Derivation
STANDING WAVE FUNDAMENTAL MODES
For a rectangular room, standing waves form at f = v/(2L) for each axis:
f₁ (length) = 343 / (2 × 10.468) = 343 / 20.936 = 16.38 Hz
f₂ (width) = 343 / (2 × 5.236) = 343 / 10.472 = 32.75 Hz
f₃ (height) = 343 / (2 × 5.844) = 343 / 11.688 = 29.35 Hz
DEGENERATE MODE ANALYSIS
Check the length-to-width ratio:
L / W = 10.468 / 5.236 = 1.9988 ≈ 2.000 (deviation: 0.06%)
When L/W = 2 exactly, f₂ = 2 × f₁. Two independent standing wave modes produce the same frequency → degenerate modes. This doubles the acoustic energy at 32.75 Hz. This technique is used in modern concert hall design to amplify specific frequencies.
COMPLETE HARMONIC SERIES (length axis)
| Harmonic | Frequency | Brain Band | Significance |
|---|
| 1st | 16.38 Hz | Beta boundary | Fundamental |
| 2nd | 32.75 Hz | Low Gamma | = Width fundamental (degenerate) |
| 3rd | 49.13 Hz | Gamma | Near AC mains (50 Hz) |
| 4th | 65.50 Hz | Gamma | = Gap resonance (Proof VII) |
| 5th | 81.88 Hz | High Gamma | Audible threshold |
| 6th | 98.25 Hz | — | Approaches 110 Hz |
| 7th | 114.7 Hz | — | ≈ 110 Hz brain lateralization |
| 8th | 130.9 Hz | — | Matches gap resonance ×2 |
COMBINED ROOM MODE (Rayleigh equation)
f(n₁,n₂,n₃) = (v/2) × √[(n₁/L)² + (n₂/W)² + (n₃/H)²]
Mode (7,0,0):
f = (343/2) × √[(7/10.468)²] = 171.5 × 0.6687 = 114.7 Hz
Mode (0,2,2):
f = 171.5 × √[(2/5.236)² + (2/5.844)²] = 171.5 × √[0.1458 + 0.1170] = 171.5 × 0.5126 = 87.9 Hz
Mode (3,2,0):
f = 171.5 × √[(3/10.468)² + (2/5.236)²] = 171.5 × √[0.0821 + 0.1458] = 171.5 × 0.4775 = 81.9 Hz
Multiple modes cluster near 80–115 Hz, creating a dense resonance band in the exact range that affects human consciousness.
COFFER (GRANITE BOX) RESONANCE
Interior dimensions: 1.977 m × 0.681 m × 0.873 m
Air cavity standing-wave modes: length = 86.7 Hz, depth = 196.4 Hz, width = 251.8 Hz.
The widely cited 438 Hz is from Tom Danley’s measurements and likely a structural resonance of the granite walls (not a simple air cavity mode). v/(2×0.392) = 438 Hz, but 0.392 m does not correspond to any clear interior dimension. This cannot be derived from first principles without a full FEM model of the coffer.
GRANITE BEAM FLEXURAL RESONANCE
f₁ = (π/2) × (h/L²) × √(E/12ρ)
√(50×10⁹ / (12 × 2750)) = √(1,515,152) = 1,231 m/s
For average beam thickness h = 1.52 m, span L = 5.236 m:
f₁ = 1.5708 × (1.52/27.416) × 1,231 = 1.5708 × 0.05544 × 1,231 = 107.2 Hz
Beam thicknesses 1.40–1.60 m span 98.8–112.9 Hz, centering on 110 Hz.