Mass Law

The mass law is a fundamental principle in sound insulation stating that the transmission loss of a single homogeneous panel increases by approximately 6 dB for each doubling of either its surface mass or the frequency. Expressed as TL = 20×log(mf) − 42 dB (for normal incidence), where m is surface mass in kg/m² and f is frequency in Hz. For random incidence (practical field conditions), the mass law prediction is approximately 5 dB lower. The mass law holds reasonably well below the coincidence frequency, where the panel’s bending stiffness provides additional resistance beyond its mass. Above coincidence, mass again governs but damping becomes important. The mass law explains why heavy walls (concrete, brick) provide good insulation and why lightweight partitions require multiple layers and air gaps to achieve comparable performance. Double-leaf partitions with an air cavity can exceed the mass law significantly because the cavity decouples the two leaves.