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Just bung the drivers onto some timber panels and glue them onto concrete blocks! C ON C R ET O Speaker System We were tempted . . . very tempted . . . to call these the greatest “ROCK” speakers ever. But that pun would fall a bit flat because these speakers are not rock – they’re concrete! More specifically, their “enclosures” are stock standard concrete building blocks – the type you'll find at very low cost in just about every hardware store. Intrigued? Read on . . . W For just a few dollars more, you can get a Class-D ampliant to build a pair of speakers, but don’t have the skills, tools or time to build proper boxes fier module to drive both, with line inputs and Bluetooth wireless audio support. for them? Concrete is actually an excellent material to make loudNo worries. We have the solution for you! Just bung the drivers into some timber panels and glue them onto con- speaker enclosures from because it’s very stiff and it’s very crete blocks. It might sound like an odd thing to do, but ‘dead’ – you don’t have to worry about it resonating at all and ‘colouring’ the sound. As a bonus, concrete blocks you’d be surprised how well it works. This bookshelf speaker system gives punchy and clean (also known as concrete bricks, Besser blocks and breeze blocks) are cheap, readily available sound, and it’s a lot of fun to build, with and have four square sides already preexcellent bass and treble out of one tiny by Allan Linton-Smith assembled. full-range driver, plus a subwoofer or two. The pair of “bookshelf” speakers, housed in half-blocks. They’ll give a good account of themselves “as is” but team them up with the full-block subwoofers and you won’t believe how good they sound for such a tiny investment. Rock on! 46 Silicon Chip Australia’s electronics magazine siliconchip.com.au At 190 x 190 x 190mm, the half-block bookshelf speakers are exactly half the size of the subwoofers because the latter are built in a standard 380 x 190 x 190mm concrete block. There are two huge advantages in using concrete blocks as enclosures: (a) they’re dirt cheap and (b) they cannot flex or move to colour the sound in any way. You might say they’re as solid as a (ahem!) rock . . . This is definitely not a new idea. Building speakers be- panels onto the $3.50 concrete blocks with silicone sealcame a bit of a fad in the 1950s. At the time, concrete was ant, and we’ve used a coaxial main driver so that no sepaconsidered by many to be the ideal material from which rate tweeter is required. That also eliminates the need for to make speaker enclosures. a crossover network. Concrete speakers have faded in popularity since then, Another big advantage of using a single driver is its phase but are seeing a bit of a resurgence. Besides being practi- coherence; that is, its ability to reproduce all frequencies cal, they also look pretty interesting, especially with nice- with mostly the same phase. ly-finished, routed timber front panels. This produces a very realistic reproduction of the origiThe concrete also helps to improve overall efficiency, nal recording for voice, instruments or complex orchestratransmitting less than 25% the amount of sound energy tion. It is essential for accurate ‘soundstaging’ (positioning that a comparable wood or MDF enclosure would. of each instrument). Many people prefer to have The small cone is very accurate smaller speakers, but they often in the upper and mid-range, but Features & specifications compromise on sound. you will also get to hear pipe orThese ones emit a solid bass • Frequency response: 90Hz-20kHz, ±6dB gan pedals and bass drum kicks and have clarity which you will • Distortion: <2%, 85Hz-2.7kHz (0.8% <at> 1kHz) if you build the extra subwoofers. fall in love with immediately. • Bookshelf efficiency: 91.5dB <at> 1W, 1m They are so cheap to make, why They also have a really smooth not build two for better bass? • Subwoofer efficiency: 88dB <at> 1W, 1m sound, partly because of the lack Driver choice of resonance and partly because • Power handling: 2x15W (Bookshelf),   2x50W (Subwoofers) of our choice of