Fullscreen HTML5Med Res (??MB)HTML4

M easy-to-build Outdoor Subwoofer By Julian Edgar any people have outside speakers in a deck or patio area, but they are often small, wall-mounted designs that lack adequate bass response. This subwoofer project can add a lot of that missing bass. Because it’s built around a fibre cement stool available from Bunnings, very little woodworking is needed, and the enclosure is weather resistant. This design includes a simple and cheap protection mechanism that makes the subwoofer very hard to blow up (that’s always a danger with small subwoofers). Depending on your interior décor, it can also be used inside. The enclosure The subwoofer enclosure is based on an elegant white cement stool available from Bunnings, called the “Marquee 350 × 350 × 450mm Stool Side Sorrento” (I/N 0596376). It costs $69 and, while completely hollow, still weighs a little under 14kg. You don’t need to modify it; you simply glue an internal panel into it and add feet. Fibre cement is a good material for speaker enclosures, as it is acoustically dead (it doesn’t ‘ring’ when tapped) and is quite stiff. This stool is even stiffer than most because it uses a ribbed wall design. The fibre cement can be painted any colour you want. If you wish to make the subwoofer using another design of stool, or even (gasp!) from MDF or similar, the key dimension is that a volume behind the drivers of about 18L is required. The drivers This passive subwoofer is designed to be used on an outside deck or patio. It is quick and easy to build, and will add substantial bottom-end to your small exterior wall speakers. It’s also largely weatherproof. » » » » » » » » » » 74 Excellent frequency response for its size Inbuilt protection against being over-driven Medium-power design suitable for amplifiers up to 100W Uses low-cost drivers Easy and quick construction Largely weatherproof, suitable for undercover outdoor use Can also be used indoors Can be painted any colour to match décor Frequency response: 35-200Hz Impedance: 4Ω Silicon Chip Australia's electronics magazine Two drivers are used, mounted in an isobaric (face-to-face) configuration. Both are 170mm (6.5-inch) WS 17 E units made by Visaton. Note that you need the 8W versions. These speakers are available worldwide – a web search will find your nearest stockist. We bought ours from RS Components (Stock No 431-8563), but they ended up coming from the UK. A major criterion in their selection is that they are cheap – about $47 each, including GST. The speakers come with full ThieleSmall specifications, allowing computer modelling of the enclosure. Still, when bench tested with the Smith and Larsen Woofer Tester hardware/software package, the tested specs of the two drivers differed somewhat from the official specifications, even after being ‘run in’. siliconchip.com.au Table 1 shows the advertised specifications, the test results and the final values used in the modelling. Note that the stated maximum cone displacement is 13mm – something we’ll come back to later. Enclosure modelling Three different designs were modelled using the freely available WinISD speaker enclosure design software. These designs were sealed, ported and a 4th-order band-pass configuration. Fig.1 shows the modelled response curves of each design approach. The aim was to achieve a response from about 150Hz down to 40Hz. Good efficiency was also important – that is, the greatest output for a given power. Given that the bare driver has a fairly low efficiency (85dB at 1W/1m), achieving maximum efficiency becomes an important part of the enclosure design. Of the three designs, the sealed approach was the worst in efficiency and had a lower -3dB point of about 43Hz. The band-pass design had a much higher efficiency, about 4.5dB louder in the critical area, so the equivalent of having 2.8 times the amplifier power! It had a modelled -3dB point of about 42Hz, but as is the characteristic of such band-pass designs, fell away quickly at the top end, being 3dB down at 110Hz. This concerned me, as many small outdoor speakers will struggle to get down to 110Hz. The ported enclosure had a -3dB point of 36Hz (substantially better than the other designs), and was nearly 3dB up at 110Hz. It peaked at about 65Hz (over 6dB up) and had a greater ‘area under the curve’ than the other two designs. It was modelled using a 100mm-long, 50mm inner diameter port. Note that I was not aiming for a flat response – since the sub can be used outside, it needs as much gain as possible without becoming crazily peaky. This modelling