Now, this is a great title. It sounds like something out of the Twilight Zone or a 1950’s Japanese Science Fiction monster movie where Godzilla meets Von Helmholtz. Well, that’s partially correct- it is something from the University of Tokyo in Japan, and the history is well known. It has to do with everything that has associations with wavelength resonances. This includes all musical instruments, except for percussion.
Acoustic levitation, as the name suggests, is the use of standing waves- specifically nodes- to lift something up. This “something” needs to be small- on the order of less than 8 mm in length and less dense than 1000 kg/m3. But this “something” can include small electronic components, medicines, and other light materials that should not come in contact with the greasy palms of humans. It would be ideal for space experiments in microgravity or in medical research.
Unfortunately, it won’t be of much use at parties to get rid of unwanted guests, especially fat ones.
Acoustic levitation works in a similar fashion to many musical instruments. Musical instruments generate a series of tones (called harmonics). As long as there is a “boundary” at the end of the musical instrument, or as long as there is a reflective source like a wall or a ceiling, the sound is bounced back to the source, going in the opposite direction. When two sound waves of similar frequency pass each other going in opposite directions, like waves at sea, there will be peaks where the sound waves add up (constructive interference) and locations of no net movement, where the sound waves cancel themselves, called nodes. In the case of musical instruments, these sound waves cause peaks and nodes inside the instruments themselves because the outputs of these instruments are acoustic boundaries.
A cork (left over from your favorite wine bottle) floating at sea will move up when the waves collide and add up constructively (in phase). The cork will bob up and down between peaks and troughs with nodes in between the troughs and the peaks.
In a laboratory, the nature of the interaction of the incident or initial sound waves, with the reflected sound waves that are now moving backward towards their source (the loudspeaker), can be precisely controlled by adjusting the phase, or start point of the sound. With enough control, one can create a node – a location of no air particle movement – in a place so that something light, such as a cork or a small electrical component, can be moved frontwards and backwards, and from side to side, while it rests comfortably on a standing wave node.
Researchers at the University of Tokyo have accomplished this feat in three dimensions, and now the cord or small particles can be moved backwards, forwards, sideways, and up and down.
It would be a neat experiment to see if an actual symphonic orchestra can be manipulated sufficiently with the correct amount of sound energy to be able to move very light particles around in front of them. This is not a Capstone essay waiting to happen for any intrepid researchers out there since this experiment would be ruined if someone moved slightly in the audience or if a door was opened.
Standing waves (used with permission from Wikipedia) are not just found in rooms or specially designed laboratories, but in all musical instruments and in our own vocal tracts. Standing waves have resonances associated with them and these resonances define the properties of the musical instrument or the vowel or nasal being articulated.
The length of my own vocal tract is about 17 cm, and given the average temperature and pressures where I live, the speed of sound is about 34,000 cm/sec. That means the first resonance associated with a quarter wavelength standing wave in my mouth as I utter the low back vowel [a] as in father’ is 500 Hz. And for those who like to do calculations, it is 34,000/(4 x 17) = 500 Hz.
Another standing wave in my vocal tract, also when articulating the vowel [a] is at three times 500 Hz = 1500 Hz. These two resonances (at 500 Hz and 1500 Hz) define the vowel [a] as indeed [a] for all to hear and understand.
So, standing waves are everywhere. They can be used for speech production, musical instruments, outer ear resonances (also a quarter wavelength standing wave generator), and, in the proper hands and condition, even for acoustic levitation.
In case you were wondering, von Helmholtz would whip Godzilla any day of the week, probably using acoustic principles and other neat stuff.