# Thermoacoustic stacks (slicer-infill blanks, spiral), resonator lengths and a speaker driver cap for tri-clamp spools

> What stack and tube length does a thermoacoustic engine need, and how do I print a honeycomb stack?

- Category: Stirling engines › Thermoacoustic & fluidyne
- Kind: Hardware
- Basis: Standing-wave thermoacoustic engine design (Swift): stack plate spacing ≈ 2–4 thermal penetration depths (δκ ≈ 0.1–0.3 mm in air at 1 bar, 100–300 Hz → gaps 0.5–1.5 mm), stack centred ~λ/8 from the closed end of a λ/4 resonator; stacks printed as a solid blank whose slicer infill IS the honeycomb
- Page: https://xemstudio.com/objects/thermoacoustic-stacks-and-resonators/ · JSON: https://xemstudio.com/api/geometry?object=thermoacoustic-stacks-and-resonators · Plain text: https://xemstudio.com/objects/thermoacoustic-stacks-and-resonators/sheet.txt
- License: dimension sheet CC BY 4.0 (Xem Studio); linked files under their publishers' licenses

## Notes

- A thermoacoustic engine has no pistons: a stack (many narrow parallel channels) with a hot end and a cold end sits inside a resonator tube; above a critical temperature gradient the gas self-oscillates at the tube's resonance. Load it with a speaker-as-microphone (the driver cap) or a piston on a linear alternator. Driven backwards it pumps heat: a thermoacoustic refrigerator.
- Print the stack from the blank: 1 perimeter, 0 top and 0 bottom layers, infill pattern honeycomb (or grid / triangles), and set the infill density to the cell size you want. Cell pitch ≈ 2 × line width / density for grid; ≈ 1.7 × line width / density for honeycomb. With a 0.4 mm line: 60% → ~1.3 mm grid cells, 45% → 1.8 mm, 30% → 2.7 mm, 80% → 1.0 mm. The density slider is the stack's channel size; no new file needed. Print 0.1–0.15 mm layers so the walls are smooth.
- Stack physics: channel half-width 2–4 × δκ, δκ = √(κ/(π f ρ cp)) = 0.26 mm for air at 1 bar and 100 Hz (0.15 mm at 300 Hz), so cells of 1–1.5 mm at 100 Hz, ~1.0 at 300 Hz. Length 20–50 mm. Centre the stack about λ/8 from the closed (hot) end. Low-conductivity walls: PETG/resin for demonstrators (hot end on a metal plate, stack behind it), ceramic honeycomb (catalytic substrate, 400 cpsi ≈ 1.1 mm cells) for real heat.
- Resonator: closed-open tube f = c / 4L (λ/4): 100 Hz → 858 mm, 150 Hz → 572, 200 Hz → 429, 300 Hz → 286, 500 Hz → 172 (air, 20 °C, c = 343 m/s). Closed-closed (two caps) is λ/2, twice as long. Tri-clamp spools stack to any length: two or three 2 in × 12 in spools make a 200–300 Hz λ/4 engine with the hot end capped. Heat: nichrome coil or flame on the capped end; cold end a water jacket or the copper-pipe cooler.
- The spiral stack is the template for the classic rolled-Mylar-and-fishing-line stack; the explicit 1.5 mm hex file is a reference for the geometry the infill approximates.
- LES: the stacks, driver cap and tube-length table from the LES thermoacoustic builds; the infill trick is how LES prints them.

