Very rough or uneven reflective faces can produce errors. Material Must Conduct Ultrasound Most metals are excellent conductors of ultrasound. Surface Finish A very flat, smooth surface is extremely important to proper coupling of the transducer. A common problem occurs when a small peak is left in the center of a bolt head after milling the fastener head flat on a lathe. This small bump prevents the transducer from achieving proper contact and greatly reduces the signal amplitude.
Several methods are in use today.
The most common method is to use a magnetic transducer and center the transducer on the end of the bolt. On large diameter bolts, above an inch in diameter, the position that gives the greatest amplitude of return echo should be used. Sometimes after tightening, due to bending of the bolt, this position may move. The position of the transducer should be changed to the location on the bolt that provides the maximum return echo signal. This assures the optimum sound path is being used, both before and after tightening.