drivers. We considered three different In keeping with the idea of • Impedance: 8Ω (Bookshelf), 6Ω (Subwoofers) full-range drivers for the Booksimplicity and cheapness, we’ve • Low cost - estimated <$200 to build all four units shelf speakers. Table 1 shows a simply glued the front and rear comparison of these units. We siliconchip.com.au Australia’s electronics magazine June 2020  47 +50 Concreto Frequency Response 10/09/18 08:39:38 Concreto THD vs Frequency, 1W <at> 1m Total Harmonic Distortion (%) 5 +30 +20 +10 +0 -10 2 1 0.5 0.2 -20 -30 10 Subwoofer Bookshelf +40 Relative Amplitude (dBr) 10/09/18 08:34:12 20 50 100 200 500 1k 2k Frequency (Hz) 5k 10k 20k 0.1 20 50 100 200 500 1k 2k Frequency (Hz) 5k 10k 20k Fig.1: the bookshelf speakers have a fairly flat response above 150Hz; the subwoofers fill in below 200Hz where the bookshelf response drops off. Fig.2: distortion is low in the critical 100Hz-2kHz range which contains a lot of human voice information as well as many musical instruments. chose the Altronics C0626 on the basis that they are a lot cheaper than the Fostex drivers and only have slightly less bass, slightly higher distortion and slightly lower efficiency. In other words, they are almost as good for about 1/4 the price. The Jaycar drivers are cheaper again, but are much less efficient, so given their relatively low 15W power handling, you’d struggle to get decent volume out of them. So that was why we didn’t end up using them, and didn’t bother measuring their actual frequency response or distortion level. However, we are using Jaycar 125mm drivers for the Subwoofers, cat no CW2192. They are also excellent value at $29.95 each (retail price, including GST) given their 50W power handling, decent efficiency of 88dB <at> 1W, 1m and low resonant frequency of 67.6Hz. In fact, in the enclosure we’ve designed, they give some output down to about 35Hz, which is impressive given their small size. build the subwoofers; they extend the bass response considerably, down to around 65Hz, with a bit of a shelf from 40-65Hz. This means that really low bass won’t quite be ‘full’, but you’ll at least hear something down to about 35Hz. Importantly, the system also provides low distortion sound, as shown in Fig.2. THD is well under 2% from 100Hz to 2.5kHz and less than 0.8% at 1kHz. It is reasonably efficient, delivering 91.5dB at 1W/1m/1kHz. You may think that it is only suitable for small rooms, but they produced a considerable amount of sound in our warehouse with only a few watts driving them. Regardless, if you want good quality sound at reasonable listening levels, these will not fail to impress. A nice little Class-D amplifier module is ideal for driving these speakers, for example, one of those I reviewed in the May 2019 issue of SILICON CHIP (siliconchip.com. au/Article/11614). They certainly could also be driven by one of our more powerful/higher fidelity audio amplifiers, such as the Ultra-LD Mk.4 (August-October 2015; siliconchip.com.au/Series/289) or the SC200 (January-March 2017; siliconchip. com.au/Series/308). These Concreto speakers will provide many hours of wonderful listening at a tiny fraction of the cost of a fullybuilt high-end hifi system. The sound is even more satisfying, knowing that you have built something a bit unusual! Performance The frequency responses of the two speaker cabinets are shown in Fig.1. Here, “Bookshelf” (the blue curve) refers to the smaller cube-shaped enclosures which house the midrange drivers with coaxial tweeters, while “Subwoofer” (the red curve) refers to the taller ported enclosures with the larger woofers. The Bookshelf speakers are pretty flat from 150Hz to 20kHz, with a moderate peak at 10kHz and a roll-off in response below about 180Hz. You can see why we decided to Table 1: full-range driver comparison. All three are rated at 15W, employ a ferrite magnet and suit a 93mm diameter hole cut-out. 48 Silicon Chip Price per pair (approximate) Impedance Rated efficiency (dB <at> 1W, 1m) Free-air resonance Vas (litres) Rated frequency response Measured response (±5dB) Measured THD+N (1kHz, 90dB) Measured SPL <at> 1W, 1m Listening tests All the staff in the SILICON CHIP office were amazed that Fostex FE103En $150 8Ω 89 83Hz 6 83Hz-22kHz 60Hz-15kHz 0.45% 92.5dB Australia’s electronics magazine Altronics C0626 $34 8Ω 95 120Hz ? 