peer-reviewedAcoustic velocities measured on standing trees using time-of-flight (TOF) devices have been found to be
between 7% and 36% higher for softwoods than those in logs using resonance techniques based on
longitudinal frequencies. This effect was explained in three different ways: (1) TOF devices on standing
trees measure outerwood containing more mature wood while resonance methods assess the whole crosssection,
(2) the variation in the velocity is due to loading conditions in standing trees, while logs are free
of loads and (3) the acoustic waves are dilatational waves in the case of TOF measurements on standing
trees and one-dimensional longitudinal waves in the case of resonance on logs. This is an important topic
considering the fact that resonance methods are considered more accurate for predicting mechanical
properties and it has been proposed that correction factors should be applied on TOF measurements.
In the present work, four hardwoods from Irish forests were studied and, on average, TOF velocities
measured in the forest above fibre saturation point (FSP) were 19.8% higher than those from resonance
measurements taken on logs immediately after felling. However, this difference reduced to 5.4% when
the measurements were repeated at a moisture content (MC) of about 18% in the laboratory. Therefore,
there is a MC effect on the velocity differences. Furthermore, higher differences were systematically
found in older specimens in wet condition. However, this age effect was small in most cases