This article presents a case study of a semi-detached residential building with a load-bearing system made of cross-laminated timber (CLT), in which sound insulation performance was assessed and improvement measures were proposed. The research objectives included the experimental determination of airborne and impact sound insulation of façade and internal elements, evaluation of the results against regulations and recommendations, and consideration of structural behaviour at low frequencies. Sound insulation measurements were conducted in accordance with ISO 16283-1, -2 and -3. The partition wall between the two dwellings achieved a high airborne sound insulation performance (R′w = 71 dB), exceeding minimum requirements and confirming effective structural design and execution. The façade achieved an R′w of 32 dB, satisfying the limit values; however, spectral analysis revealed a less favourable response in the low-frequency range, the cause of which could not be conclusively determined based on the measurements performed. The inter-storey floor exhibited an impact sound level of L′n,w 44 dB, complying with residential building requirements, while low-frequency impact transmission remains a challenge for the CLT systems. It follows from the above that a high R′w value and a low L′n,w value do not necessarily guarantee optimal acoustic protection in all cases. The results show that high acoustic performance in timber buildings can be achieved through appropriate design, early involvement of an acoustic specialist, and high-quality construction. In addition, the findings indicate that the reduced low-frequency sound insulation performance of the façade may result in increased sensitivity to external low-frequency noise sources, such as heat pumps, which should be carefully considered in future acoustic design and system placement.
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