GE USM 36 Series Technical Reference And Operating Manual page 28

1 Introduction Important information on ultrasonic testing
The ultrasonic wave is attenuated in any material. This
sound attenuation is very low, e.g. in parts made of fine-
grained steel, likewise in many small parts made of oth-
er materials. However, if the sound wave travels larger
distances through the material, a high cumulative sound
attenuation can result, even with small attenuation coef-
ficients. There is then a danger that echoes from natural
flaws appear too small. For this reason, an estimate
must always be made of the effects of attenuation on the
evaluation result and taken into account if applicable.
If the test object has a rough surface, part of the incident
sound energy will be scattered at its surface and is not
available for the test. The larger this initial scattering, the
smaller the flaw echoes appear, and the more errors oc-
cur in the evaluation result.
It is therefore important to take the effect of the test ob-
ject's surfaces on the height of the echo into account
(transfer correction).
1-8 Issue 2 (12/2013) USM 36