Influence of framing density and insulation materials on the airborne sound insulation of prefabricated timber wall assemblies

Lightweight timber assemblies are increasingly adopted in modular, prefabricated construction, yet their acoustic performance remains a challenge. In particular, the framing density of wall systems influences both structural behaviour and airborne sound transmission. This study presents experimental results on the combined influence of framing density and insulation materials on the airborne sound insulation of prefabricated timber wall assemblies. Two wall configurations were tested under laboratory conditions: one with standard stud spacing and another with increased framing density. For each configuration, five insulation materials were considered: loose cork granules, expanded cork boards, mineral wool boards, wood fibre boards, and cellulose boards. All assemblies included identical sheathing layers to isolate the effects of framing and insulation. Airborne sound insulation was measured according to ISO 10140-2:2021, and results were analysed in terms of the sound reduction index across the frequency spectrum. The results indicate that framing density and insulation material jointly influence acoustic performance, with contrasting effects depending on frequency. At mid frequencies (≈500 Hz), walls with lower stud density systematically outperformed denser configurations, with differences up to 8 dB depending on the insulation material. At low frequencies (≈125 Hz), increased framing density improved insulation by up to 8 dB across all materials. The findings highlight a trade-off between stud density and acoustic behaviour: reduced framing enhances mid-frequency performance, directly impacting standardised airborne sound insulation indices, while denser framing provides more rigidity with advantages at low frequencies. At high frequencies, behaviour across different stud densities was similar, with no significant differences. The insulation material further modulates these effects, with mineral wool, cellulose, and wood fibre boards showing superior performance across both ranges. These findings highlight the need to carefully balance structural and acoustic requirements in timber modular construction. Future work will extend the study to a large wall area and additional wall typologies.