Provisional material references + theoretical RT60 estimates

Acoustic Material Octave-Band Absorption Coefficient (125 Hz – 4000 Hz) & Air-Gap Reference Matrix

Single-number Noise Reduction Coefficients (NRC) average only 250, 500, 1000, and 2000 Hz—completely ignoring the 125 Hz bass band where small rooms suffer severe modal ringing. Use this 24-material reference matrix and live Sabine vs. Norris-Eyring RT60 calculator to compare rigid fiberglass (Owens Corning 703/705), mineral wool (Rockwool Safe’n’Sound & Comfortboard 80), open-cell foam, PET felt, and room boundaries with and without an air gap.

24-Material Reference Matrix — Product Reports Not Yet Verified

In standard ASTM C423 / ISO 354 reverberation-room tests, mounting conditions are designated as Type A (mounted directly flush against a hard surface) or Type E-X (mounted over an air cavity of X millimeters). Note that laboratory Sabine coefficients can slightly exceed 1.00 on thick three-dimensional panels due to edge diffraction (sound waves bending into the exposed perimeter edges of the panel) and the Sabine formula break-down at high absorption.

On narrow screens, swipe the table horizontally to see all material data.

The Porous Absorber & Reverberation Physics Behind the Matrix

The inherited values in rows #2 and #3 differ at 125 Hz, but matching original reports have not been identified here. Their ratio is arithmetic on provisional inputs, not evidence that an air gap multiplies a product's absorption or improves a room by that amount. Use the following models to understand relevant variables, then check product-specific evidence.

1. Quarter-wavelength particle velocity (u(x) ∝ sin(2πx/λ)): The ideal standing-wave pattern at a rigid wall illustrates why distance from the boundary matters. Adding a porous layer changes the acoustic problem; the empty-room velocity expression alone does not predict a coefficient or a fixed gain. See Panel Air Gaps: Theory and Limits.
2. Delany-Bazley Gas Flow Resistivity (σ in Pa·s/m²): The governing parameter of any porous absorber is its specific airflow resistance per unit thickness (σ = ΔP / (u · d)). For thin 50–100 mm wall panels, moderate-to-high flow resistivity (10,000 to 25,000 Pa·s/m²) provides friction over a short path. Thick 250–400 mm corner traps may benefit from lower flow resistivity: penetration depth, impedance, mounting and frequency all matter. Compare material-specific measurements before choosing a construction (see Porous Absorber Gas Flow Resistivity).
3. Sabine vs. Norris-Eyring RT60 Equations: Wallace Sabine’s formula assumes a lightly damped diffuse field (α_avg < 0.20), whereas Carl Eyring’s logarithmic formula accounts for discrete energy loss at each boundary reflection in well-treated studios:

Total Equivalent Absorption Area in Metric Sabins (A_f):
  A_f = Σ (S_i × α_i,f)   [in m² Sabins]

Area-Weighted Mean Absorption Coefficient (α_bar):
  α_bar = A_f ÷ S_total

Sabine Reverberation Time Equation (1898):
  RT60_Sabine = (0.161 × V_m3) ÷ A_f = (0.161 × V_m3) ÷ (S_total × α_bar)   [in seconds]

Norris-Eyring Reverberation Time Equation (1930, for damped rooms where α_bar > 0.20):
  RT60_Eyring = (0.161 × V_m3) ÷ ( − S_total × ln(1 − α_bar) )              [in seconds]

To coordinate your porous panel layout with your room’s standing-wave pressure nodes, Schroeder frequency, and 38% listening triangle, open the Small Room Standing-Wave Mode, Schroeder & 38% Placement Planner.