The speaker used, the Dayton Audio RS100-4, had been briefly introduced in the article dedicated to the sizing of the transmission line. It is a 4-inch wide-band, rigid aluminium cone and 25mm long excursion voice coil. The declared frequency response (80-20000 Hz) allows it to be used for a one-way design. The black anodized cone, solid aluminum center phaser and 6-hole die-cast basket make this component modern and attractive. Although it belongs to the Reference series of the American brand, the price is quite inviting: about forty euros for the single driver.
The cabinet is made of 16 mm thick MDF. For those who do not know, the abbreviation stands for Medium Density Fiberboard: an agglomerate of pressed wood fibres that is often used for the construction of loudspeakers thanks to its excellent characteristics of dimensional stability and homogeneous composition. The smooth surface and the absence of grains make it particularly suitable for lacquering, which is precisely the finish I chose for the monitors. Another interesting property of this material is the ease of machining and this is even more true in the case of milling, both manual and with a CNC machine.
I made all the cuts and milling with a 1050 watt Kress electric tool mounted on a CNC pantograph which, although hobbyist type, has a strong anodized aluminum structure and recirculating ball-bearing guides on the three axes. The 400 x 620 mm workbench is quite limited, but it still allowed me to work several panels at the same time (Photo 1).
As can be seen in Photo 2, the side panels constitute the entire sides of the enclosure and contain the milling slots (6 mm deep) to house all the other panels. The building plans that I propose (Fig. 1) are slightly simplified so that they can be made from the wood cutting service offered by practically all DIY stores. In this case the “full size” panels are the front and back. The internal panels, as there are no milling cuts, must be 110 mm wide.
The only difficult steps for the hobbyist could be the inevitable holes for the speaker (and any flush mount milling) and the opening. There are many tutorials on the web to make these workings with a manual electric milling machine. For the loudspeaker a compass guide may be sufficient, while for the opening I suggest the construction of a jig and the use of a milling cutter with bearing.
I glued the panels with vinyl glue following a rule learned a long time ago from a carpenter friend: the glue that holds is the one that comes out. This doesn’t mean spreading glue all over the laboratory; with a bit of experience you can get the minimum necessary overflow, maybe applying the glue with a brush. The excess glue must be removed promptly with a cloth. It is essential to have a good number of clamps of the right size and some pieces of wood (even from scrap) to be placed between the clamp and the panels.
In Photo 3 you can see a test assembly of the various panels; note the milling also on the top, front and rear panels. I suggest to carry out the operation in two steps, gluing first the internal, upper and lower panels and the side panels and, in a second step, the front and rear panels. In this way you have time to place the foam inside the box as shown in Figure 2.
Although with a slightly different sequence I also did the gluing in two steps (Picture 4). Each bonding step takes at least 24 hours to dry. In order not to introduce leaks in the TL and given the very short route I did not drill holes to pass the connection cable. Although the openings for the loudspeaker and the terminal block are quite large it would be preferable to insert the cable (2 x 2.5 mm) before closing the cabinet completely.
If you intend to veneer the case, no special finishing operations are required, but only good sanding to eliminate any steps in the panel joint. If, on the other hand, you want to proceed with rounding the edges (Photo 5 and 6), and with lacquering, you need an orbital sander, a good jar of wood filler and fine sandpaper.
Before starting the work I prepared some cardboard caps to prevent dust and paint from dirtying the material already placed inside the cabinet. I first milled all the edges with a 6.35 mm concave radius cutter with a bearing above the cutting edge and then milled the edges of the side panels (excluding the one on the bottom) with a 45° 45 mm cutter also with a bearing above the cutting edge. On the front panel I started the milling immediately above the opening. The result is shown in Photo 7.
These last two machining operations were carried out with a spindle moulder milling machine. In Photo 8 you can see the main milling cutters used; the concave radius cutter is mounted on the spindle moulder shaft.
For the final stages of the finish, which requires several coats of primer and paint, both polyurethane based, I turned to a professional lacquererer.
The loudspeaker and the terminal plate are fixed with M4 screws, with Allen head, screwed on mother screws inserted in the wood from the inside (Photo 9). The holes for the mother screws, which I did not dimensioned as they could differ according to the type used, are 6.5 mm. I prefer those with flat head and hexagonal insert, which I bought from an online hardware store.
