<!-- SPDX-License-Identifier: CC-BY-4.0 -->

# Decision Record

## Decisions

- Treat the hydraulophone as a water-driven acoustic instrument centered on
  playable water jets, player-flow interaction, and hydraulic/acoustic coupling.
- Keep water supply, jet interface, acoustic coupling, drainage, frame, and
  safety systems as separate design subsystems.
- Keep this first packet at V5 L1 because no measured water-loop, jet coupon,
  acoustic coupon, pressure review, or sanitation plan exists.
- Record every artifact as `concept_only` or `pending_measurement`.
- Do not create CAD, DXF, jet geometry, pressure values, flow rates, tuning
  tables, resonator geometry, pump specs, or fabrication instructions in this
  pass.

## Open Questions

- What real hydraulophone reference, measured exemplar, or original test target
  should govern the first prototype?
- Should the first coupon test a finger-blocked jet, a key-controlled wet valve,
  or a simpler water-flow/acoustic-cavity interaction?
- What water-loop architecture can be tested safely with relief, drainage, and
  splash containment?
- What cleaning and drying protocol is acceptable before any public handling?
- Which acoustic behavior is being measured first: pitched tone, timbral water
  noise, resonator response, or playability of flow interruption?
- How will electrical isolation be handled if pumps, sensors, or lights are
  introduced later?

## Promotion Gates

- L2 requires selected references or measured design targets, a reviewed safe
  water-loop plan, and a non-dimensional coupon test plan.
- L3 requires reviewed fabrication authority, leak/splash validation, and
  repeatable jet/acoustic response evidence.
- L4 requires empirical playing data, safety observations, recordings or
  measurements, and documented design revisions.

## Next Work

- Select reference evidence for the hydraulophone mechanism and safety posture.
- Draft a one-jet coupon test plan with relief, shutoff, drainage, and splash
  controls.
- Separate acoustic observations from hydraulic noise in the first measurement
  log.
- Record cleaning, drying, leak, and slip-risk observations before upgrading
  readiness.