was just a starting point – it’s easy to look at lines on a PC screen and think that they represent reality, not just a model of reality. However, software modelling is a good way of getting into the region of what is wanted. What counts as a subwoofer? A traditional subwoofer works only at very low frequencies – for example, below 40Hz. That is, the subwoofer provides output below the frequencies of a conventional woofer. However, over time, this definition has become blurred. Computer sound systems, for example, typically use two small satellite speakers that might work down to only 200Hz, with the separate ‘subwoofer’ providing the frequencies below this. Many high-level and custom car sound systems use a subwoofer, but again, it typically provides the bass component below 100-200Hz. So rather than developing only very low frequencies, a subwoofer has come to be known as any separate speaker that provides the bass. If you’re listening to music, how low a frequency response is actually required? The response of human hearing is stated as being 20-20,000Hz. However, age reduces this range, and 20Hz can arguably be more clearly felt as vibrations than heard. A pipe organ can produce notes at just 16Hz, but few pipe organ recordings have this low frequency content. There’s not much point in including sounds that no speakers will reproduce! A bass guitar and a double bass go down to 41Hz, which is clearly audible. A bass drum can produce frequencies of 20-120Hz, usually centred around 40Hz. The lowest note on a standard piano is 27.5Hz. So, while having as low a frequency response as possible is desirable, in the real world, a speaker system that can reproduce down to 40Hz will give the vast majority of what is needed. Whether you then call that speaker a subwoofer is up to you! This subwoofer is not designed to fill large outside areas with booming bass. To do so, it would need to be about ten times as big, ten times as expensive and ten times as powerful! Instead, it’s designed to add bottom-end and body to normal background music played at quiet-to-moderate levels in outdoor areas. Fig.1: the model outputs (predicted frequency responses) for three different enclosure designs: sealed (blue), band-pass (red) and ported (green). The ported design was chosen. Table 1 – WS 17 E 8Ω specifications Specification Listed Speaker A Speaker B Used in modelling DC resistance 6.3W 6.1W 6.2W – Sensitivity <at> 1W/1m 88dB 85dB 85db – Resonant frequency 41Hz 45Hz 43Hz 44Hz Qms 2.83 3.45 3.22 3.4 Isobaric configuration Qes 0.81 1.10 1.01 1.1 The two drivers are mounted in an isobaric configuration – that is, Qts 0.63 0.83 0.77 0.8 Vas 31L 25L 27L 26L siliconchip.com.au Australia's electronics magazine June 2025  75 Photo 1: the Bunnings fibre cement stool. Making it into a subwoofer doesn’t change its appearance much – it will just have a slightly larger gap at its base. Source: Bunnings Photo 2: this halogen incandescent light bulb is used as the speaker protector. Its resistance rapidly rises as the current flow through it increases, limiting the maximum speaker power. Source: Narva face-to-face with a small, trapped air volume between them. The drivers are wired out of phase so that as one pushes, the other pulls. The advantage of an isobaric configuration is that the drivers act as if they are working in a larger enclosure volume, and most importantly from our perspective, the power handling of the drivers doubles from a nominal 60W (90W peak) to 120W (180W peak). The use of paralleled drivers explains the need for selecting 8W designs – the two drivers then form a nominally 4W amplifier load. The WinISD software can model isobaric configurations. Test and development To initially test the design, a disc of 22mm-thick weatherproof particleboard was cut so that it would sit within the upturned stool, about 130mm down from the end. This placement gives room for the drivers and the port to project from both sides of the baffle. Two holes were cut in the particleboard disc – one for the drivers and the other for the port. The hole for the port was made a tight fit so that different lengths of 50mm ID PVC plastic pipe could be trialled. The gap between the edge of the particleboard disc and the inner wall of the stool was temporarily sealed. A layer of polyester quilt wadding 76 Silicon Chip was placed inside the enclosure, with care taken that it did not block the port. The wadding prevents sound reflections off the hard interior surfaces. The subwoofer stool could then be tested upside-down. A frequency generator app (Signal Gen from Media Punk Studios) on an iPhone was used in conjunction with an audio amplifier to test the subwoofer on sine wave sweeps from 150Hz down to 25Hz. Always test at low volumes – you can easily blow up drivers with sinewave testing! Different port lengths were trialled, with a 190mm-long port giving better results to my ears than the modelled 100mm port. Yes, that’s a big difference, implying the enclosure tuning point has moved from about 47Hz down to about 35Hz. Testing of the completed enclosure showed an actual enclosure tuned frequency of 38Hz. Using shorter ports than 100mm gave a much peakier response – something the software modelling had shown would be the case. For example, using a 50mm ID port that was only 40mm long gave a modelled +10.5dB peak at about 75Hz. Therefore, if you’re not unduly concerned about one-note bass and just want it louder, use a shorter port like this. Extensive testing with music followed, and this showed something else. Because these are not expensive drivers with huge cone travel, driving the sub hard could bottom-out the drivers’ suspensions. This is important to understand, because many people Speaker resonance and one-note bass Many people confuse a good bass response with a pronounced bass resonance. I remember when I was young and trying to make my car sound system perform well. I’d fitted a new amplifier, new speakers and a new head unit (a cassette player in those days!). In one particular song, a note from the bass guitar caused the whole car to vibrate… something I thought was really cool. What was happening was that a major speaker resonance was being triggered, and that excited the car. Nowadays, I’d see that as a shortcoming! A speaker resonance is where, for a given power input, the audio output of the speaker sharply peaks. That is, at a particular frequency, the speaker is much more efficient at turning electrical input power into an audio output. The problem with a subwoofer having a pronounced resonance is that the output at that frequency will dominate the rest of the content. This is often termed ‘one-note bass’. One-note bass is the thump, thump you often hear in poor sound systems – all the bass, no matter its actual frequency, is reproduced as much the same-sounding thump. Australia's electronics magazine siliconchip.com.au Photo 3: the underside view of the baffle with the speakers, port, protection light bulb and terminal strip temporarily mounted. In the final design, some of the parts were orientated slightly differently. Photo 4: a closeup showing the 8mm flanged nuts used as spacers between the speaker mounting lugs, the protection light bulb mounted on its bracket and the terminal strip. over-drive subs without realising that they are doing so. When developing a subwoofer, always test it with a crossover and without the other speakers running. That is, listen to just the sub working at only low frequencies. This way, you can hear what is really happening, without the sound being masked by the other speakers. the speaker system, they will dominate the sound. For example, without a sub crossover, the lower midrange can be over-emphasised. Crossovers can be achieved by using a series inductor in the subwoofer speaker feed (not so good), or much better, using an electronic low-pass or band-pass filter. Subwoofer crossovers Two solutions were developed to prevent the sub from being overdriven. The first was to temporarily place a heavy lid on top of the upturned sub, raised from the stool body by 10mm spacers. This replicates how the sub will actually be used – inverted and placed on the ground on short feet. The use of this lid (or in use, the presence of the ground) better acoustically loads the drivers, reducing their displacement peaks. Technically, we’re also adding another chamber and port (the gap around the periphery), but that made little difference to the sound except that upper frequencies were better suppressed. You don’t want these coming out of a sub anyway. The other solution was electronic – or at least, electric. Many professional speakers used in commercial settings have a very simple approach to protecting (especially) the tweeters. They place a normal incandescent light bulb in series with the drive to the speaker. A subwoofer – especially a small one like this – must be ‘crossed over’. It must not be fed frequencies outside the range of those you’re trying to reproduce. The reason for this is that a small sub can easily reproduce higher frequencies, and if you have the sub producing them as well as the rest of