120Hz-20kHz 70Hz-15kHz 0.65% 90dB Jaycar CS2310 $25 4Ω 83 122Hz 3.3 90Hz-18kHz siliconchip.com.au these little speakers with 100mm (4in) drivers could produce such a huge sound. Not only that, but the realism, separation and positioning of the instruments and singers are truly first class. When using a quality DAC and amplifier, the music was fabulously rich, and we were able to pick out each instrument as if they were there. But don’t believe us; you be the judge. Build it and enjoy the rewards of listening to a concerto... err... Concreto. It will be worth the effort! Construction Select your concrete bricks carefully. We bought ours from Bunnings, and selected the ones with the smoothest surfaces and minimal cracks and chips. You might want to spend a few extra dollars and buy some spares, because they are heavy and are easily dropped or bumped. Make sure they are completely dry (especially if you take them from your backyard); otherwise, the silicone sealant won’t adhere too well. For the subwoofer bricks, decide which side is to be the front and the back, mark them with a pencil and then grind or chisel 2-3mm from the back of the centre piece as shown in Fig.3. We used an angle grinder fitted with a diamond blade, but you can also use a “scutch” or masonry chisel. Check how much you’re taking off with a straightedge, and use a credit card as a feeler gauge. When finished, you should be able to hold the straightedge across the front surface of the brick and slide the credit card between it and the centre section, where you removed the material. FRONT REAR GRIND OR CHISEL 2 – 3 mm Fig.3 (above): you FROM THIS AREA TO ALLOW will need to remove SOUND TO TRAVEL TO THE PORT some of the cross connector where the rear panel sits to allow air to flow from the driver to the port. It doesn’t have to be pretty because it’s covered by the rear panel. Timber panels We used premium pine planks, dressed all-round (DAR), 184mm x 1.8m x 19mm thick. You can then easily cut these to 184mm and 390mm lengths to make the panels for the Bookshelf and Subwoofer speakers respectively. You may be able to get the hardware store to cut these 184 Fig.4 (below): dimensions of the front and rear baffles for both the midrange and subwoofers. We used 19mm DAR pine but many other timbers could be used. 184 184 92 100 93mm DIAM. 115mm DIAM. 184 92 ALL DIMENSIONS IN MILLIMETRES REAR BAFFLE 92 SUBWOOFER REAR BAFFLE 92 MIDRANGE FRONT BAFFLE SUBWOOFER FRONT BAFFLE 78mm DIAM. HOLE IF JAYCAR PT3012 TERMINAL BLOCK USED, OR 76.5mm DIAM. HOLE IF ALTRONICS P2017 USED 78mm DIAM. HOLE IF JAYCAR PT3012 TERMINAL BLOCK USED, OR 76.5mm DIAM. HOLE IF ALTRONICS P2017 USED 390 92 92 184 92 54mm DIAM. HOLE IF BINDING POST PLATE IS USED siliconchip.com.au 90 54mm DIAM. Australia’s electronics magazine 92 54mm DIAM. HOLE IF BINDING POST PLATE IS USED June 2020  49 THESE HOLES 3.0mm DIAM. 10 35 20 20 30 10 10 THESE HOLES 8.0mm IN DIAMETER 10 (70 x 70mm SQUARE OF BLANK PC BOARD) ALL DIMENSIONS IN MILLIMETRES Fig.5: here's the plate we made to house the speaker terminals on the back panels. We used scraps of blank PCB material; aluminium or other thin (rigid) plates would work! pieces for you; many will do it for free, or a nominal charge. If they won’t, and you don’t want to do it yourself, you could seek out a kitchen cabinet maker, who would surely take on the job for a modest fee. Note that you can use any type of timber which is 19mm thick or more, such as MDF or plywood. But we think the DAR pine looks pretty special in this application. Assembly is pretty easy, but do not rush it and allow plenty of time between steps so that you don’t make any mistakes. Once you’ve cut the panels to size, the next step is to cut the holes, as shown in Fig.4. Ideally, you should use a hole saw for the port holes in the Subwoofers, as they are fully exposed You could use a jigsaw to cut the other holes, as long as you don’t make them