ShapechangingInstruments.com is always looking for new sources of inspiration. Here are some of the lesser known areas of acoustics that can help inspire.
Wikipedia defines it as the investigation of sound production, dispersion and reception in animals. (including Humans) Animals serve as good inspiration for thinking about new ways to generate sound, and also process sounds. Snapping shrimp work with the relationship between sound, pressure and heat to create a unique and awesome effect.
Thermoacoustic engines and refrigerators work on the relationships between temperature, density and pressure variations on the acoustic longitudinal waves. For thremoacoustic engines, a heat source is used to resonate a vessel. That vessel works as a resonator and has an elastic band across its opening which can be used to drive an engine. You can even get a kit, Thermoacoutsitc Stirling Engine, which displays this property. Penn State is one of the leading research facilities in thermoacoustics.
Photoacoustic spectroscopy is the measurement of the effect of absorbed electromagnetic energy (particularly of light) on matter by means of acoustic detection. This is similar to ultrasound, but newer in the medical area. The absorbed energy from the light causes local heating and through thermal expansion a pressure wave or sound. This can be used to evaluate materials behind a surface layer.
Acoustic ecology is the study of the relationship, mediated through sound, between living beings and their environment. The World Listening Project is the best place to hear field recordings from around the world. It opens to the door to consider how sound interacts with life everywhere.
The study of sound underwater is complicated by the variable sound of speed in water. Pressure also varies by depth within the water column. The higher speed of sound in water allows sound to propagate much further, with documented studies measuring sound across distances of hundreds of kilometers. Additionally, the multipath nature of sound means echos and distortions arrive at multiple times.
Aeroacoustics is the study of how air flow transforms into noise. The Lighthill analogy provides a mathematical understanding of how fluid flow can transform into noise. It is built on top of the navier-stokes equation for fluid dynamics.
There is ongoing research on how music effects human health. This research is on the fringe of academic acoustics. For underwater sound, it is possible to generate sounds that resonate the lungs of human divers. This can be used to discourage divers from an area, and at larger amplitudes, it can damage harmful. The coupling of sounds into the body is more difficult in air than in water, but internal organs do have resonant frequencies