Question

An architect is designing a recording studio for a musician. The studio needs to be soundproof to keep out noise. The architect plans to use soundproof panels for the walls, ceiling, and floor. Each soundproof panel is made from two sheets of hard foam with a vacuum in between them, as shown in the diagram.

Diagram of soundproof panel with hard foam sheets and vacuum between them.
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Describe three ways the soundproof panel works to keep out noise.

Answer 1

1) The vacuum layer stops airborne sound transmission. Sound is a pressure wave that needs a material medium (air or a solid) to travel through. With a vacuum between the two skins there is essentially no air to carry the pressure wave from one side to the other, so very little airborne sound gets through the panel.

2) Large impedance mismatch and reflection at the faces. The hard-foam faces present a big change in acoustic impedance relative to the studio air, so much of an incident sound wave is reflected back into the room instead of being transmitted. The two solid faces therefore act like mirrors for sound at many frequencies.

3) Mass, decoupling and damping reduce what little energy can pass. The two separate hard-foam skins add mass (mass-law: heavier barriers transmit less sound) and are mechanically decoupled by the vacuum gap, so structural vibrations are not easily carried straight through. Any vibration that does enter the foam is partly dissipated by internal friction in the foam (converted to heat), so transmitted energy is reduced further.

(Practical note: seams, frames and rigid connections provide flanking paths for sound, so the panel must be well-mounted/sealed and isolated at edges to achieve good overall soundproofing.)