# Erhu Prototype Design

Issue: `tonykoop/instrument-maker#150`

Status: L2 V5 build-packet candidate. Use this packet to plan and measure an
erhu-inspired prototype, not to cut a production-ready instrument.

## Canonical Geometry

The active acoustic setup is the string segment from qianjin to bridge.
For this prototype packet:

| Parameter | Value | Status | Notes |
| --- | ---: | --- | --- |
| nominal speaking length | `400 mm` | canonical starter | Qianjin-to-bridge span. |
| setup range | `380-420 mm` | canonical starter | Adjust after measured response. |
| bridge position | `TBD` | measure-fit | Depends on membrane, bridge, and resonator response. |
| qianjin height/offset | `TBD` | measure-fit | Set from playable action and string clearance. |
| resonator body | `TBD` | prototype variable | Hex/octagonal/round body requires reference review. |
| membrane | `TBD` | prototype variable | Thickness, tension, and attachment dominate response. |
| bridge | `TBD` | prototype variable | Height, mass, foot geometry, and contact matter. |
| string set | `TBD` | prototype variable | Do not freeze gauge/tension until tested. |

Do not use total instrument length, neck length, or any nut-to-tail distance as
the active vibrating-length basis. For this packet, the only promoted scale is
qianjin-to-bridge.

## Non-Canonical Archive Note

Round 8 included a long-scale blind-pass assumption around `680 mm`. That value
is useful only as archived comparison material for explaining why this packet
was corrected. It must not be copied into future erhu packets as active
geometry, CAD authority, or setup guidance.

## Prototype Scope

This packet is intended to answer first-build questions:

- Can the prototype sustain a stable bowed tone across both strings?
- Does the `380-420 mm` qianjin-to-bridge range allow playable left-hand
  pitch control and vibrato?
- Which bridge, membrane, and string-set combination gives a usable response?
- What measured body, bridge, qianjin, and membrane values should be promoted
  into the next packet?

## Round 21 B2 Measurement Layer

The next planning layer separates three evidence chains that are easy to blur:

| Chain | Governing file | What can be claimed now | What remains blocked |
| --- | --- | --- | --- |
| qianjin-to-bridge scale | `qianjin-measurement-plan.md` | A starter `380-420 mm` contact-point measurement window. | Final installed length, action, and pitch-response claims. |
| resonator and membrane authority | `resonator-authority-gates.csv` | Resonator geometry is an unresolved prototype variable. | Body, membrane seat, wall thickness, and joinery CAD/DXF. |
| string/source assumptions | `string-source-assumptions.csv` | Two-course erhu-style stringing is planned for testing. | Gauge, tension, source recommendation, and target tuning claims. |

Promotion requires direct installed measurements and response evidence. A
reviewer should fail the packet if any active artifact treats violin-family
nut-to-bridge scale, total instrument length, or a generated silhouette as the
erhu speaking-length authority.

## V5 Fabrication Authority

The current fabrication authority is deliberately narrow:

| Artifact | Authority | Scope |
| --- | --- | --- |
| `cad/erhu-qianjin-scale-params.csv` | design table | 380/400/420 mm qianjin-to-bridge setup marks only. |
| `drawings/erhu-qianjin-scale-starter.dxf` | retained setup drawing | Existing mainline DXF evidence for the setup strip; not full instrument geometry. |
| `cad/erhu-qianjin-scale.scad` | source-only CAD starter | OpenSCAD source for review; not executed in this lane. |

Body shape, membrane seat, neck joinery, peg geometry, string set, qianjin
height, and bridge geometry remain measured/reference-reviewed work. Generated
images are concept support only and must not be treated as CAD, DXF, cut
templates, measured drawings, or setup evidence.

## V5 Provenance

Tool and MCP provenance is recorded in `cad/mcp-session-log.md`. Rows with
`session_id=unknown` identify existing or source-only artifacts whose original
tool session is not available. That log is review evidence only; it does not
promote the packet beyond L2.

## Required Measurements Before Promotion

Record these values before changing readiness beyond prototype-only:

| Measurement | Required evidence |
| --- | --- |
| qianjin-to-bridge length | Caliper or ruler measurement in millimeters. |
| qianjin contact point | Photo and note showing the actual string contact point, not just the wrap edge. |
| bridge height and mass | Measured bridge with photo and notes. |
| resonator and membrane opening | Measured/reviewed drawing before body CAD/DXF promotion. |
| membrane material and tension method | Material source, attachment method, and observed response. |
| string gauges or product IDs | Source evidence plus installed tuning/tension notes. |
| open-string pitch and stable range | Tuner/audio capture under repeatable bowing. |
| action at qianjin and bridge | Clearance measurements and playability notes. |

See `validation-loop.csv` and `risks.md` for the V5 gates that block L3/L4
promotion.