Floating things with sound might seem like science fiction or something you could only find in a physics lab, but actually many people have shown that its easy to diy. There are simple designs:
and there are not quite as simple designs:
In this project I'm aiming for something in-between along with some cool demonstrations. If you just want a quick overview then check out the post on my website.
The physics behind acoustic levitation gets a little complicated but the general idea is that if you take a sound wave and reflect it back into itself, this will setup a stationary wave. Somewhere along that wave there will be a condition (or several of them) where the forces are directed towards a single point, and if the acoustic power is high enough, this can overcome gravity and we get stable levitation.
Step 1: The Components
The frequency of sound used to levitate objects must be high enough so that the object doesn't fall out of the levitation zone (also called pressure node) during a single cycle of the wave. Therefore these frequencies are typically in the ultrasound region(< 20 kHz), so we need a special speaker to generate the sound. Luckily these can be found pretty cheap as they are used for range finder applications. These speakers are quartz crystals which usually resonates at 40 kHz, and the cheapest place I found to get them is to desolder them from cheap HC-SR04 range finder modules, but you can also buy them directly. If you want more power then you can buy higher quality transducers from farnell, I measured them to have twice the acoustic power.
Next we need a way to generate the 40kHz signal to power the speakers. If you have a signal generator then set it to the resonance frequency and crank up the amplitude as high as it goes. If not then you can simply use an arduino. It is possible to power the speakers straight from a digital IO pin however the 5V amplitude does not generate much power, so I would recommend using a step up converter module to generate 18V and power a mosfet driver IC such as the TC4427. The will turn the 40KHz 5V signal into a 40kHz 18V signal which can then power the transducers.
To double the power you can use two speakers facing each other. I designed a 3-piece jig to hold the speakers and change their separation by turning the center screw. It turns out that this has very little effect but it did make for some nice results. The jig can be 3D printed and transducers glued on with hot glue.
As you can see the wiring it pretty simple and there is no need to worry about the polarity of the transducers here as I will explain in the next section.
Step 2: The Setup
To test the setup you can connect one of the transducers to act as a speaker, and the other to act as a microphone which can viewed on the oscilloscope.
The wavelength of the sound can be calculated by λ = c / ν, which is ~0.9 cm for a 40 kHz wave travelling at 340 m/s. This means that the levitation zones are spaced out by roughly 5 mm.
You can see that on my setup, the screw has a pitch of 1 mm so roughly 4 turns causes 180 degree phase shift between the transmitted signal and the received signal.
Initially I thought that the transducers would have to be a set distance apart so that they would constructively interfere to double the output power. However, It turns out that this is not the case. When waves interfere, the energy is not lost, if there is destructive interference then somewhere else there will be constructive interference.
For levitation this means that the transducers can be placed any distance apart which is vaguely close together (a few cm) and this will be good enough for levitation.
Step 3: Floating Things
The best things I found to levitate are polystyrene bead. When the setup is switched on, placing the beads in between the transducers will cause them to get pulled inside and they will appear frozen in mid-air!!
This is an absolutely stunning demonstration and does not fail to amaze anyone from kids to professors.
Another amazing demonstration is placing a piece of dry ice between the transducers. The dry ice spills out cold air which is drawn into the low pressure nodes and condenses out water vapor.
Step 4: Floating Sound With Sound
Now this is the not quite so simple acoustic levitation, using two of these rigs its actually possible to make a speaker in which the air itself is the diaphragm. Imagine taking a pressure node from one rig and moving it back and forth using a second pair of transducers. The pressure node can now act as the diaphragm in a speaker.
You can see how I constructed a setup to try this; the two pairs of transducers are perpendicular to each other, one is driven from a constant 40 kHz source (arduino) and the other from a function generator. There is also a microphone next to the setup to record the output sound.
Using an oscilloscope, the frequency of the yellow pair is measured to be exactly 40,006.8 Hz. My signal generator has a resolution of 1 Hz so I set it to output three different frequencies to try and generate three different tones.
The frequencies are :40,007 Hz, 40,010 Hz and 40,030 Hz, and you can see the traces captured on the oscilloscope.
I should mention here that the transducers on their own do not make any sound that you can hear, but have a listen to what the microphone picks up when the three offset frequencies are fed in on the white transducers (video on next step).
There is sound!! It looks like the microphone picks up: a sub 1 Hz, ~ 3 Hz and ~ 23 Hz sounds
The physics here is actually pretty simple, when two waves of different frequencies (f1 & f2) interfere their superposition leads to 2 new frequency components; one at the average frequency ([f1 + f2] / 2), and one at the difference between the original frequencies (f1 - f2), called the beat frequency.
Which is exactly what the microphone picks up:
40,007 Hz - 40,006.8 Hz = 0.2 Hz
40,010 Hz - 40,006.8 Hz = 3.2 Hz
40,030 Hz - 40,006.8 Hz = 23.2 Hz
Wikipedia has a very good explanation of this.
So as long as the frequencies stay close to the resonance frequency of the quartz transducers, what we have here is a speaker made out of thin air.
Step 5: Results
Step 6: Conclusion
Personally I think this is the best demonstration of standing waves at a higher level. It is visually amazing and can easily be described mathematically with the help of an oscilloscope.
The components required are extremely cheap and the setup uses barely any power, so it could make a pretty sweet desk ornament.
The electronics may seem complicated but in total you need 2 modules, 2 transducers, 1 chip and less than 10 wires which are hard to mess up, so do not let that put you off from trying this.
The 3d printed case is absolutely not necessary, the transducers can be held close together with pretty much anything, I'm sure cardboard and super glue would work amazingly well.
Finally I hope you've enjoyed this! I'm happy to answer questions and let me know if anything isn't clear enough :)