2014

Uosukainen, Seppo

Absorption of acoustic wave is due to viscous and thermal losses and relaxation processes in fluid: - Absorption is emphasized near boundaries, especially in small holes, slits and narrow tubes, - Viscous losses dominate with small free space dimensions: -Perforated / microperforated plates, - Absorbing materials: "Holes" and "slits" described by fluid bulk properties (flow resistivity, porosity, tortuosity, viscous andthermal characteristic lengths). Sound transmission of a wall is due to two parallel transmission paths: - Non-resonant and resonant transmission, - Below coincidence frequency non-resonant transmission dominates except in the resonance region. With double walls above double-wall resonance, sound insulations in dBs are summed with reasonable amount of absorption in cavity if mechanical coupling is insignificant.