Engineers in Japan have figured out how to pick up objects from reflective surfaces using acoustic levitation. Although they can't yet do so reliably, the advance could help unlock the full potential of the manipulation of physical objects using nothing but sound.
Biomedical engineering, nanotechnology and the development of pharmaceuticals are some of the fields in which manipulating objects without touching them is potentially really useful. We can already do this with a technology called optical tweezers, which use lasers to generate sufficient radiation pressure to levitate and move extremely small particles.
Acoustic tweezers - where pressure generated with sound waves can be used to move particles - have the potential to be an even more powerful tool. They could be used to manipulate a wider range of materials, and at larger sizes - up to the millimeter scale.
However, despite being first discovered in the 1980s, there are significant limitations hindering acoustic tweezers from broad practical application. To start off, you need a reliable 'trap' made up of sound waves.
Hemispherical arrays of acoustic transducers can be used to create the sound trap, but controlling them in real-time is tricky, since you need to create just the right sound field to lift an object and move it far from the transducers.
It gets even trickier if there is a surface that reflects sound, since this can complicate the sound field.