## Sizes

| spool | bore_mm | stack | lengths_mm | quarter_wave_150hz_mm | half_wave_150hz_mm |
|---|---|---|---|---|---|
| 1.5 | 34.8 | blank + infill (table) | 30 / 40 / 50 | 572.0 | 1143.0 |
| 2 | 47.5 | blank + infill (table) | 30 / 40 / 50 | 572.0 | 1143.0 |
| 3 | 72.9 | blank + infill (table) | 30 / 40 / 50 | 572.0 | 1143.0 |

## Where the files are

- Wikipedia: [Thermoacoustic heat engine](https://en.wikipedia.org/wiki/Thermoacoustic_heat_engine) — web — CC BY-SA 4.0 (text)
- G. W. Swift (LANL): [Thermoacoustics: A Unifying Perspective for Some Engines and Refrigerators (textbook; DeltaEC software free)](https://www.lanl.gov/org/ddste/aldps/materials-physics-applications/condensed-matter-magnet-science/thermoacoustics/) — web, software — LANL (verify at link)
- NASA (NTRS, public domain): [Thermoacoustic Stirling heat engine for space power](https://ntrs.nasa.gov/search?q=thermoacoustic%20Stirling) — PDF — US Government work, public domain
- Long Earth Society: [Engine builds](https://longearthsociety.com/engineering/) — web, video — LES site

## Free STEP/STL (16 parts, CC BY 4.0, Xem Studio)

Index: https://xemstudio.com/files/hardware/thermoacoustic-stacks-and-resonators/index.json