The accessories used are Dayton Audio and below are the codes:
Binding post plate: SBPP-BK
Premium binding posts: BPP-G
Set 4 black speaker spikes: DSS2-BK
The dimensions of the driver are shown in Figure 3, while for the binding post plate, if you want to flush mount it, I suggest to refer to the plans (the technical drawing downloadable from the manufacturer’s website shows an incorrect radius of curvature). The holes for the nuts of the spikes are 5/16 inch, but an 8 mm drill bit is more than fine. For stability reasons I used only 3 spikes per box and drilled the holes 2 cm from the edge (Picture 10). These last ones, involving also the front and back panels, have to be made when the gluing is finished (unless you have a cnc router and intend to build the cabinet as in the original design).
The material used to dampen the line is 30 mm thick open-cell polyurethane foam. I bought it from a local dealer, but it is easily available online. I have tried several glues and found that a simple vinyl glue provides an excellent sealing result without affecting the material at all. If you have a staple gun you can use it to hold the material in place where it tends to lift. To avoid this inconvenience I cut the foam with a band saw and left the width slightly wide so that the material would remain “stuck” between the side panels making the gluing phase much easier.
For aesthetic reasons, I “closed” the end of the duct with a rectangular 1 cm thick pore-crosslinked polyurethane foam. This is a material with very low flow resistance commonly used for air filtration and which, if necessary, can be washed with soap and water. I had prepared a groove to position this material 1 cm inside the front panel (Photo 11), but to cover a defect in the lacquering I had to position it practically flush (Photo 12).
I placed the speakers at the ends of a solid 1.5 m wide TV cabinet. I borrow a comfortable armchair from the children’s room and sit just under two meters away with the speakers at ear height and oriented towards the listening position. The small size of the monitors, and the close listening, make the placement in the room much less critical than with large speakers. I have a whole afternoon to spare and already have a few titles in mind: I start with Café blue by Patricia Barber. In Mourning Grace it is immediately evident how the sound engineers wanted to paint the soundstage, with very wide drums acting as a stage for the other instruments and Patricia Barber’s persuasive and sometimes hissing voice. In the minimal Inch worm and Ode to Billy Joe I appreciate the contrast between the naturalness and reverberation of the voice. The snaps of fingers are compact and clean, right in front of my nose. I enjoy the enchanting Too rich for my blood and Nardis’ captivating drum solo without taking notes and I switch to Tinariwen’s Aman Iman. Here, the realism of the voices, thanks mainly to the good recording, becomes really remarkable, but what impresses me most is the stability and height of the soundstage. In the pizzicato, the high register of the acoustic guitar appears realistic and complete. By now I’m in the desert with the Tuareg (or Imazighen, free men, as they prefer to be called) and I listen to the whole CD. I switch to the piano with Solitude by Solal Martial and once again these little monitors surprise me for the transparency in the middle range. I wonder how they do with the saxophone and decide to listen to a recording of Danish SteepleChase that I particularly like: Trouble in mind by Archie Shepp and Horace Parlan. Suddenly I remember that I should evaluate this project mainly for the particular type of loading of the woofer and its low frequency performance, but I realize that it is the mid-high range that continues to capture my attention. I still have some time and listen to some tracks from a collection of Queen’s and in fact it comes out a very well damped punch, definitely monitor oriented. Sure, no hits in the stomach, but the bass is extended enough to make you suspect the presence of a small subwoofer. I’m definitely satisfied with this listening session, not at all fatiguing, and with the surprisingly well-balanced performance over the entire frequency range that can be reproduced by the small transducer; after all, it is a component of the Dayton Audio Reference series. What impressed me the most is undoubtedly the sound stage, wide, especially in the vertical dimension, and very stable. I suppose this last characteristic is to be attributed to the point source which is particularly appreciated in the near field, where the distance between the speakers starts to be significant compared to the listening distance. While I’m reflecting on how little is enough to set up a sound system with a correct and pleasant sound, a sentence by Gian Piero Matarazzo during the listening test of the Revel Performa F288Be (AUDIOreview n.407) buzzes in my head: “what if the natural music doesn’t include the tweeter at all?