overtighten these as the foam tape will ensure a good seal. With the driver in place, attach the bottom panel. Do not glue it; simply screw it down with the generous number of fixings. This will allow you to access the driver later if it needs to be replaced. Finishing the box I routed all external edges with a 12mm radius bit. If you do not have a router, that doesn’t matter. Use 80 and then 120 grit sandpaper to round the edges until they look and feel smooth. I then used “builder’s bog” to fill all the countersunk screw holes. Once this dried, I sanded those areas and then applied a second coat of bog to get those areas really smooth. Do not fill the holes in the bottom panel, though! You need to be able to remove it. Once I was satisfied that the enclosure was smooth enough and all screw holes – except those in the bottom panel – were now flush with the MDF, I coated the box in DuraTex. I first applied a thin coat, then after one hour, a second, thicker coat using a 10mm nap roller. DuraTex is a textured paint sold for use on professional speakers. It is tough and textured so that it takes life’s bumps without showing too much. It also helps to hide any imperfections in our work. Finally, I screwed on the feet and the sub was complete. As promised earlier, next month I’ll describe an active crossover that’s perfect for use with this subwoofer (or any two-way SC or three-way speaker system). A router makes finishing the edges much easier, but it can also be done with sandpaper. Any gaps and cracks can be filled by using a mix of wood glue and sawdust, or wood filler. The finished subwoofer had primer applied and was then painted black. You could also just apply a lacquer or polish depending on how you want it to look. 74 Silicon Chip Australia’s electronics magazine siliconchip.com.au