Photo 5: two of these Visaton 170mm (6.5in) drivers are used in the subwoofer, mounted in an isobaric configuration. Source: Visaton siliconchip.com.au Speaker protection Australia's electronics magazine Low-voltage incandescent light bulbs have low resistance (eg, 0.7W) when they are cold but about 15 times as much resistance when they are hot – that is, when the filament is glowing brightly. Therefore, as the current flowing through the bulb increases, so does its resistance, limiting the power getting to the protected speaker. We decided to take a similar approach to protect the subwoofer drivers. Many different light bulbs were tested, including those with different voltages and wattages, and multiple bulbs wired in series, parallel and series/parallel configurations. The aim was to limit power to the subwoofer such that the speakers could not be overdriven on a variety of music. However, a lot depends on the amplifier you are using, the type of music you play – and how loudly you play it! While the light bulb approach has audio downsides (it is a non-linear compressor), and so is frowned on by purists, it works very effectively. Depending on the music type and power level, the bulb may not glow at all, glow just a little, or light brightly. Furthermore, it has a short-term memory in that if the power is repeatedly high (eg, you are loudly playing a song with lots of bass), the filament stays warm and so limits the power earlier. To put this another way, if the subwoofer is constantly being overdriven, the sub output drops a lot – it June 2025  77 Light bulb based speaker power limiters The question that is always asked by people wanting to use light bulbs as speaker power limiters is how to choose the correct bulb for the application. The bottom line is that it is nearly impossible to do it theoretically – testing is the only practical way. The difficulty with trying to specify the required light bulb theoretically is that the resistance posed by the bulb constantly varies with filament temperature. In turn, this is governed by the light bulb’s characteristics, amplifier power, the type of music being played and the impedance curve of the speaker being protected. If we were using a sinewave as the signal, it would all become much simpler – but we aren’t. Even the bulb’s maximum continuous power dissipation (ie, the wattage rating) isn’t as useful as it might first appear. In our application, the bulb is required to dissipate large amounts of power only on very short transients. Therefore, the power limiting that occurs depends in part on the response speed of the filament. Furthermore, the filament has a thermal memory. If it is dissipating large amounts of power in successive bursts (eg, there is a rhythmic bass beat), the filament stays hot between the bass notes. It therefore has a higher constant resistance and so the expected high pulses of amplifier power are not fully developed! Based on the voltage swings of the audio signal, it would appear obvious that a bulb with a higher voltage rating (eg, 60V) should be used – however, such a bulb has a higher cold resistance, so it will reduce amplifier output power all the time. That is not what we want. In the case of the project described here, the 24V 55W halogen bulb worked well. A similar result could also be achieved using multiple 24V 18W tail-light bulbs. However, they were more expensive to buy than the single 55W bulb. It’s a fascinating area, and we’d love to hear about any successful results readers have gained using light bulbs for speaker protection. Two light bulbs used to limit the power in a Bose Lifestyle speaker system. is actually louder when the amplifier is turned back down. By watching the filament lamp during testing, you can also get a very good idea of when the speaker is being driven too hard and so reduce the maximum power the sub will ever see. (More on setting up the amplifier later.) If, in ‘normal’ use, some crazy dude gets hold of the amplifier knob and cranks it right up (eg, when you’re away on holiday and the kids decide to host a party!), the light bulb will protect the speaker. If the worst comes to the worst, the bulb will likely blow before the drivers do, cutting off the subwoofer output. We have nominated using a single Narva 24V 55W bulb (part number 48701). This bulb costs about $10 from automotive parts shops. It is difficult to over-drive the sub on normal music with this bulb wired in series, using any amplifier up to about 100W. However, if you want to be less conservative, using two of these bulbs in parallel