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Foldback speakers are essential to any live performance in a large venue when high power amplification is employed. They enable each musician to hear his or her own playing, over and above the general sound level. Build these and save a bundle of dollars compared with commercial units. By JOHN CLARKE FOLDBACK LOUDSPEAKER BUILD A T HIS VERSATILE FOLDBACK loudspeaker is suitable for stage musicians, vocalists, entertainers and performers. It uses readi­ ly available loudspeaker drivers and can be built using quite basic hand and power tools. Foldback loudspeakers, commonly called “wedges” because of their shape, are used by musicians so that they can hear them­selves over and above the general noise level in a performance venue. Often there are several foldback loudspeakers on stage, for example, one for the keyboard player, one for the vocalists and one or two for guitarists (bass and lead). Without foldback loudspeakers, the performers would have to rely on the sound projected to the audience via 72  Silicon Chip conventional “front of house” loudspeakers. However, they may find it difficult to hear from their location behind the speakers and there can also be a considerable delay before the sound reaches them. This delay will cause the performance to become slow and very deliberate as the performers attempt to sing or play and then wait to listen. With foldback speakers, the performance can be kept tight and lively. Foldback loudspeakers are designed with quite different criteria compared to conventional types and as a consequence they look and sound different. While conventional loudspeakers pro­ject the sound away from the performers and toward the audience, a foldback loudspeaker does the opposite and projects the sound toward the performers themselves; this provides the perfect recipe for acoustic feedback. To avoid this problem, foldback loud­ speakers are designed with a sloping front baffle to project the sound directly toward the performers’ ears. This means that they beam the sound to the rear of the microphones which are usually designed to have a minimum pickup from the rear, so as to mini­mise feedback. The design described here enables the loudspeaker baffle to be set to 35° or 55° to the horizontal, depending on how the box is placed on the floor. This will suit either close-up use (35°) or more distant listening (55°). Sloping the baffle is done for two Two 200mm “Redback” woofers (one shown) and a single Motorola KSN-1141A piezoelectric horn are used in the Foldback Speaker. The woofers are protected by steel mesh grilles. other reasons. First, the sound is beamed at the listener so that they receive the brighter “on-axis” sound. Second, it reduces floor reflections back up to the microphones where their rejection of sound is far less than directly from the rear. Angling the baffle does not solve all acoustic feedback problems though. Feedback can still occur when the microphones are spaced out on the stage, producing the potential for a micro­ phone to receive off-axis sound from an adjacent foldback loud­ speaker. However, this new foldback design has lobes and nulls in its off-axis response and by placing adjacent microphones in the “nulls” of the foldback speaker, acoustic feedback can be further reduced. The SILICON CHIP foldback loudspeaker is 546mm wide, 335mm high and 408mm deep. The box is made of 18mm MDF (medium density fibreboard) and covered in loudspeaker carpet. The loudspeakers are protected with steel mesh grilles and moulded plastic corners provide protection for the enclosure against rough handling. Two 200mm woofers and a single piezo ceramic speaker with a 50mm x 150mm wide dispersion horn are used. The woofers are placed either side of the vertically mounted horn. This arrange­ment produces a symmetrical on-axis frequency response ideal for foldback. If the loudspeaker box is mounted on end, the woofers will be arranged vertically and the horn horizontal. This allows the speaker to be used conventionally for projecting sound toward an audience. The specified woofers (Altronics Cat. C-3060) have a power rating of 60W and a nominal impedance of 8Ω. Their Thiele-Small parameters are Vas Specifications Frequency range: 45Hz to 20kHz <at>-6dB Nominal Impedance: 16Ω Power rating: 200W into 16Ω (equivalent to 400W into 8Ω) Mass: 18kg Dimensions: 546mm (W) x 408mm (H) x 335mm (D) Baffle slope: 35° or 55° Fig.1: only two components are used in the crossover network for the Foldback Speaker. The 1.8mH coil (L1) prevents high frequencies from being fed to the woofers, while the 0.33µF capacitor (C1) reduces the output of the tweeter by about 6.4dB. November 1999  73 Fig.2: these graphs of Fig.2 show the CALSOD response predictions for the woofer and tweeter combination. The solid line on the upper graph is the on-axis frequency response while the dotted line is the impedance plot. The solid line on the lower graph is the phase response. Fig.3: this graph shows the predicted horizontal off-axis response at 30°. The notch in the re­sponse at around 1.2kHz occurs at different frequencies depending on the off-axis angle. 