BIM Room Acoustics (RT60 + rectangular-room modes)

What it is

Architectural room-acoustics first pass for a BIM space: from the room dimensions and a surface-absorption schedule it computes the total absorption and the reverberation time RT60 by both Sabine (0.161 V/A) and Eyring (high-absorption form), the Schroeder transition frequency, and the EXACT rectangular-room mode frequencies up to a cutoff - classified axial / tangential / oblique with the modal density. The lowest axial mode is c/(2L) exactly, Eyring collapses to Sabine as absorption -> 0, and the enumerated mode count is checked against the Weyl asymptotic V(4pi/3)(f/c)^3. Honest scope: Sabine/Eyring assume a diffuse field; the modal analysis assumes a rigid rectangular room; single absorption band per run (repeat per octave). A design aid, not a measurement (ISO 3382 is a measurement standard) - for the modal region of a non-rectangular room use the companion acoustic FDTD plugin. Deterministic, in-browser, no API/key/network/AI.

Honest scope

Authorities cited

• W. C. Sabine, Collected Papers on Acoustics, Harvard 1922 - RT60 = 0.161 V/A (the 0.161 = 24 ln10 / c).
• C. F. Eyring (1930), Reverberation time in "dead" rooms, J. Acoust. Soc. Am. 1, 217 - the -S ln(1 - alpha_bar) absorption form for high absorption.
• M. R. Schroeder (1962), Frequency-correlation functions of frequency responses in rooms, JASA 34, 1819 - the transition frequency f_s = 2000 sqrt(T/V).
• H. Kuttruff, Room Acoustics, 6th ed., CRC 2016 - rectangular-room eigenfrequencies f = (c/2) sqrt((p/Lx)^2+(q/Ly)^2+(r/Lz)^2), axial/tangential/oblique classification, and the Weyl mode-count asymptotic.
• ISO 3382-1/2 - acoustics, measurement of room acoustic parameters - cited as the measurement practice this design-aid companions, not a benchmark.