A specific operating range, optimized phase spread, called Frequency prime (f90), occurring where there's a phase difference from the shallow I.D. to shallow O.D. defects of 90° (aka base frequency). Additional frequencies are selected for mixing, landmarking (supports), flux density considerations, etc. Minimum base frequency = 4.8. Maximum base frequency 2.1 x f90 = 10. f90 = The frequency which generates a 90° phase separation between a shallow inside and a shallow outside originated defect.