--- instrument: Bowed Frame Drum family: membranophone subfamily: frame-drum origin_archetype: Irish bodhran scale: chromatic sympathetic (tone bar tunable) bore_profile: N/A (open frame cylinder) target_fundamental_hz: 130 target_note: C3 tuning_mechanism: head-tension screws + tone-bar length trim sheet_metal_challenge: rolled cylindrical frame + internal tone-bar suspension bracket round: 5 lane: 05 version: v0.1.0-blueprint date: 2026-05-21 --- # Bowed Frame Drum — Design Document ## Design Thesis A bodhran-scale (~450 mm OD) frame drum with a rolled sheet-metal cylinder frame instead of the traditional laminated wood hoop. Inside the frame a flat CR-steel tone bar is suspended on two neoprene grommets; it vibrates sympathetically when the head is struck, adding an overtone shimmer tuned to approximately C4 (260 Hz). An optional rosined-wheel assembly — a 50 mm brass wheel on a cantilevered arm, rosined edge touching the head's inner rim — produces a sustained bowed drone in the 80–400 Hz range when the arm is engaged. The sheet-metal frame offers three advantages over wood: (1) dimensional precision from the slip-roll, (2) weld-in grommet anchor points without splitting risk, (3) matte-black finish gives a modern aesthetic while remaining acoustically transparent (the frame is a passive radiator; head mass dominates). ## Geometry ``` Frame OD : 450 mm Frame ID : 448 mm (1 mm wall each side) Frame depth : 90 mm Head playing OD : 430 mm (head slightly larger, clamped over rim) Rim strip width : 12 mm (top + bottom, bent from 1 mm CR) Tone bar : 400 mm × 25 mm × 1 mm CR flat bar Tone bar mount : 2× brackets at 180° spacing, 50 mm from each end of bar Grommet ID : 5 mm (bar thickness 1 mm; bar snaps into slot cut in grommet) Rosined wheel OD : 50 mm, brass, pivots on M5 stainless bolt Rosined wheel arm : 60 mm × 15 mm × 1 mm CR, two 45° bends ``` ## Acoustic Intent | Mode | Target frequency | Note | Source | |------|-----------------|------|--------| | Head fundamental f₀₁ | ~130 Hz | C3 | Bessel J₀ first zero, tension tuned | | Head overtone f₁₁ | ~220 Hz | A3 | Bessel mode 11; appears naturally | | Tone bar bending mode 1 | ~260 Hz | C4 | Euler-Bernoulli free-free beam | | Rosined wheel drone range | 80–400 Hz | — | Slip-stick excitation bandwidth | The tone bar is intentionally detuned ~2 semitones above the head fundamental so sympathetic ring decays slowly and adds air rather than masking. Trimming the bar shorter raises pitch; adding blu-tack mass on the bar lowers pitch (field-adjustable). ## Sheet-Metal Authority Chain ### Frame ring - Class: **cylinder** - Material: 1.0 mm cold-rolled steel, ASTM A1008 - Blank: 1414.7 mm × 90 mm (π × 450 mm OD × depth) - Seam: butt-TIG, full-penetration, fixture in V-block jig - Spring-back: minimal on thin stock; overbend ~2° in slip-roll to compensate ### Rim reinforcement strips (×2) - Class: **flat panel, rolled in-place** - Blank: 1414.7 mm × 12 mm - Operation: roll with frame; TIG-weld to top and bottom frame edges - Provides: bearing surface for head wrap + stiffening ring against oval distortion ### Tone bar - Class: **flat plate, no forming** - Blank: 400 mm × 25 mm - Operation: laser or plasma cut; edges deburred; no bends - Suspend via 2× neoprene grommet mounts ### Tone bar mount brackets (×2) - Class: **bracket, 1× 90° bend** - Blank: 25 mm × 30 mm - Bends: one 90° fold to form L; one leg rivets to frame ID wall, other leg cradles grommet ### Rosined wheel arm - Class: **bracket, 2× bends** - Blank: 80 mm × 15 mm - Bends: 45° down at 20 mm from pivot end; 45° back at 50 mm (creates offset so wheel clears frame and contacts underside of head near rim) ## Seam Strategy Butt + TIG throughout. Rationale: frame is structural (head tension ~60 N distributed), butt-TIG is cleanest on 1.0 mm CR, lap joint would create inner ridge that snags head wrap. ## Tuning Mechanism Head tension is set by 6× M4 × 30 mm cap-head screws threading into 6× M4 weld-nuts on the frame OD, pulling a wrap-ring down over the head. Quarter-turn increments give ~8–12 Hz pitch change at the fundamental. Tone bar pitch trimmed with angle grinder (shorten) or mass-loading. ## Ergonomics Drum held in left arm, played with short tipper (traditional bodhran technique). Frame depth 90 mm provides grip without blocking playing hand. Rosined wheel arm on a spring-loaded pivot — press with thumb to engage, release to disengage. Mass budget: frame ~700 g, head ~80 g, hardware ~120 g, total ~900 g.