54l, Qts 0.323, Qes 0.398 and Qms of 1.72. Their resonant frequency (Fs) is 34.7Hz and sensitivity is around 89dB at 1W and 1m. The two woofers are connected in series to provide a nominal 16Ω impedance. This means that we can parallel up several foldback loudspeaker units together without overload­ing the driving power ampli74  Silicon Chip fier. Two foldback units will produce an 8Ω load, three units a 5.3Ω load and four units in parallel a 4Ω load. The tweeter is a Motorola KSN­ 1141A piezoelectric horn which incorporates protection circuitry to allow its use with amplifi­ers rated at up to 400W. The protection comprises an incandescent lamp and a positive temperature coefficient (PTC) resistor in series. These components increase their resistance at high power levels to protect the tweeter element. A common design approach when using a piezo tweeter is to simply connect the speakers without a cross­ over. The natural rolloff of the woofer at higher frequencies and the tweeter at lower frequencies are supposed to compensate each other and produce a smooth response. This rarely works well and inevitably the response is markedly louder in the 1kHz to 5kHz region as the sound level is summed from both woofer and tweeter. Usually the tweeter is also quite a bit more sensitive than the woofer and so such systems are often excruciatingly bright at the higher fre­quencies. For our design, we used a series 1.8mH inductor to roll off high frequencies to the woofers. While this will produce a theo­retical rolloff of 6dB/octave above 1.4kHz, in practice the rolloff will be somewhat less than this due to the woofers’ own voice coil inductance. In addition, to compensate for the sensitivity mismatch between the tweeter and woofers, the tweeter is fed via a series 0.33µF capacitor. Since the piezo tweeter itself has a ca­pacitance of 0.3µF, there is a capacitive voltage divider effect which reduces the signal level by a factor of 0.48 or 6.4dB. The speaker circuit diagram is shown in Fig.1. Note that the tweeter is connected out-of-phase with the woofers, to ensure a flat response. An in-phase connection pro­ duces a null in the response at the crossover frequency. The phase was predicted using a computer simulation and took into account the distance between drivers, the offset behind the baffle and the phase response of the drivers and crossover. Some readers may be puzzled by the two jack sockets shown on the circuit. One allows the cable from the amplifier to be connected while the second allows another foldback loudspeaker to be connected in parallel. In practice, you could “daisy chain” four of these foldback speakers together, to provide a nominal 4Ω load to the driving amplifier. Design software The crossover and low frequency ported response was mod­elled using Fig.4: the overall dimensions for the Foldback Loudspeaker. Use a jig saw to cut out the holes and rebate the woofer holes using a router. an Australian developed loudspeaker design program called CALSOD 1.40 (Computer Aided Loudspeaker Sys- tem Optimisa­tion and Design). This allowed the response to be adjusted for optimum smoothness and match- ing between drivers. The program allows off-axis predictions to be made using multiple drivers. November 1999  75 Fig.5: cut the two sheets of MDF as shown in this diagram, to make the sides, base, back, top and baffle. The material can be cut using either a jig saw or a circular saw. You can model and optimise cross­ overs, produce phase and response curves and also position the drivers on the baffle. Copies of this DOS based program can be obtained from Audio­ soft, 13 Beatty St, Ivanhoe, Victoria 3079. Fax (03) 9497 4441. Email audiosoft<at>netwide.com.au. The price for the budget version CALSOD 1.40 with on-disk users manual is $119 including postage and handling. Professional versions which allow importing measured data (CALSOD 3.10) are available from $369. The graphs of Fig.2 show the CALSOD response predictions for the woofer and tweeter combination. The solid line on the upper graph is the on-axis frequency response while the 76  Silicon Chip dotted line is the impedance plot. This shows the expected double hump at low frequencies for the ported design while the value rises rapidly above 5kHz due to the parallel tuned circuit formed by the woofers’ inductance and the capacitance of the piezo tweeter. Note that the impedance does fall back to lower values for fre­quencies above 16kHz, as can be seen at the top righthand corner of the graph. The solid line on the lower graph is the phase response. The abrupt changes from -180° to 180° at 40Hz and 7kHz does not mean that there is a sudden phase change; it is simply drawn that way so the phase plot fits on the graph. Fig.3 shows the predicted horizon- tal off-axis response at 30°. The rolloff above about 3kHz is an estimate for the