- stack-blank-1.5in-30: [STEP](https://xemstudio.com/files/hardware/thermoacoustic-stacks-and-resonators/stack-blank-1.5in-30.step) · [STL](https://xemstudio.com/files/hardware/thermoacoustic-stacks-and-resonators/stack-blank-1.5in-30.stl) — stack blank Ø34.2 × 30 for the 1.5 in bore: slice with 0 top/bottom layers, 1 perimeter, honeycomb or grid infill; density sets the cell size (see table)
- stack-blank-1.5in-40: [STEP](https://xemstudio.com/files/hardware/thermoacoustic-stacks-and-resonators/stack-blank-1.5in-40.step) · [STL](https://xemstudio.com/files/hardware/thermoacoustic-stacks-and-resonators/stack-blank-1.5in-40.stl) — stack blank Ø34.2 × 40 for the 1.5 in bore: slice with 0 top/bottom layers, 1 perimeter, honeycomb or grid infill; density sets the cell size (see table)
- stack-blank-1.5in-50: [STEP](https://xemstudio.com/files/hardware/thermoacoustic-stacks-and-resonators/stack-blank-1.5in-50.step) · [STL](https://xemstudio.com/files/hardware/thermoacoustic-stacks-and-resonators/stack-blank-1.5in-50.stl) — stack blank Ø34.2 × 50 for the 1.5 in bore: slice with 0 top/bottom layers, 1 perimeter, honeycomb or grid infill; density sets the cell size (see table)
- stack-honeycomb-1.5in-40-c1.5-reference: [STEP](https://xemstudio.com/files/hardware/thermoacoustic-stacks-and-resonators/stack-honeycomb-1.5in-40-c1.5-reference.step) · [STL](https://xemstudio.com/files/hardware/thermoacoustic-stacks-and-resonators/stack-honeycomb-1.5in-40-c1.5-reference.stl) — explicit 1.5 mm hex honeycomb stack 1.5 in × 40 (reference geometry; the blank + infill route is faster)
- stack-spiral-1.5in-40-g1.0: [STEP](https://xemstudio.com/files/hardware/thermoacoustic-stacks-and-resonators/stack-spiral-1.5in-40-g1.0.step) · [STL](https://xemstudio.com/files/hardware/thermoacoustic-stacks-and-resonators/stack-spiral-1.5in-40-g1.0.stl) — spiral (rolled-sheet) stack 1.5 in, 40 long, 1.0 mm gap (also a template for rolling film + fishing-line spacers)
- driver-cap-1.5in-40mm-speaker: [STEP](https://xemstudio.com/files/hardware/thermoacoustic-stacks-and-resonators/driver-cap-1.5in-40mm-speaker.step) · [STL](https://xemstudio.com/files/hardware/thermoacoustic-stacks-and-resonators/driver-cap-1.5in-40mm-speaker.stl) — 1.5 in ISO 2852 cap with a socket for a 40 mm speaker (thermoacoustic refrigerator driver / engine load)
- stack-blank-2in-30: [STEP](https://xemstudio.com/files/hardware/thermoacoustic-stacks-and-resonators/stack-blank-2in-30.step) · [STL](https://xemstudio.com/files/hardware/thermoacoustic-stacks-and-resonators/stack-blank-2in-30.stl) — stack blank Ø46.9 × 30 for the 2 in bore: slice with 0 top/bottom layers, 1 perimeter, honeycomb or grid infill; density sets the cell size (see table)
- stack-blank-2in-40: [STEP](https://xemstudio.com/files/hardware/thermoacoustic-stacks-and-resonators/stack-blank-2in-40.step) · [STL](https://xemstudio.com/files/hardware/thermoacoustic-stacks-and-resonators/stack-blank-2in-40.stl) — stack blank Ø46.9 × 40 for the 2 in bore: slice with 0 top/bottom layers, 1 perimeter, honeycomb or grid infill; density sets the cell size (see table)
- stack-blank-2in-50: [STEP](https://xemstudio.com/files/hardware/thermoacoustic-stacks-and-resonators/stack-blank-2in-50.step) · [STL](https://xemstudio.com/files/hardware/thermoacoustic-stacks-and-resonators/stack-blank-2in-50.stl) — stack blank Ø46.9 × 50 for the 2 in bore: slice with 0 top/bottom layers, 1 perimeter, honeycomb or grid infill; density sets the cell size (see table)
- stack-spiral-2in-40-g1.0: [STEP](https://xemstudio.com/files/hardware/thermoacoustic-stacks-and-resonators/stack-spiral-2in-40-g1.0.step) · [STL](https://xemstudio.com/files/hardware/thermoacoustic-stacks-and-resonators/stack-spiral-2in-40-g1.0.stl) — spiral (rolled-sheet) stack 2 in, 40 long, 1.0 mm gap (also a template for rolling film + fishing-line spacers)
- driver-cap-2in-40mm-speaker: [STEP](https://xemstudio.com/files/hardware/thermoacoustic-stacks-and-resonators/driver-cap-2in-40mm-speaker.step) · [STL](https://xemstudio.com/files/hardware/thermoacoustic-stacks-and-resonators/driver-cap-2in-40mm-speaker.stl) — 2 in ISO 2852 cap with a socket for a 40 mm speaker (thermoacoustic refrigerator driver / engine load)
- stack-blank-3in-30: [STEP](https://xemstudio.com/files/hardware/thermoacoustic-stacks-and-resonators/stack-blank-3in-30.step) · [STL](https://xemstudio.com/files/hardware/thermoacoustic-stacks-and-resonators/stack-blank-3in-30.stl) — stack blank Ø72.3 × 30 for the 3 in bore: slice with 0 top/bottom layers, 1 perimeter, honeycomb or grid infill; density sets the cell size (see table)
- stack-blank-3in-40: [STEP](https://xemstudio.com/files/hardware/thermoacoustic-stacks-and-resonators/stack-blank-3in-40.step) · [STL](https://xemstudio.com/files/hardware/thermoacoustic-stacks-and-resonators/stack-blank-3in-40.stl) — stack blank Ø72.3 × 40 for the 3 in bore: slice with 0 top/bottom layers, 1 perimeter, honeycomb or grid infill; density sets the cell size (see table)
- stack-blank-3in-50: [STEP](https://xemstudio.com/files/hardware/thermoacoustic-stacks-and-resonators/stack-blank-3in-50.step) · [STL](https://xemstudio.com/files/hardware/thermoacoustic-stacks-and-resonators/stack-blank-3in-50.stl) — stack blank Ø72.3 × 50 for the 3 in bore: slice with 0 top/bottom layers, 1 perimeter, honeycomb or grid infill; density sets the cell size (see table)
- stack-spiral-3in-40-g1.0: [STEP](https://xemstudio.com/files/hardware/thermoacoustic-stacks-and-resonators/stack-spiral-3in-40-g1.0.step) · [STL](https://xemstudio.com/files/hardware/thermoacoustic-stacks-and-resonators/stack-spiral-3in-40-g1.0.stl) — spiral (rolled-sheet) stack 3 in, 40 long, 1.0 mm gap (also a template for rolling film + fishing-line spacers)
- driver-cap-3in-40mm-speaker: [STEP](https://xemstudio.com/files/hardware/thermoacoustic-stacks-and-resonators/driver-cap-3in-40mm-speaker.step) · [STL](https://xemstudio.com/files/hardware/thermoacoustic-stacks-and-resonators/driver-cap-3in-40mm-speaker.stl) — 3 in ISO 2852 cap with a socket for a 40 mm speaker (thermoacoustic refrigerator driver / engine load)

