Tomography in standing trees: revisiting the determination of acoustic wave velocity

Key message

Wood acoustic velocity determination is affected by the wavelength and the detection algorithm used. The Fisher algorithm is optimal with high signal attenuation; otherwise, the Hinkley algorithm should be used.

Abstract

The quality of acoustic tomographic images in standing trees is mainly function of the accuracy of the acoustic velocity computation. Improving the acoustic velocity determination is, furthermore, of great interest because acoustic tools are widely used in nondestructive testing of wood. Four different signal processing algorithms were used (1) to study the effect of the signal dynamic on the velocity determination, (2) to determine the validity range of each computation method, and (3) to compare the behavior between a homogeneous material and wood.  The experiments were performed using the conventional experimental protocol for the ultrasonic characterization of materials in a tank (normal incidence transmission at 500 kHz). A polyurethane resin (homogeneous material) and two wood species (Bagassa guianensis Aubl., Milicia excelsa (Welw.) C.C. Berg) were used for the experiments.

Computed velocity increased as the noise level increased. The Hinkley method appeared to be the most exact when the noise level exceeded 10 dB. The Fisher method was that most suitable for very noisy signals. No difference was found between the resin and the wood samples. A combination of the Fisher and Hinkley methods in the same algorithm would yield the most accurate acoustic velocity determinations in the tomography of standing trees.

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