# 40 mm loudspeaker envelope (the speaker in the thermoacoustic driver cap)

> How big is a 40 mm speaker, and how does it sit in a thermoacoustic driver cap?

- Category: Stirling engines › Thermoacoustic & fluidyne
- Kind: Hardware
- Basis: Overall Ø40 × 25 and the step diameters measured from the manufacturer's published model of a 40 mm speaker (Adafruit #3968); drawn as a stepped envelope, nothing copied
- Page: https://xemstudio.com/objects/loudspeaker-envelope-40mm/ · JSON: https://xemstudio.com/api/geometry?object=loudspeaker-envelope-40mm · Plain text: https://xemstudio.com/objects/loudspeaker-envelope-40mm/sheet.txt
- License: dimension sheet CC BY 4.0 (Xem Studio); linked files under their publishers' licenses
- Use in every chat: add https://xemstudio.com/mcp as a connector (https://xemstudio.com/use-with-ai/)

**Exact dimensions.** Every file is drawn at true size with no printing tolerance. The holes and pins that meet another part are listed below under Mating features, with the fit each joint needs; open those by your printer's or process's clearance before making the part.

## Notes

- A stepped envelope of a common 40 mm speaker: the space it takes, not its parts. Front rim Ø40 × 3 with the cone behind it, a Ø34 × 4 step, the Ø28 basket and the Ø20 magnet; 25 mm overall.
- In the 2 in driver cap it sits rim down in the Ø40 × 6 socket, cone toward the gas through the cap's Ø28 port; the basket and magnet stand above the cap.
- As an alternator: the gas pressure moves the cone, the voice coil moves in the magnet's field and gives a voltage at the terminals. A small speaker gives milliwatts to a fraction of a watt; see the build lists for the model's acoustic power at the cap.

## Mating features (exact size; open press / slide / run / socket by your process clearance, leave designed and clamp as drawn)

| feature | exact | mates with | fit | files |
|---|---|---|---|---|
| front rim | Ø40 × 3 | Ø40 × 6 socket of the 2 in driver cap | socket | all |
| basket and magnet | Ø34 / Ø28 / Ø20 | space above the driver cap (no contact) | designed | all |

## Sizes

| feature | value |
|---|---|
| step 1 | Ø40 from 0 to 3 |
| step 2 | Ø34 from 3 to 7 |
| step 3 | Ø28 from 7 to 17 |
| step 4 | Ø20 from 17 to 25 |

## Where the files are

- Adafruit (CAD parts repository, MIT): [#3968 Speaker 40mm: overall Ø40 × 25 and the step diameters measured from the published model; nothing copied](https://github.com/adafruit/Adafruit_CAD_Parts/tree/main/3968%20Speaker%2040mm) — STEP — MIT; dimensions only
- Long Earth Society: [Engine builds](https://longearthsociety.com/engineering/) — web, video — LES site

## Free STEP/STL (1 parts, CC BY 4.0, Xem Studio)

Index: https://xemstudio.com/files/hardware/loudspeaker-envelope-40mm/index.json

- speaker-envelope-40mm: [STEP](https://xemstudio.com/files/hardware/loudspeaker-envelope-40mm/speaker-envelope-40mm.step) · [STL](https://xemstudio.com/files/hardware/loudspeaker-envelope-40mm/speaker-envelope-40mm.stl) — 40 mm loudspeaker envelope: front rim Ø40 × 3 (cone side at z = 0), Ø34 × 4, basket Ø28 × 10, magnet Ø20 × 8; 25 overall

## Questions

**Is this the speaker's real shape?**

It is its envelope: every step is the largest diameter of the speaker over that height, so anything that clears the envelope clears the speaker.

## Fits with

- [Thermoacoustic driver cap: ISO 2852 cap with a Ø40 speaker socket, 1.5, 2 and 3 in](https://xemstudio.com/objects/thermoacoustic-driver-cap/)
- [Speakers, amplifiers and microphones: 3 in, 40 mm, enclosed, STEMMA speaker, MAX98357, I2S/PDM mics](https://xemstudio.com/objects/adafruit-audio-speakers-mics/)
- [Build list: Thermoacoustic traveling-wave loop, 2 in tri-clamp](https://xemstudio.com/objects/list-ta-loop-2in/)
- [Build list: Thermoacoustic standing-wave engine, half-wave, propane, 2 in tri-clamp](https://xemstudio.com/objects/list-ta-standing-half-wave-2in/)