Introduction: Visualize the Sound!

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About: I usually enjoy handicrafts with my three daughters. Paper-crafts, robots, woodworking, glassblowing… Let's enjoy together!

"Have you ever seen the speed of sound?" The speed of sound is approximately 340 meters per second. It is usually too fast for our eyes to track. This project started with a simple idea: "I want to visualize the sound!" If I line up many sensors that light up when they catch sound, and clap my hands at one end, the sound should look like a wave of light traveling down the line. Furthermore, if I arrange them on a 2D plane, I should be able to see the sound expanding outward. To observe this radial propagation of sound waves, I decided to design a new, mass-producible sensor device from scratch.

Supplies


The specifications for components like resistors and LEDs are not strict, so please choose ones that are readily available.

  1. Resister (R3,R4) 10kohm
  2. Resister (R2) 1Mohm
  3. Resister (R1) 4.7Kohm
  4. Capacitor (C1,C3) 47uF
  5. Capacitor (C4) 100uF
  6. Capacitor (C2) 1uF
  7. Transister (Q1,Q2) 9014
  8. LED (D1,D2,D3,D4,D5,D6) Any
  9. LED (D7) Any
  10. DCDC (U2) QX2303L50T
  11. Inductor (L1) 47uH
  12. Schottky Diode (D8,D10) Any
  13. Microphone (MK1) Any
  14. Switch (SW1) Any
  15. Battery
  16. Battery box

Step 1: Circuit Design - Lighting Up With Constraints

First, I designed a circuit that turns sound into light. The basic mechanism is simple: sound picked up by a microphone acts as a trigger, sending current through a transistor to light up an LED. However, there is a key constraint in this design: It must run on a single AA/AAA battery (1.5V). Usually, LEDs require two or more batteries to light up brightly, but I insisted on a single battery for the following reasons:

  1. Mass Production Costs: Since I planned to make over 200 units, I wanted to minimize the number of batteries.
  2. Safety: Since I might experiment with small children, I wanted to avoid button cells (swallowing hazard) and use standard, accessible AAA batteries. By incorporating a "boost converter" circuit, I succeeded in making the LEDs shine brightly with just one battery.

for the booster circuit, I selected "QX Micro Devices QX2303L50T" as step-up DC-DC converter.

here is data sheet

https://www.lcsc.com/datasheet/C219027.pdf

Step 2: Detailed Design - Optimizing for Mass Production

To tile the entire plane, a large number of sensors need to be prepared. More is better, but there are budget constraints. Considering the size of the gymnasium for the experiment, I decided to create 200 sensors this time.

200 devices. Soldering them all by hand was not realistic. So, I incorporated design features to "make my life easier."

  1. Full SMD (Surface Mount Device): I switched all components to surface mount types so I could outsource the entire PCB manufacturing and assembly process.
  2. No Wires: I designed the battery holder to be soldered directly to the back of the PCB. This eliminated the need for tedious wire stripping and routing. (Only the battery holder is non-SMD. I can tolerate doing 400 solderings myself.)
  3. Calculated Angle: The battery holder doubles as a stand. When placed on the floor, the microphone and LEDs are positioned at the perfect angle for the observer. The result is a compact device, roughly the same size as a AAA battery, that is very easy to handle.

Step 3: Verification Experiment 1 (Linear Array)

The 200 sensors are complete! First, I lined them up in a straight line to verify if I could see the speed of sound. The experiment took place in a gym about 20 meters deep. At 340m/s, sound travels from one end to the other in just 0.06 seconds. You might think, "Can we really see something that fast?" But the human eye is amazing—I could clearly see the light zipping across the line. Checking with smartphone slow-motion video (240fps), I captured about 16 frames (0.5 seconds) of the light traveling. The video clearly captured the phenomenon. The experiment was a success!

Step 4: Verification Experiment 2 (2D Matrix)

Now for the real deal: "2D Visualization." I laid out the sensors on the gym floor in a matrix of 20 vertical by 10 horizontal units, spaced about 1 meter apart. Imagine this: to place 200 sensors on the floor, you have to squat and stand up 200 times. This was physically the hardest part of the experiment! When I clapped my hands in the "center" of the sensor field... "I saw it!!" I reviewed the video in super slow motion (1/32 speed). It beautifully captured the sound spreading outward from the center in an arc. Just like ripples spreading on water, I was able to visualize the invisible energy of sound traveling through space using particles of light.

I cannot express the emotion I felt when I finally saw this "sound wave."

Why don't you try taking a peek into the world of the speed of sound?