The first version used internal ribbing and bitumen paint to reduce enclosure resonance. The second version used an external carbon fiber shell. Both approaches were a bit cumbersome. For the third version we wanted to fully use the capabilities of 3D-printing. Therefore, a high-density metal-filled filament was used and internal gyroid-shaped support was used even where overhanging surfaces would not have required it. In addition, height and tilt can be adjusted using three threaded rods that form a tripod. The finished enclosure with three 14 mm trapezoid-threaded nuts bonded to it weighs 1.2 kg.
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Using additive manufacturing (AM) has many benefits over traditional construction methods, such as design freedom, fast product development and integration of functions into one part. There are drawbacks as well. The plastic AM parts tend to be low in mass and not very stiff. Air-tight walls are sometimes difficult to achieve, too. Adding mass by increasing fill density of the print is not a good solution, since it adds build time and material cost. Stiffeners and bitumen paint were used in (Version 1). However, the stiffeners were cumbersome to paint with bitumen and it did not add significant weight. For Version 2, we used the vent as a part of the mechanical structure and used a thicker wall. Dry carbon fiber tow was wound around the enclosure and then wetted with epoxy resin resulting in a unique unidirectional carbon fiber surface finish. The composite shell adds mass and stiffness to the enclosure. The loudspeaker sits on four feet printed from TPU material, which allows rotating the speaker.
The following changes were made to Thingiverse:
Updating the driver dimensions and screw pattern to the latest Alpair 7 MS.
The weight of the loudspeaker will try to bend the speaker stand. It was therefore changed from a shell-like structure to a solid.
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The first version of 3D-Fi speakers are spherical (180 mm diameter) with 3 liter internal volume. The box is vented with two rectangular ports on both sides. The ports act as stiffeners and also give more space to assemble the connectors and amplifier inside the enclosure. Internal wall stiffeners are used in order to maximize internal volume as opposed to simply increasing wall thickness, since a small external size is typically desired while internal volume needs to be high enough for the driver to work properly. We use bitumen paint to both seal the enclosure and also to add mass. The enclosure is printed as one part using UPM Formi3D cellulose composite filament. Metallic nuts are pressed on the backside of the flange to receive the machine threaded screws that hold the emitter. Our setup uses a laptop PC as a source which allows equalizing the frequency response at the digital source. The signal is transferred via USB to a USB-powered DAC/pre-amplifier with volume control and a power switch. The analog signal is then transferred to one of the speakers where it is amplified using a two-channel chip amplifier board that is powered by a 65 W laptop charger. The amplified signal of the other channel is then transferred to the other speaker for reproduction. The design uses 6 cm full-range emitters without any analog filters or corrections.
A reference system with Genelec 8040 speakers was used for comparison. The sound of the 3D-Fi speakers is very unique and quite tricky to get the most out of. The full-range emitters are very sensitive to off-axis listening and the listening distance also changes the sound markedly. Even slightly tilting one’s head has an effect. It seems that finding the best spot is challenging and takes time. It’s hard to remember not to move an inch while listening to these speakers. However, all the effort pays off, because there is a reward at the end. The sound stage is unbelievably good and there is a huge presence from such a tiny speaker. The Genelecs sound distant and all over the place compared to the very precise and point-like sound of the 3D-Fi speakers. Obviously the bass is not very deep and the sound pressure levels achievable with 6 cm cones is limited. On the other hand, they seem to tolerate significant bass boost without distortion and in normal listening the cone travel stays in check. Overall, a very difficult set of speakers to get into and they are quite picky with the type of music played, but once a good recording is found together with the right listening conditions…Bliss!
Acknowledgements: We wish to thank UPM for the materials and support. J-P Virtanen took the studio pictures and Markus Markkanen the ones in the library.