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Sonic Ramjet

This document describes a low-frequency sound-based engine—referred to as a sonic ramjet—that generates enough thrust to move a small model craft. The design contains no moving parts beyond the speaker diaphragm itself. The thrust comes strictly from sound waves exiting the nozzle.

Overview

• Engine: A 3D-printed resonance chamber (the "engine") that channels 40 Hz sound into a compressed jet.
• Propulsion Method: Stacking and resonating a 40 Hz tone approximately three times to form a uniform wave jet.
• Inspiration: Sonic booms, deep-bass cars that rattle entire houses, plus a chamber vibration effect that reduces friction (conceptually inspired by water striders).

A single-engine STL is provided here. Additional files for the multi-engine “sound resonance vehicle” setup will be shared separately.

• Video Demonstration – Small Resonance Vehicle
• 3" Engine Blowing Paper Demo

STL Files

Variants

1. 1.5" Version – Designed for a 1.5" speaker.
2. 3" Version – A more robust design intended for a 3" speaker, but can be scaled up to larger sizes (e.g., 4", 5", etc.).

Important: Always confirm that the speaker’s outer diameter matches the model’s speaker mount. If you plan to use a speaker of a different size, simply scale the model accordingly in your slicer.

Note: The 3" design file is more durable and recommended for larger scaling.

Printing Orientation: Print the engine in a vertical orientation (nozzle facing up or down) to preserve the internal geometry and reduce support material.

Basic Requirements

1. Speaker
   • Typically 1.5" or 3", but can be scaled to 4"+ if desired.
2. Amplifier
   • Any small amplifier that can output ~10 W or more.
3. 40 Hz Signal
   • A sine wave generator is the most direct way to achieve consistent 40 Hz performance.
   • Using a Bluetooth module is convenient for streaming test tones from a phone or music device.
4. Scaling
   • If you choose a speaker size different from 1.5" or 3", adjust the STL dimensions accordingly before printing.

BOM

Below are the components used in a reference build, though substitutes can work similarly:

1. Audio Amplifier
   • Model: TPA3116 D2 XH-M543.
2. Bluetooth Receiver
   • Model: XY-BT-Mini (or similar).
3. Speaker (1.5")
   • Example: SOTAMIA 40 mm (4 Ω / 5 W).
   • Note: If files were previously scaled for ~52 mm speakers, use a 133% scale to match that larger diameter if you already have 52 mm units.
4. Speaker (3")
   • Example: SOTAMIA 3" portable full-range speaker (2 Ω / 5 W).
5. Battery
   • Recommended: 8.4 V Li-ion 2S pack (made from 18650 cells).
   • The amplifier can handle higher voltages, but the Bluetooth module is typically rated for 3.7–5 V. A buck converter or separate power supply may be necessary.
6. Bearings (Optional)
   • For a “sound buggy,” using small bearings (e.g., 11 mm × 5 mm) on the wheels can reduce friction. These can often be purchased online at lower cost than at local hardware stores.

Updates

• 3" Version Added
  • Scales readily if using 4" or larger speakers.
  • Contains a mount in the center for experiments with plasma arcs or similar high-voltage tests (untested prototype stage).
  • Performance:
  • Not as optimized as the 1.5" design; it is a single solid piece intended for preliminary trials.
  • Generates surprising thrust at 8.4 V.
  • Stronger performance may require a longer, more tapered chamber.
  • The larger speaker (3") is more efficient than the smaller one, and higher voltage to the amplifier can yield more thrust. Use caution at or above 12 V.

Assembly Instructions

1. Mounting the Speaker

• 1.5" Version
  • Tape the speaker to the chamber’s rear opening using electrical tape, ensuring no speaker vents are blocked.
• 3" Version
  • Use 4× M3 screws in the dedicated mounting holes. A gasket or thin foam can help seal edges for more efficient airflow.

2. Bluetooth Module Wiring

• Identify L, R, and G on the module’s underside.
• Solder wires from these pads to the amplifier’s left/right audio input and ground.

3. Amplifier & Speaker Connections

1. Connect the Bluetooth receiver’s L, R, G to the amplifier input wires.
2. Connect the speaker leads (positive and negative) to the speaker output on the amplifier.
3. Ensure polarity is correct: + on the speaker to + on amp output, – to –.

4. Power Considerations

• The Bluetooth module requires 3.7–5 V, while the amp can handle higher voltages.
• Common approach:
  • 8.4 V from a 2S Li-ion pack to the amplifier.
  • A buck converter (or separate 5 V supply) feeding the Bluetooth module.
• If unfamiliar with Li-ion packs and DC–DC conversion, consider two separate power sources:
  • A small 3.7–5 V for the BT module, and 8–12 V for the amplifier.

5. Volume Control

• The TPA3116 D2 (XH-M543) amplifier typically includes two knobs for L/R volume.
• 3D-printed knobs may be helpful for fine-tuning output.

6. Operation

1. Connect the amplifier to its power supply (e.g., 8.4 V Li-ion pack).
2. Power or connect the Bluetooth module to its 5 V (or appropriate) supply.
3. Pair the module with a phone or send a 40 Hz sine wave from a dedicated generator.
4. Adjust volume on the amplifier until the engine produces a strong, resonant 40 Hz tone.

Safety & Sound Levels

Both the 1.5" and 3" engines create loud, low-frequency waves. Hearing protection is recommended. Exercise caution with:

• High-Voltage Arc Experiments (if exploring plasma).
• Higher Voltages on the amp, as they can produce extreme sound pressure levels.

This sonic ramjet concept demonstrates how low-frequency acoustic energy can be harnessed to generate a measurable thrust, even using small, readily available parts:

• No mechanical moving parts beyond the speaker diaphragm itself.
• Impressive immediate results—the design is effective in table-top propulsion demos.
• Potential expansions into plasma-assisted propulsion for advanced experiments.

The maker community is encouraged to explore, modify, and scale these designs, sharing new discoveries and improvements.

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