Absorption of periodically aligned absorber strips in concrete structures

At present, new buildings are frequently built by thermally active concrete ceilings due to the requirements of thermal comfort and energy saving. Thus sound absorbers (sails, baffles) are installed below the ceiling in order not to deteriorate thermal efficiency. This frequently collides with architectural design asking for planar surfaces. A periodic arrangement of sound-absorbent strips in concrete ceilings shows higher sound absorption than is to be expected from surface-averaged sound absorption. This is well known, since the ceiling behaves like an acoustic phase grating. The challenge for acoustic consultants is to design periodic ceiling structures in a way that the sound absorption is adequate to keep thermal losses as low as possible. Based on the socalled Rayleigh expansion, frequently published so far, a simple model was derived to predict sound absorption at normal, oblique and field incidence. The model was validated by means of measurements and calculations for two types of absorbers: a micro-perforated resonator and a porous absorber made of recycled glass. The measured and computed results comply at normal incidence, whereas discrepancy between measurements and calculations is clearly higher at field incidence. It is assumed that this behavior is mainly due to the edge effect. The findings of a demonstration project in situ are: sound absorption achieved by means of periodic strips in concrete ceilings is adequate for usual office use, thermal losses are very low, but implementation at the construction site requires further improvement. Part of this text was previously published in the proceedings of INTER-NOISE 2008.