gives more subwoofer power but still some protection. Of course, you can choose to delete the protection bulbs entirely and use only a lowpower amplifier. However, we suggest using the single light bulb. Despite the combined drivers being rated at 180W peak, this does not mean that if the sub is used with an amplifier having less power than 180W, the sub requires no protection. Remember, the protection is primarily to protect the drivers from being over-driven rather than their voice coils being burned out. Warning: if the subwoofer is continuously overdriven, the protection lamp will become very hot. The subwoofer should always be placed on a firm, level and non-combustible surface like tiles, concrete or similar. It should not be placed on dry grass. Construction Construction is easy, and should take you only about an hour, spread over two days. The steps are: 1. Make the baffle and trial-fit the speakers, port, protection lamp and terminal strip to it. 2. Disassemble the baffle, removing all the parts. 3. Glue the baffle, port and feet in place and let the glue harden for 24 hours. 4. Fit and wire the speakers, protection lamp and terminal strip. 5. Test it. 78 Silicon Chip Australia's electronics magazine siliconchip.com.au Photos 6 & 7: the baffle and port glued into place with Liquid Nails. The port protrudes from both sides of the baffle, while the two drivers are positioned face-to-face in the opening. The towel under the stool protects this surface when working on the subwoofer. Once the glue has hardened, you can paint the inside of the enclosure, the baffle and the port tube. Photo 8: a close-up of the protection lamp and terminal strip wiring. Note the cable going through the baffle to the other speaker. The first step is to use a jigsaw to cut a disc of particleboard about 285mm in diameter. Measure the maximum diameter of the opening in the stool’s base – the disc should be just smaller than this. The stools are handmade and so vary a bit in size. We used 22mm-thick, moisture-­ resistant particleboard (a flooring offcut), but if you seal it on all sides with paint before gluing it into place, slightly thinner MDF should be fine. Don’t use material less than about 18mm thick – the peripheral glue needs plenty of ‘meat’ to adhere to. Cut a 165mm hole in the baffle for the drivers to sit in, and mark and drill small diameter pilot holes for the particleboard screws that will hold the drivers in place. Next, use a hole saw to make the opening in the baffle for the 50mm ID PVC pipe. The required hole diameter is 56mm – if you don’t have a hole saw of this diameter, make a smaller hole and then file it to size. The exact location of these holes is not critical – just ensure the speakers and port clear the inner walls of the stool. We recommend proper safety precautions when cutting MDF, such as using a respirator and cutting in a well-ventilated location. Drill a small hole for the inside speaker’s cable to come through from the other side of the baffle. Cut a suitable speaker gasket from a thin foam rubber sheet before dropping the first speaker into place. In the final assembly, you can use silicone if on the baseplate and enlarge this hole to 3mm. Be very careful when drilling these holes – it is easy to damage the lamp (eg, by dropping it). The lamp mounts on a small right-angle bracket that is attached to the baffle with a wood screw. Wiring connections to the lamp are by two solder lugs that are attached with the 3mm screws. Ensure the baffle assembly will fit inside the stool without the port fouling the inside wall. When you are happy that everything will fit correctly, remove the baffle and disassemble it. If using non-weatherproof MDF, paint the baffle on both sides and on all cut edges. Spread ‘water clean-up’ Liquid Nails (or equivalent) building adhesive at an appropriate height around the inside of the stool where the baffle will sit. Be generous with this glue – you want no leaks and for the baffle siliconchip.com.au you don’t want to make a gasket. The other speaker fits on top, so they mount face-to-face (see Fig.2). These speakers have an external gasket, so they seal to each other very well – no further gasket or sealant is needed between them. Use 6mm spacers between the two sets of speaker mounting holes so that the speakers are clamped firmly together, but the mounting tags are not overly bent. I used 8mm flanged nuts that had the required 6mm depth. The particleboard screws go through both sets of speaker mounting tags and the spacers, holding the two speakers firmly to