A sound damping sheet for car panels only works if it’s actually bonded flat against the metal. Air gaps under the material basically cancel out the damping effect, since the whole mechanism depends on direct contact transferring vibration into the
Posted by nakamichicaraudio 34 minutes ago (https://picgiraffe.com/does-sound-dampening-actually-make-a-car-quieter-or-is-it-mostly-placebo)

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