Automotive NVH
Doubling the mass buys six decibels. Splitting the same mass into two skins with foam between them buys far more — everywhere except at the resonance it creates.
— dB
Attenuation through the full stack at the chosen frequency
The double-wall gain is applied as a clean 18 dB per octave above resonance, which real panels never quite achieve and which ignores the dip at the resonance itself — right at that frequency the assembly performs worse than its mass alone, and this model does not show that trough. Coincidence at high frequency is also absent, so the numbers above roughly 2 kHz on a stiff substrate are optimistic. Mass law is a diffuse-field infinite-panel result; a real headliner is a small, curved, edge-bonded panel and flanking through the pillars usually governs the cabin anyway.
Automotive Headliner Sound Transmission Loss Predictor — free, with the formula and a worked example, at Textile School.