the baffle. Push the 190mm PVC pipe through the hole in the baffle; the pipe projects about equally from each side. Next, mount the protective light bulb. To do this, enlarge the existing hole in the bulb’s bottom tang to 3mm. Nip off one of the nipple protrusions Fig.2: the two drivers are mounted face-to-face, being inserted into the baffle from the underside of the enclosure. Australia's electronics magazine June 2025  79 to be held in place very firmly. Ensure the glue is the water clean-up type or it will be difficult to remove the excess! (We suggest “Liquid Nails Fast Grab”, Bunnings I/N 1230096). Carefully drop the bare baffle into place and push it down onto the glue, ensuring the baffle stays level. Apply more glue around the gap between the baffle and the inside of the stool and smooth the glue with a wet finger, wiping up any excess with a wet cloth that you repeatedly rinse in water. Insert the vent, also sealing it into place with the glue. Let the glue harden for at least 24 hours. You may choose to paint the interior of the stool at this stage – ie, the baffle, glue and visible inside wall of the stool. I did so using black spray paint. A single layer of quilt wadding, about 500 × 500mm can now be glued in the bottom of the enclosure (the top when it is orientated normally). This step is optional – I am not sure it makes a great deal of difference, but it will possibly reduce ‘hollow sounding’ reflections. Place the gasket on the underside of the first speaker or, if not using a gasket, apply silicone around the upper edge of the hole. Solder the speaker’s connecting cable to its terminals, then feed this wire through the baffle hole, pushing it from the inside by placing your hand through the speaker hole. Ensure you know the polarity of your connections, eg, by using coloured wires or a cable with a trace on one conductor. Put the first speaker into place and then mount the second speaker on top, remembering to include the spacers. Insert the four screws and tighten in a series of steps using a diagonal tightening pattern. Wire the two speakers out of phase – the inside speaker’s positive connection goes to the outside speaker’s negative and vice versa (see Fig.3). Seal the wiring hole in the baffle. Next, install the protective light bulb. It mounts on a small bracket that is attached to the baffle with a particleboard screw (drill small diameter pilot holes for the screw). Do not place the lamp against the PVC vent. If you have touched the bulb’s glass at any point in the installation process, wipe it with a cloth moistened in methylated spirits. This removes any oils that may have been deposited on the glass from your fingers, which could potentially weaken the glass. The terminal strip mounts next, again with a particleboard screw into a pilot hole. Wire the drivers to the terminal strip with the light bulb in series (it doesn’t matter which lead it goes in). Because the outside driver pushes air outwards when the cone moves backwards (rather than the normal forwards), the negative terminal of the outer driver connects to the positive speaker connection terminal. Mark this with a (+). Should the vent be flared? The vent uses straight rather than flared ends. Flared ends reduce port noise (sometimes called ‘chuffing’). However, considering the size of the drivers and their isobaric configuration, a large diameter vent has been used, so air velocities in the port are relatively low. No port noise could be heard even without flares. Photo 9: testing the subwoofer with Niles wall-mounted speakers on a deck under construction. The ceiling is 2.7m high, and the deck area is about 10 × 5m. On a deck this large, two subwoofers spaced about 5m apart will give the best results. 80 Silicon Chip Australia's electronics magazine siliconchip.com.au Testing The next step is to test the subwoofer. Place it upright on its feet on a firm, flat surface. Feed the subwoofer through an amplifier and crossover, and ensure other speakers cannot be heard. Use a frequency generator app to perform a sweep from 200Hz down to 20Hz. The speaker should be audible down to about 35Hz, and there should be no buzzes, whistles or rattles. If there are, isolate where the sound is coming from (eg, a loose port or leak around the frames of the drivers) and then fix the problem. If you hear a buzz, ensure it’s not something in the room becoming excited, rather than the subwoofer itself. If the bass output is poor, check the speaker phasing, ensuring the speakers are wired out of phase. Now switch to the type of music you