From Material Properties to Acoustic Response: Multilayer Varnish Coating Alters the Acoustic Vibration Behavior of Spruce Wood

Varnish coatings affect the vibro-mechanical properties of wood by altering its material properties, but the underlying mechanisms remain to be further elucidated. This study combined microstructural and performance characterization with vibration signal analysis to investigate the influences of multi-layer essential oil varnish coatings on the dynamic response characteristics and acoustic vibration properties of Norway spruce wood. The results showed that, after coating, the varnish–wood composite system exhibited increased density (ρ) and damping (tanδ) but decreased stiffness (E), leading to lower resonance frequencies, faster vibration attenuation, and reduced relative acoustic signal energy (single-sided coating: −24.33%; double-sided coating: −53.39%). The sound intensity reflection coefficient increased while the transmission coefficient decreased. Mediation analysis based on the MEMORE (Mediation and Moderation for Repeated-Measures Designs) procedure indicated that the coating affected the acoustic vibration performance of wood by altering its material properties, with the full mediation effect of tanδ not being confounded by mathematical relationships. This study demonstrates that surface coating alters the acoustic properties of wood through mass, stiffness, and damping effects, providing a reference for the design of coatings and performance regulation of wood used in musical instruments.