There are quite a few rather unconventional methods of propulsion, but perhaps one of the more curious approaches involved Helmholtz resonance, as demonstrated by [Junsun Hwang] et al. with a paper in Science Advances and associated summary article by EPFL’s School of Engineering.

Although probably better known from something like musical instruments, Helmholtz resonance can be used for more than creating or deadening noise. If stimulated with an external acoustic source that matches the chamber’s resonance frequency the result is a jet of air at the neck of the chamber. This acoustic actuation can thus be used for a number of applications.

In the paper a number of such applications are demonstrated, including a boat with three of these chambers for propulsion and steering, as well as a microflier (see above image) that when placed above an ultrasonic phased area will hover due to the production of this jet of air.

This microflier concept was then adapted with angled resonator chambers so that they could drive a propeller. Naturally, the produced thrust is only a fraction of a Newton so it’s essential to make these structures as light as possible, in the order of micrograms. These microfliers were created using high-resolution 3D printing, with a few iterations attempted to determine the optimal configuration.

In the case of the boat the ultrasonic transducers were directly placed on the bottom of the resonance chamber, but in the case of the microfliers the weight limitations necessitate these transducers to be external. Even if not the most practical kind of flying robot, as a demonstrator of this application of Helmholtz resonance for acoustic propulsion it’s pretty cool.

Most instruments, with maybe the exception of pianos, have consumable parts. Guitars need string changes, bows need rosin, saxophones need reeds. [Co:Creation Lab]’s balloon banjo requires both fishing line and a party balloon, lest it be semi-silent.

If you want to change the resonance, just increase or decrease the amount of air inside the balloon. Neither the balloon nor the strings need tools to adjust, but we might argue that tuning pegs are tools.

The balloon banjo was the result of a design constraint: a parent should be able to build it with their child. To that end, there are no metal screws, no recurring snap-fits in the assembly, and a single, internal, one-time-use snap fit for the adult’s tactile satisfaction. Instead, the balloon banjo uses 3D-printed screws. A six-year-old can turn them with their fingers, and it’s difficult to over-tighten them and strip the plastic threads.

Be sure to check it out in action after the break. If you would rather use extant plastic to make music, the Bottlephone 2.0 is calling.