A Simple Acoustic Levitator Teaching Tool

This project would not be possible with the inspiration provided by Dr. Asier Marzo original article.

https://www.instructables.com/Acoustic-Levitator/

A while ago, we published an instructable on several methods to build a simple acoustic levitator. At the time, we did not include some of the changes we made to the final form of the project. The final form was used by a group of homeschoolers to help understand standing waves as part of a STEM initiative. They were intrigued with the idea of levitation and how far they could get styrofoam pellets to "float." They actually created several games out of "floating" pellets. The first game was to see how many pellets they could stack, and the second game was to figure out how high they could get a pellet to levitate above the stage. They modified the device so that there were two soundstages so they could compete with each other.

This instructable deals with some of the changes we made to the device. The construction of this project is the same as described in our original instructable, found here:

https://www.instructables.com/A-Simple-Stand-for-a...

The key differences are:

The addition of a separate microcontroller to calculate distances between the stages the contain the speakers, larger sound stages and a longer central beam.

When we did this for the first time, we just gave the kids the formula for speed of sound at room temp. They had just finished a unit on standing waves.

So from this starting point:

343m/sec = 343,000mm/sec

Wavelength (mm) = Wave velocity (mm/s) / Frequency (Hz)

So,

(343,000mm/sec) / 40,000Hz = 8.575mm

The frequency we were driving the speakers at was 40 Hz

So they started look for standing waves at ~8.5mm. They started by putting styrofoam pellets on the bottom speaker and adjusting the top speaker until the pellet started to "dance." They then started to try and place a pellet in the approximate center of the two speakers. This allowed them to find a distance that worked. They then used the measurement display to double the distance between plates. (Twice the measured value.) They were able to use this method to get distances between the plates of between four and six times the values of the initial calculation.

One of the students found this article in Make Magazine that let them use an oscilloscope to get greater distances. The students printed another sound stage, used a microphone salvaged from an SR-04 and were able further refine the distance between the plates.

https://makezine.com/projects/micro-ultrasonic-lev...

The question:

Over a couple of days, some questions that emerged were:

1. How far apart can the speakers be apart and still have a pellet of styrofoam levitate?
2. Does the size of the stage make any difference in how far apart the speakers can be place?
3. if we disable the top speaker and replace it with a solid surface (note card) can levitation still be obtained?
4. How high can a pellet be levitated above the bottom speaker?
5. If you stack pellets of styrofoam does the order you place them have any impact?
6. Does the size of the pellets have any impact on the distance between pellets in a stack?
7. If you use the dish why can you levitate pellets at the same level? How many can you levitate?

What was interesting to us, was that the kids returned to the project several times. Some ideas the kids proposed were the idea for a larger stage, and what would happen if a soundstage were a parabola.

The central beam was a 357 mm (14 inches) 8020 series 10 pice of extruded aluminum. This beam was too long to be 3D printed, so we used cut aluminum extrusion.

The sound stage is larger.

It incorporates a built in measuring device to determine the distance between the speakers.

You may have opted to used the parabolic dish which focused the sound. The design is focused at approximately 89mm. It provides an interesting contrast to the flat reflector surfaces.

The SLT files for the stages with a distance sensor can be found at: Printables or Thingiverse

If you do not want to include the distance sensor, the file is at: Printables or Thingiverse

A ruler works just fine for measuring the distances between the stages.

The bracket for the feathering terminal block is at: Printables or Thingiverse

The parabolic dish version can be found at: Printables or Thingiverse

The parabolic dish can be printed without supports if it orientated with the long side of the support bracket down to the build plate.

We measure the distance between plates by using an Adafruit VL6180X Time of Flight distance sensor. The upper plate has a mounting space for the sensor. The lower plate has a finger that is used for a distance target.

The version that we constructed used a Feather M0 with an OLED display to display of the distance between the plates. The code and a description of the wiring for the VL6180X can be found at:

Learning Guide for adafruit-vl6180x-time-of-flight

The students may undertake a separate software project to calibrate the sensor.

The mount used is at Featherwing Terminal Block Mount

The parts we used:

https://www.adafruit.com/product/2772

https://www.adafruit.com/product/4650

https://www.adafruit.com/product/2926

The parabolic dish in one of the more interesting variations on this project we tried. It has the ability to suspend multiple styrofoam pellets at the same height. (Our record was four.)