As discussed earlier, the two worst enemies of loudspeaker builders are cabinet vibrations and sound emanations. Aside from choosing the right building materials, a few other measures can help you deter these troublemakers from their task and make them work for you.
From a board cut exactly according to the inside dimensions of the cabinet, and not necessarily (in fact, preferably not) made from the same material as the cabinet, the inside is cut out in such a way that only a 2-3-cm edge remains (depending on the size of the box). This board is glued a few centimeters above or below the center of the box, on the inside. It divides the cabinet walls into various-sized sections, whose respective resonances are then shifted to various higher frequencies and are no longer affected by the bass (in a large box) or the bass mid-range speaker (in a small box). For tall boxes, several ring reinforcements can be used. The gross box volume must be increased to account for the space taken up by the ring reinforcements.
Naturally, the matrix of the Duetta is also a kind of ring reinforcement. It is just a little more complicated. That’s why it is reserved for do-it-yourselfers (at least for affordable projects) who are willing to spend a few extra minutes to achieve better results.
Soft-fiber insulation board
Boards made of wood fibers and binding agents, pressed loosely to widths of 5 to 12 mm, are sold in home improvement stores as 1.0 x 1.5 meter boards. Now they are also available as mats for laminate or parquet flooring, and many hardware stores will cut them to size for you at prices ranging from 3 to 10 euros per square meter. Glued to the inside surface of the cabinet walls, they form a sandwich bond with the wall board, creating a slightly reduced resonant tendency. The insulation plates should be cut about 3 cm narrower than the interior of the box so that they don’t bump into each other. You can cut them using a box cutter and a wooden board or a metal straightedge. Once you have notched it by running the blade along the length of the metal straightedge, it is easy to snap off the soft-fiber insulation material to the right size. You don’t need to increase the box volume when you use soft-fiber insulation boards because the sound velocity is (virtually) decreased in proportion to the space that they take up.
Stores offer a wide range of insulation materials that can be inserted into the box and that will dampen various frequency ranges according to their density. They are primarily used to suppress “standing waves” created in the cabinet (these are sound waves whose length corresponds exactly to the distance between two cabinet walls; they reflect back on each other and create a droning sound in the walls). They can also be used to suppress mid-range tones that are radiated from the speaker into the box. Depending on the material’s density and how much is filled in, the cabinet will need to be 10 to 20% smaller, since deflections in the insulation material virtually reduce the sound velocity by that percentage. For tested assembly suggestions, this cabinet reduction is already included in the calculations (or at least it should be).
The construction plan for the box should (hopefully) tell you where to put in which insulation material. If this is not the case, you can use the following rule of thumb: closed cabinets are completely filled with loose insulation; bass reflex boxes should always leave the area behind the reflex channel clear. Exceptions to these rules will always be noted by the developer.