Armed with @Dave Merrett's inspiration I fired up OpenSCAD and created a first -guess model that could be 3D printed. At present this an octagon with eight cavities.

The purpose of this first prototype is two-fold:

• exercise OpenSCAD and the 3D printing process for our desired shape of object, and provide a center of attention for discussion. Having something in your hands is a great prompt for a brainstorming session

• actually test it with our measurement scenario

I started out using Helmholtz' law and naively created spherical ended cavities and varied the area of the inlets to change the resonant frequency of each cavity. It is likely that those decisions are actually incorrect, and I should instead have adjusted the cavity volume itself, as the cavity is the primary resonant structure. On reflection we the cavity should have a perpendicular cylindrical end rather than semi-spherical. Further experimentation will certainly help.

This first prototype was printed using ABS. This was also a mistake, because the 3D printers we have access to need a lot of coaxing to print anything large in ABS, and in this case, the prototype has warped, although it is intact enough to be able to start characterisation with. Being able to print in ABS is probably essential eventually for improving outside use robustness, but we need to get the concept proven first, so the next several prototypes will be printed using good-old PLA.