Moisture Content Effects on the Resonance Frequency and Loss Factor of Sitka Spruce and an Offset Correction Model

This study investigated Sitka spruce (Picea sitchensis (Bong.) Carr.) specimens from the same batch over a moisture-content range of 2%–12%. Under free–free boundary conditions with non-contact magnetic excitation, the first- to fifth-order resonance frequencies and loss factors were obtained, and the responses of resonance frequency, loss factor, dynamic modulus of elasticity, specific dynamic modulus of elasticity, acoustic radiation quality constant, and acoustic conversion efficiency to moisture content were analyzed. moisture-content offset correction models were established for the first- to fifth-order resonance frequencies and loss factors. The results showed that, with increasing moisture content, resonance frequencies decreased, loss factors increased, and the dynamic modulus of elasticity, specific dynamic modulus of elasticity, acoustic radiation quality constant, and acoustic conversion efficiency decreased. The interval decreases in the mean first-order resonance frequency decreased from 1.61 Hz in the 2%–4% interval to 0.84 Hz in the 10%–12% interval, while the loss factor increment declined from 1.11 × 10−3 to 5.80 × 10−4. The mean differences between measured and model-calculated values were −0.011 to 0.021 Hz for the first-order resonance frequency and −1.1 × 10−4 to 5.0 × 10−5 for the loss factor, indicating accurate conversion to a unified target moisture content.