Preparatory Notes for ASNT NDT Level III Examination - Ultrasonic Testing, UT

2.6.3 Frequency selection
There is no ideal frequency; therefore, frequency selection must be made with
consideration of several factors. Frequency determines the wavelength of the
sound energy traveling through the material. Low frequency has longer
wavelengths and will penetrate deeper than higher frequencies. To penetrate
a thick piece, low frequencies should be used. Another factor is the size of the
grain structure in the material. High frequencies with shorter wavelengths
tend to reflect off grain boundaries and become lost or result in ultrasonic
noise that can mask flaw signals. Low frequencies must be used with coarse
grain structures. However, test resolution decreases when frequency is
decreased. Small defects detectable at high frequencies may be missed at
lower frequencies. In addition, variations in instrument characteristics and
settings as well as material properties and coupling conditions play a major
role in system performance. It is critical that approved testing procedures be
followed.