attenuation in sound level at this angle. The notch in the re­sponse at around 1.2kHz occurs at different frequencies depending on the off-axis angle. Construction The overall dimensions of the fold­ back loudspeaker are shown in Fig.4. It is made from one 600 x 900mm sheet and one 450 x 1200mm sheet of MDF. This is cut as shown in Fig.5, to make the two sides, the base, back, top and front pieces. We used a jig saw and a straight edge guide to make all the cuts although you could also use a circular saw. The baffle is made from the material remaining after the base and front pieces have been cut from the 450 x 900mm sheet. Adjust the jig saw or circular saw so that it is set for a 35° cut. This will enable the baffle to mate flush with the inside of the front piece. Cut the baffle edge at 35° using a straight edge as a guide. Measure 300mm from the inside of this bevelled edge and draw a line across the 510mm length. Now cut this edge at 55°. Assemble the base, back, sides and front pieces together using 8g x 30mm countersunk wood screws. Do not glue the pieces at this stage. Fit the front baffle and top piece in place and check for fit. The bevelled edges may require some adjustment using a plane to produce a good fit. Now secure them with screws. Check that all right angle edges are square and that the straight edges provide a close fit. Mark out the pieces, indicating their orientation and positioning to adjacent pieces. This will make it easier to reassemble later on. Now disassemble the pieces. Cutting the speaker holes Mark out and cut the baffle as shown in Fig.4. Use a router to rebate the woofer hole and a jig saw to cut out the holes. The cutout for the tweeter horn was made deliberately small to allow a greater amount of wood between the woofer and tweeter hole. The 63mm diameter cutout is to allow the piezo element of the horn to fit through the baffle. Check that the speakers and port tubes fit into their respective holes. Now assemble the box using PVA glue on all mating surfaces. Assemble one side, the base, back, top and front first, followed by the baffle, using the screws to secure the pieces in place. Now glue the second side in place. This assembly method will ensure that the baffle can be glued on its side. Wipe any excess glue off with a damp rag. We fitted braces made from 31 x 13mm timber to the inside of the base and back. A 500mm length was used along the base spanning from side to side and a 290mm length along the back spanning from base to top. These were located offset from centre. For extra strength in the enclosure, we also recommend using 12 x 12mm cleats on all right angle joints and along the baffle to side joints. These Fig.6: this diagram shows how the main pieces are fitted together to form the box. The inside of the box is also fitted with braces and cleats, for added strength – see text. should be secured with PVA glue and screws. Smooth the box edges with a rasp or plane to produce a small 2-3mm chamfer and round off the corners neatly. This will allow the plastic corner protectors to fit correctly. Test these for fit before finishing this process. It is probably unnecessary to sand the box since the carpet covering will mask any imperfections in the surface. However, remove any large protrusions from the box surface such as glue runs, screw heads, etc. You will need to drill a 25mm hole in each side of the box for the 6.35mm socket adaptors. The same sized holes are also required if Neutrik panel sockets are used. Standard 6.35mm sockets, while relatively inexpensive, are really not rated for driving high powered speakers. Also they are not airtight and may introduce extraneous noises as the air passes through them. While you can seal them using a cover and silicone sealant, Neutrik locking chassis jack sockets are preferable because they are sealed and attach more solidly to the case. Standard 6.35mm sockets also have a tendency to fall inside the speaker box if the securing nut becomes loose and falls off. Alternatively, you could use Neutrik “speakon line” sock­ ets. These leaded sockets feature a 30A rating, locking plug, rugged line plugs and solid wire clamping on the plug. Covering it with carpet We covered the whole speaker box in carpet and attached it with contact adhesive. The carpet covers the box in four sec­tions. It is cut out using a cutting mat, metal straight edge and a sharp utility knife (eg, Stanley knife). Start by cutting a 510 x 300mm piece and coat the baffle with contact adhesive. Place the carpet over the baffle and then immediately lift it off again. This will reveal the baffle cutout areas on the carpet which do not require coating with adhesive. Coat the required areas on the carpet and recoat the baffle where the contact adhesive has lifted off. Wait for the adhesive to dry, then place the carpet in position over the baffle, making sure that it is oriented correct­ly. Now press the carpet down firmly until it is fully attached. The carpet will then need to be trimmed