On the use of dynamic vibration absorbers to counteract the loss of sound insulation due to mass-spring-mass resonance effects in external thermal insulation composite systems

•External Thermal Insulation Composite Systems (ETICS) suffer from a reduced sound reduction index in the low frequency range.•This reduced sound reduction index is caused by a mass-spring-mass resonance effect.•Dynamic vibration absorbers are proposed to counter the decrease of the sound reduction index.•Experimental results are given to prove the proposed solution.•Both ISO 10140-2 and laser Doppler vibrometry measurements are presented. One of the common ways to increase the energy performance of existing buildings is to add a thermal insulation layer to their façade elements, which in turn is covered by a thin protecting cement plaster. Such an arrangement is commonly known as External Thermal Insulation Composite System (ETICS). This article studies the effect of an ETICS implementation induced mechanical resonance mechanism, which results in a loss of performance in the sound insulation spectrum. A mitigating approach is proposed, which is based on an array of dynamic vibration absorbers that were designed to counteract the decrease of the sound reduction index. Using a wavenumber domain approach combined with unit cell FEM-modelling employing Bloch-Floquet boundary conditions, numerical predictions are presented for the sound reduction index. A number of different dynamic vibration absorber designs were tested on a silicate cement brick wall in a transmission loss facility. The predictions are in good agreement with experimental data obtained by classical, microphone based acoustic isolation measurements and laser Doppler vibrometry.