Bandpass Filter Comparator for Confirming the Absence of Serious Internal Voids in Wood Poles

Structural timber is frequently exposed to weather and is thus subject to decay and consequential strength loss which eventually requires the replacement of the affected member. When decay is internal, destructive investigation is the only direct way of detecting it. This is obviously undesirable and a frequently used alternative is to tap each member with a hammer. A conclusion is reached by listening to the sound emitted. Because this sound is composed of many different frequencies which vary in magnitude and spectrum from one member to another, the result of the test is uncertain. Research has revealed that a definite sonic frequency range exists for a class of members such as poles used for electricity distribution purposes. Part of this range is common to all members regardless of condition, but there is another part which is greatly affected by the presence of hollowness or decay. A double bandpass filter arrangement has been constructed to register the presence and magnitude of frequencies relating to hollowness or decay and to measure this magnitude against the magnitude of frequencies always present. The member under test is excited by a series of blows of equal force and a ratio established between the signals from each filter, the A low ratio indicates a solid section. A high ratio indicates that the section may be hollow or decayed.