## How it fits

### Goes in or on
- [Tri-clamp spool (two ferrules on a tube), 1.5 and 2 inch](https://xemstudio.com/objects/tri-clamp-spool/): stack in the bore, spools as resonator

### Connects to
- [Tri-clamp end cap and gasket, 1.5 and 2 inch](https://xemstudio.com/objects/tri-clamp-end-cap/): capped hot end

### Also needed
- [Tri-clamp clamps (printable two-bolt), hot-joint gaskets and heat shields: 1.5, 2 and 3 in](https://xemstudio.com/objects/tri-clamp-clamps/): clamp per joint
- [Regenerator and heat materials: stainless mesh counts, steel wool, scrubbers, mica, fiberglass, ceramic, firebrick](https://xemstudio.com/objects/regenerator-and-heat-materials/): mesh heat exchangers

## Questions

**How do I 3D print a thermoacoustic stack?**

Print the stack blank with 1 perimeter, 0 top/bottom layers and honeycomb or grid infill; the infill density sets the cell size (0.4 mm line: 60% ≈ 1.3 mm grid cells, 45% ≈ 1.8 mm, 80% ≈ 1.0 mm). No custom geometry needed.

**How long should a thermoacoustic engine tube be?**

Closed-open (λ/4): L = 343 / (4f): 286 mm for 300 Hz, 429 mm for 200 Hz, 858 mm for 100 Hz in air; closed-closed tubes are twice that.

**What channel size should the stack have?**

2–4 thermal penetration depths: about 1–1.5 mm in air at 1 bar and 100–300 Hz; finer at higher frequency or pressure.

**Where does the stack go in the resonator?**

Centred about one-eighth of a wavelength from the closed (hot) end: 107 mm for a 400 mm λ/4 tube.

**Can a thermoacoustic engine make electricity?**

A speaker in the driver cap becomes a generator (milliwatts), or a piston on the linear alternator kit for real power; NASA's TASHE used a travelling-wave loop with a linear alternator.

## Fits with

- [Tri-clamp spool (two ferrules on a tube), 1.5 and 2 inch](https://xemstudio.com/objects/tri-clamp-spool/)
- [Tri-clamp end cap and gasket, 1.5 and 2 inch](https://xemstudio.com/objects/tri-clamp-end-cap/)
- [Tri-clamp displacers and engine heads: 1.5, 2 and 3 in spools](https://xemstudio.com/objects/tri-clamp-displacers-and-heads/)
- [Regenerator and heat materials: stainless mesh counts, steel wool, scrubbers, mica, fiberglass, ceramic, firebrick](https://xemstudio.com/objects/regenerator-and-heat-materials/)
- [Moving-magnet linear alternator kit (free-piston Stirling power take-off)](https://xemstudio.com/objects/linear-alternator-kit/)
- [Copper pipe (ASTM B88 Type L and M), 1/4 to 2 inch](https://xemstudio.com/objects/copper-pipe-type-l-m/)
- [Speakers, amplifiers and microphones: 3 in, 40 mm, enclosed, STEMMA speaker, MAX98357, I2S/PDM mics](https://xemstudio.com/objects/adafruit-audio-speakers-mics/)
- [Stirling engine configurations: which type needs which parts on this shelf](https://xemstudio.com/objects/stirling-configurations-guide/)