Influence of length on acoustic time-of-flight (ToF) measurement in built-in structures of Norway spruce timber

Time-of-flight (ToF) measurements were conducted on twelve 76×226-mm² in cross-section, 4.52-m long specimens of Norway spruce timber pieces from a dismantled 19ᵗʰ-century building in Barcelona (Catalonia, Spain). Two commercially available acoustic devices were used: the USLab ultrasound device with conical 22 and 45 kHz sensors and the MicroSecond Timer (MST) stress wave device with spike sensors. ToF were obtained for the full-length (4.52 m) specimens in an end-to-end arrangement and for lengths of 0.5, 1, 1.5, 2, 3 and 4 m on the same surface and on opposite surfaces. The differences between the velocities obtained from end-to-end and semi-direct measurements were less than 4.5%. Apparent velocity dependence on length was observed in short-distance measurements. This could be corrected by determining a time lag (tL) from a linear regression between the ToF and the distance. Estimation of the modulus of elasticity (MOE) from the dynamic MOE (MOEdyₙ) is acceptable (r²=0.45–0.69) depending on the measurement procedure, and the best results for in situ timber are obtained in edge surface velocity. Although modulus of rupture (MOR) estimation from MOEdyₙ improves slightly when knottiness is included, it has low predictive capacity.