to reveal the cutouts on the baffle. For the woofer cutouts, trim the carpet to the outer diamet­er of the rebate using a sharp knife. The port cutouts will need to be recut to the November 1999  77 Parts List 2 200mm woofers (Altronics C-3060) 1 Motorola KSN1141A piezo ceramic speaker and horn 2 50mm adjustable speaker ports 2 200mm speaker grilles 1 strap handle 7 box corners 2 6.35mm jack sockets (Altronics P-0071) plus mounting cup (Jaycar HS-8025) or 2 x Neutrik locking chassis mount jack sock­ets (Jaycar PS-0196) or 2 x Neutrik “Speakon” sockets (Altronics P-0790, Jaycar PS-1094) 1 1.8mH air-cored inductor 1 0.33µF 200V polyester capacitor 1 5-way 30A mains terminal strip 1 piece of 1m x 1.8m x 3mm speaker carpet 1 1m x 500mm piece of speaker wadding 1 1200 x 450mm sheet of 18mm Medium Density Fibreboard (MDF) 1 600 x 900mm sheet of 18mm MDF 1 790mm length of 31 x 13mm dressed timber (pine or meranti) 1 250ml tin of contact adhesive 1 100ml container of PVA adhesive 1 2m length of speaker sealant or adhesive backed draught exclud­er 46 8g x 25mm countersunk bronzed wood screws (for corner protec­tors speakers and ports) 2 10g x 25mm cheese head bronzed wood screws (for handle) 50 8g x 30mm countersunk wood screws (for securing box panels) 3 6g x 20mm cheese head wood screws (for terminal strip and inductor) 4 6g x 20mm countersunk bronzed wood screws (for 6.35mm sockets) 1 1.5m length of red 15A hookup wire 1 1.5m length of black 15A hookup wire 78  Silicon Chip Fig.6: this wiring diagram shows how the 5-way terminal block is used to terminate the leads from the jack sockets, the woofers, the tweeter and the crossover components. outer diameter of the port mounting flange. This can be done by placing the port in position and then cutting the carpet to the flange diameter. Similarly, the tweeter horn can be fitted and the carpet cut around its perimeter. The second and third pieces of carpet required are for the sides and are initially cut to 335 x 408mm. Glue these to the sides and then trim the carpet along the sloping edge so that the carpet will fold over to meet the baffle. Cut holes in the carpet at each side for the 6.35mm jack socket or adaptor. Insert the socket (or adaptor if this is used) and trim the carpet around its perimeter. The final piece of carpet needs to be 550 wide by 1100mm and fits over the top, back, base and front of the box, starting at the top of the baffle and going around to the base of the baffle. Trim the carpet at one end so that it meets the carpet already applied to the baffle and sides. Apply contact adhesive along those edges where the carpet joins will be and on the carpet itself. Fit the carpet in place when the glue has dried. Now coat the bulk areas of the box on the top, back, base and front and the carpet and fit it in place. The corner protectors can now be fitted with countersunk screws. These mount on each right angle corner. We cut another corner protector to provide two separate flat pieces and these were fitted along the sloping edge on each side of the box to allow it to be stood on side for normal loudspeaker use or for stacking during storage. Attach the handle onto the front of the box close to the baffle and central to the width of the box. This should provide the best balance point for carrying the box. Wiring Use 15A hookup wire and solder a 300mm length of red wire to the positive terminal of each speaker and a 300mm length of black wire to the negative terminals. Then connect a 300mm length of red wire to each tip connection of the jack sockets and a black wire 300mm in length to the ring terminals. The crossover components are mounted using a 5-way terminal block Above: both the terminal block and the inductor are secured to the bottom of the box using wood screws. Note that the inside of the box is lined with wadding which is stapled in place but this must be clear of the port holes. which is secured to the inside of the box with two wood screws. Mount the inductor using a wood screw also. Wire up the components by passing the wires through the holes allocated for each component as shown in the diagram of Fig.6. Apply a layer of speaker wadding around the edges inside the box and secure it in place with staples. Make sure the wad­ding is clear of the port holes. Fit the jack sockets to the side of the case with screws and secure the speakers with sealant between the baffle and speaker mating surfaces. Cut the two 50mm ports to a length of 125mm and secure them in place. Fit the protective mesh grilles over the woofers using the supplied clamps and wood screws. Then vacuum the outside of the box to remove wood shavings and sawdust. Any contact adhesive on the outside of the carpet can be removed with mineral turps and an old toothbrush, before it sets hard. Then you are ready to have a SC listening test. Enjoy. A 2-metre length of speaker sealant (or adhesive-backed draught excluder) is used to seal the loudspeakers and the port tubes, to prevent air leaks. November 1999  79