normally play. Do not run any high-power tests with the frequency generator. Ensure you can see if the light bulb is glowing brightly – eg, in dim conditions, it will cast a visible ring of light around the open base of the enclosure. Increase the amplifier gain until the filament is just glowing on bassy passages. Now, while not changing the amplifier gain, select music of the type that has as much bass as you will ever listen to. The light bulb(s) should light quite brightly on these greater bass passages, showing the protection is working, and the drivers should not bottom out. If the bulb is glowing brightly a lot of the time, the amplifier gain is too Fig.3: wire the two speakers out of phase so that as one pushes, the other pulls (and vice versa). The protection light bulb is inserted in one conductor of the main feed (either is OK). siliconchip.com.au Parts List – Outdoor Subwoofer 2 Visaton WS 17 E 8W speakers [RS Components 431-8563; there are many other suppliers] 1 Marquee Sorrento 350 × 350 × 450mm Side Stool [Bunnings I/N 0596376] 1 300 × 300mm 22mm-thick weatherproof particleboard sheet 1 190mm length of 50mm internal diameter PVC DWV pipe 4 25mm diameter, 10mm thick white rubber feet 1 cartridge of water clean-up Liquid Nails [Bunnings I/N 1230096] 1 Narva 48701 24V 55W automotive light bulb [Car parts supplier] 1 500 × 500mm section of dressmaker’s quilt wadding [Spotlight] 1 small terminal strip (eg, 2-way) 1 packet of self-tapping particleboard screws various lengths of wire & cable assorted small hardware items (screws, washers, nuts etc) 4 6mm spacers (particleboard screws must fit through) 1 thin foam rubber sheet 1 small steel right-angle bracket high. Of course, depending on the amplifier power, you may not see the bulb light at all. Results The prototype speaker had good output from 35Hz to 200Hz. Furthermore, the response was pleasingly smooth across this range. The measured impedance did not drop below 4W at any point in the frequency range. Using the specified protection lamp, amplifier powers up to 100W should be fine when playing normal program material. Setup Remember that the subwoofer needs its own amplifier, and that amplifier needs either a built-in subwoofer crossover or to be fed only low frequencies via an electronic crossover. Lowcost Class-D subwoofer amplifiers with built-in crossovers are readily available, but it can be much cheaper to use a surplus, bridgeable stereo amplifier with an adjustable subwoofer crossover on its input. The location of the subwoofer is important. Placing the subwoofer at the intersection of a wall and the floor causes greater loading of the speaker’s drivers. As a result, the energy of the speaker is better communicated to the air, so the bass output will increase. Placing the sub at the intersection of two walls and the floor increases this effect even more. Conversely, placing the sub on a pedestal in the middle of the room or outdoor area will reduce the output. Large changes in output are achievable by these various placements. Australia's electronics magazine The other aspect of placement is that when used outside, the closer the sub is to you, the more its presence will be felt. Unlike in a small room, where the loudness of the sub doesn’t vary much wherever you are in the room, outside the low frequencies are clearly louder when you are closer to the sub. Therefore, place it near where you will most often be sitting. When placed on the floor against a wall, the sub was effective over about a 25m2 area – it works fine in an open area about 5 × 5m. If your deck or patio area is larger than this, you could use two of these subs. To limit cone movement, the open end of the sub should always be placed on the ground (spaced upwards by its feet). If the sub is to be used upside-down with the opening facing upwards, a heavy panel should be placed over the opening, again spaced upwards by about 10mm. The sub is weatherproof in that the concrete stool can cope with rain or being sprayed with a hose. However, the sub is open underneath, so it should not be placed on grass or any other surface where there is moisture present all the time. If washing a deck or other outside area where the sub is placed, move it first. You may occasionally wish to spray some insecticide into the interior of the enclosure (including through the port) to prevent bugs and spiders making colonies. Finally, if you can obviously hear that the sub is working, it’s probably too loud – it should be just part of the music, not a separate, identifiable entity. SC June 2025  81