Rectangular-room eigenmode solution f = (c/2)·sqrt((nx/Lx)² + (ny/Ly)² + (nz/Lz)²) at c = 343 m/s, with axial/tangential/oblique classification, pile-up clustering below 200 Hz, the Bolt-area proportion test (Bolt, 1946, JASA), and Schroeder crossovers at two ASSUMED RT60 values. Pure geometry — no material data is involved, which is why RT60 is bracketed rather than stated.
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