Circular economy - RD Physics

What’s wrong with having batteries in your portable boombox?

Wireless electronics, such as bluetooth speakers, are extremely popular nowadays. All such devices must have a power source and typically it is a lithium-ion battery. However, the demand for battery raw materials is rising at an alarming rate:

The supply of some of these [battery] materials, in particular cobalt, natural graphite and lithium, is of concern today and for the future in view of the large quantities needed and/or very concentrated supply sources.

European Battery Alliance (EU)

As we have discussed earlier, when launching our Circular Sound program, the best solution for reducing reliance on critical raw materials is to reduce their use. The RD Physics BB1 boombox enables you to do just that. It is designed to use existing external power sources and therefore no new batteries are needed. Battery service-life or battery replacement is no longer a concern.

Alternatives to dedicated batteries

The BB-project started by looking at how power tools are sold without accompanying batteries. The idea being that the user needs only one battery (plus spares) that fits all tools. While this approach reduces the amount of batteries needed, it is also used to tie the customer to a specific brand. We wanted a universal solution and therefore the USB-C standard was chosen. The BB1 and BB2 boomboxes use a USB-C port as an interface to feed power to the amplifier. The boombox can be connected to any USB port: power banks, phone chargers, laptops, extension cords, solar panels etc. Obviously, the input voltage and current draw is limited, which leads to limited sound pressure level (SPL).

The weather-proof USB-C port is located at the top.

Frequency response of BB1 at maximum drive level.

BB2 boombox with Dayton Audio RS100 drivers.

Frequency response of BB2 at arbitrary drive level.

Components

What you will need to build your own batteryless boombox:

Geometry files for 3D printing (free under Creative Commons License at Thingiverse)
3D printer big enough to fit a 235 mm diameter sphere
Slightly over 1 kg of filament depending on your settings
Two active drivers. Either Peerless 3″ (BB1) or Dayton Audio 4″ (BB2)
One 6½” Dayton Audio passive resonator
A Sure (Wondom) bluetooth board with additional cables set
USB-C panel mount plug (from eBay) and 6 mm DC plug
Wood screws (4.2 mm for the drivers and resonator, 3 mm for the BT board)
Drawer handle, IKEA Eneryda 703.475.16
Damping material (bitumen or similar automotive damping mat and fibrous wadding, for example pillow stuffing)
Optional: Wall mount bracket, Genelec 4000-410B
Minimal soldering capabilites

The enclosure for the BB1 and BB2 can be downloaded from the link above. Assembling everything takes 30 minutes.

3D printed enclosure ready for assembly.

How to build the BB1/BB2 bluetooth speaker

Start by soldering the 6 mm DC plug to the USB connector. Red (+) goes to center pin and black (-) to outer shell.
Connect DC power and speaker cables to bluetooth board and fasten the board inside the enclosure by tightening the screws via the driver openings.
Mount the USB connector and handle.
Line the inside of the enclosure with bitumen or similar visco-elastic damping material. Heat will aid in conforming to internal shapes. Make sure the damping material is fully bonded to the walls.
Bring the speaker wires through the driver openings and solder them to the drivers. Make sure polarity is the same for both drivers. Then fasten the drivers using wood screws.
Fill the enclosure with fibers (cotton, polyester, wool etc.) and fasten the passive resonator.
Optional: Attach the wall mount bracket.
Connect a USB port and the bluetooth board powers on automatically. Pair your signal source with the device (“WONDOM”). Enjoy!

BB1 ready to rock.

Assembly instructions

Concept and sound test

RD Physics CX1 – A coaxial loudspeaker

RD Physics has been developing speakers with full-range drivers for some time now and while they have their inherent benefits, it is time to look what coaxial drivers have to offer. The starting point was a spherical shape, which is known for its benefits. However, the limitations in build volume favored a shape closer to a rectangular cuboid. The shape of the CX1 has the largest possible roundovers, with the constraints imposed by driver size and maximum baffle dimensions. This is to reduce edge diffraction. The sides are compound curved to maximize stiffness. There is also internal ribbing to stiffen the enclosure without taking up internal volume like a sandwich structure would. The enclosure is made in two parts; the front baffle has a separate cover that conceals the driver flange and mounting screws. The driver is a proprietary SEAS unit designated MP15 (15 cm diameter). The idea is to have an external active crossover and bi-amp the loudspeaker via the Neutrik SpeakOn 4-pin connectors at the back.

3D printing a loudspeaker using Desktop Metal Forust method

The geometry files were sent to Forust for 3D printing. The chosen colour is “natural” with the artificial wood grain introduced during manufacturing. The result is a structure that looks like plywood. Parts can be ordered without the grain and with darker colours, too. The grain is more interesting, however, because various surface texture effects can be achieved by aligning the layers at low angles relative to the principal axes of the printed shape giving a zebra stripe effect.

Post-processing of 3D printed wood

The parts printed with the Forust method can be sanded smooth, but it is not like sanding natural wood. The surface can be varnished, but not stained. The Forust material does not absorb wood stain. It does not tolerate ethanol and perhaps other solvents either. Long-term exposure to water should be avoided, otherwise there will be is a sticky brown residue on the surface. Although a wooden look can be mimicked, post-processing is not similar to wood. Instead, it resembles the wood-filled polymers used in our previous builds. This is not a serious drawback, it just means that 3D printing skills are more useful than woodworking skills. In terms of aesthetics this is the closest thing available for increasing the acceptance of 3D printed loudspeakers in the audio community, where wood veneer is the go-to solution.