too rough, as the speaker surrounds will cover the cuts. Hint: if you are using a jigsaw, cut from the inside of the panels, so any ‘bruising’ is hidden. One important thing to note is that the size of the holes FRONT BAFFLE MIDRANGE DRIVER SEALED MIDRANGE ENCLOSURE in the rear panels vary depending on which type of speaker terminals you’ll be fitting. If you’re building the home-made speaker terminal panels from a piece of blank PCB laminate and a pair of binding posts, cut 54mm holes. If you’re using the Jaycar PT3012 terminals instead, make the holes 78mm diameter, or for Altronics P2017, 76.5mm diameter. Once you’ve made all the cut-outs, sand the port holes nice and smooth, and clean off any burrs from the other holes. You might like to dress the edges with a router or plane. This makes the speaker look much better, although it isn’t absolutely necessary. Next, if you didn’t splurge on the pre-built speaker terminals, make up the connector plates from blank PCB material, and standard binding posts mounted 30mm apart. Make sure you’ve sanded away any imperfections in the panels, then paint, stain or lacquer the panels. We used a red stain and a lacquer finish. Allow them to dry completely, and you are ready to assemble everything. Assembly This is pretty straightforward; you just need to proceed carefully, so you don’t damage anything; especially the delicate speaker cones and surrounds. Start with the backs of the enclosures. Dust off the concrete blocks, make sure they are clean and dry then apply a 3-5mm bead of silicone sealant as shown in Fig.9. Keep the bead close to the inside edge. This prevents it from appearing on the exterior when you press the panel into place. Do not put any silicone on the centre piece at the back of the subwoofer bricks; otherwise, sound cannot travel from the driver to the port! However, you do need to seal the centre section at the front so as to direct all the sound to the back and then around to the port. Use an adhesive type silicone. We used one called Parfix Kitchen & Bathroom Silicone Sealant, again purchased at the local Bunnings. Take your prepared rear timber panel and gently lay it on the silicone bead. Once you are happy with its position, slowly press it down onto the brick. If any silicone squirts out the edges, quickly wipe it away with a damp cloth. Add weight on top (eg, a pile of books) to keep the panel in position and leave it to cure for at least 24 hours. FRONT BAFFLE PORT SUBWOOFER ACOUSTIC WADDING REAR ACOUSTIC WADDING BAFFLE BINDING POSTS REFLEX SUBWOOFER ENCLOSURE ACOUSTIC WADDING REAR BAFFLE BINDING POSTS 2-3mm GAP FOR SOUND TO REACH PORT CHAMBER, ALSO WIRES TO REACH BINDING POSTS Fig.6 (left): the sealed midrange enclosure is made from a “half block” and midrange driver as this semi-section shows. Fig.7 (right): the ported subwoofer is made from a “full block” and subwoofer driver with a tiny air gap between the halves. 50 Silicon Chip Australia’s electronics magazine siliconchip.com.au PINE TOP/FRONT CONCRETE C ONCRETE BLOCK B LO CK SILICONE BEAD (NOT TOO CLOSE TO THE OUTER EDGE) Fig.8 (above): front side view showing how the “baffle” is secured to the concrete block. Make sure you use plenty of silicone so the join between the block and panel is airtight . Fig.9 (below): similarly, here’s how the rear panel is attached to the block. Make sure air can flow between the two block halves, as explained in the text . Make sure its position is correct because once it cures, you will not be able to shift it! Repeat this procedure with all the other enclosures. After 24 hours (or more, if you are in a cold climate), repeat this procedure with the front baffles (see Fig.8). Just remember to add the silicone across the centre of the subwoofers this time. After another 24 hours, you are ready to mount the drivers. But first, cut 60-70cm lengths of speaker wire and solder them to each driver. When the silicone on the cabinets is completely cured, pack the subwoofers with acoustic wadding. We used Acousta-Stuf Polyfill, from Parts Express; see: http:// siliconchip.com.au/link/aayq It is also available from Jaycar. Pack