A technique used during the late 1920’s for testing welds for porosity by evaluating the sound produced with the tapping of a hammer. A stethoscope with a rubber tip made contact on the surface of the plate. This apparatus excluded extraneous sounds and minimized damping of the oscillations at the contact point on the metal. The drum, pipe or plate being tested is tapped with a small hammer at the spot to be tested.

The presence of a serious defect in a vessel could often be determined by its ring. A “flat” or dull sound indicated to the trained ear the presence of a defect.

In past times, blacksmiths used this principle when hammering metal cold on the anvil to test it. In “sounding” a tank, however, this technique was difficult due to the forced and natural vibrations of the tank, and the tendency for the natural vibration to drown out all other sounds. This technique has been superseded by radiographic and other nondestructive testing technologies, except perhaps in cases where this equipment is not available.

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