Acoustic Sound Reduction (Rw & Ctr dB)
Sound attenuation in glass is governed by mass law, stiffness, and the critical coincidence frequency. Standard monolithic glass suffers an acoustic dip between 1000 Hz and 2500 Hz where human speech is concentrated; acoustic PVB laminated interlayers dissipate this resonance through shear damping.
1. The Coincidence Dip Phenomenon
When sound waves strike glass at an angle matching the bending wave velocity of the pane, acoustic energy passes straight through. A 6mm monolithic pane drops ~8 dB in performance at 2000 Hz. Acoustic PVB introduces a micro-viscoelastic core that flattens this dip.
2. Asymmetric Glass Configurations
In double glazing, identical panes (e.g. 4mm + cavity + 4mm) resonate at the same frequency, amplifying noise transmission. Specifying asymmetric thicknesses (e.g. 10mm Toughened + 16mm Cavity + 8.8mm Acoustic Laminated) ensures the two plies have different critical frequencies, maximizing noise cancellation.
3. Traffic and Urban Correction (Ctr)
The standard weighted sound reduction index (Rw) measures general speech and railway noise. For urban commercial facades subject to low-frequency diesel engine rumble, the Rw + Ctr rating must be specified.
Critical Specification Pitfalls to Avoid
- •Over-specifying glass thickness while neglecting perimeter acoustic frame gaskets (a 1% perimeter air leak reduces acoustic performance by up to 10 dB)
- •Assuming thicker monolithic glass is better than acoustic laminate (12mm monolithic Rw 34 dB vs 8.8mm acoustic lam Rw 37 dB at 25% lower weight)