this in loosely behind the drivers. In a pinch, you can also use small (dry) towels. Push the speaker wires through so that they are sticking out the hole in the back panel, then mount the drivers using wood screws. You’ll get the neatest result if you first mark and drill small pilot holes, using the driver surrounds as templates. Try to orientate the drivers all the same way; it generally looks best to have the screws in the diagonal corners, as shown in our photos. Solder the wires sticking out the back of the enclosures to the inside of the speaker terminals, then mount those terminals on the rear panels in a similar manner. Your speakers are finished! Note that you can stack the smaller speakers on top of the subwoofers, or you can locate them separately. The placement of the subwoofers is not critical. Depending on the surface your speakers are going to be PINE REAR PANEL CONCRETE C ONCRETE BLOCK B LO CK siliconchip.com.au DO NOT PUT SILICONE BEAD IN THIS AREA, TO LEAVE A SMALL GAP BETWEEN UPPER & LOWER CHAMBERS Front view showing the baffle secured to the half block and the acoustic wadding inside. The four screwheads could be painted black so they don’t stand out as much. Australia’s electronics magazine June 2020  51 The finished midrange speaker, here shown from the under-side, with a 170 x 170mm piece of thin felt glued to the block to ensure it doesn’t scratch underneath surfaces. placed on, you might like to glue a 190 x 190mm square of felt or similar protective material on the underside of each of the blocks. Concrete scratches most other surfaces quite nicely! Driving them As the drivers in the smaller bookshelf speakers are fullrange units, and the subwoofers only respond to bass frequencies, you can drive each pair from separate amplifiers. Parts list – (for one pair each of Bookshelf speakers & Subwoofers) 2 full Besser blocks, 390 x 190 x 190mm    [eg, Bunnings 3450457] 2 half Besser block, 190 x 190 x 190mm [eg, Bunnings 3450458] 2 DAR pine planks, 184mm x 1.8m x 19mm 2 100mm (4in) 15W 8Ω twin cone speakers [Altronics C0626] 2 5in (125mm) woofer/midrange speakers [Jaycar CW2192] 2 pairs of pre-mounted speaker terminals [Altronics P2017, Jaycar PT3012] OR 2 pairs of red/black binding posts AND 2 70 x 70mm squares of fibreglass laminate (blank PCB material) 16 20mm-long wood screws (eg, No.9/4.5mm thread) 1 3m length medium-duty speaker cable (figure-8) 1 pack of acrylic speaker damping material [eg Jaycar AX3694 or from Parts Express; see text] 4 squares protective felt (or similar), ~170mm x 170mm 1 tube of neutral-cure clear silicone sealant 52 Silicon Chip Similarly, the completed subwoofer, here seen from the back to show the small plate housing the terminals. We suggest that you don’t wire them in parallel as the Subwoofers have lower sensitivity than the Bookshelf speakers, and therefore require a slightly higher signal level to get matching levels. Two stereo amplifiers, each capable of 25W into 8Ω or a bit more into 6Ω should do the job. The Subwoofer drivers can handle up to 50W each, so if you like playing really bassy music, more powerful amps are the go. But you’re likely to get more power into the Subwoofers anyway, given their slightly lower impedance (6Ω vs 8Ω for the Bookshelf speakers). The Class-D modules we mentioned earlier are suitable, as long as you power the ones driving the subwoofers from a sufficiently high supply voltage (20V+). Connect the amplifier outputs to the four sets of speaker terminals, then use RCA Y-cables to connect the outputs from your preamp to the left/left and right/right pairs of power amplifier inputs. You can then play some music and adjust the individual amplifier volume controls until the bass and treble levels sound well-matched. We allowed about two hours playing various types of music at reasonable volume to “run in” the speaker drivers before we took measurements; you may find that these drivers are a little stiff straight out of the box. You should notice an improvement in the sound with time, as you use them, especially in the bass response. SC Australia’s electronics magazine siliconchip.com.au