Coincidence Frequency

The coincidence frequency (also called the critical frequency) is the frequency at which the wavelength of bending waves in a panel equals the wavelength of the incident airborne sound wave projected onto the panel surface. At this frequency, the panel vibrates efficiently in response to the airborne excitation, causing a significant dip in transmission loss (10–20 dB below the mass law prediction). Below the coincidence frequency, the panel’s bending stiffness resists excitation; above it, the mass law governs. The coincidence frequency depends on the panel’s material properties (bending stiffness, surface density) and thickness. For 12.5 mm plasterboard, it is approximately 3000 Hz; for 150 mm concrete, approximately 120 Hz. Strategies to mitigate coincidence effects include using constrained layer damping (viscoelastic compounds), multiple layers of different thickness, and decoupled double-leaf constructions. The coincidence effect is a fundamental consideration in sound insulation design.