When sound leaves an ultrasonic transducer it isn't just one wave produced from a single point but a whole wave front produced from multiple points along the face of the transducer!
The intensity of the beam varies in the material adjacent to the transducer due to the constructive and destructive interference of these individual waves which is known as the near field. This phenomena makes it extremely difficult to detected discontinuities accurately as most modern day techniques use echo amplitude in their criteria.
The size of the near field, that is, the distance that these flucuations in the material occur is dependant and can be controlled by the probe frequency, transducer crystal diameter and material velocity.
This online tool below will calculate the length of the ultrasonic near zone given the following
- Probe Diameter
- Material Velocity
- Probe Frequency
Probe near zones should always be evaluated using the appropriate industry standard, however, knowing the effect of material velocity, probe frequency and probe diameter will always benefit an NDT technician when working with angle inspection or the immersion technique.
The above calculator uses the following equation:
NZ = Length of the nearzone
D = Diameter of the probe
F = Frequency of the probe
V = Velocity of the material
Suppose you wish to calculate the nearzone of a probe of 5mHz frequency, 10mm diameter and a material of 2330m/s velocity:
NZ = ????
D = 10mm
F = 5mHz = 5,000,000 Hz
V = 2330m/s = 2,330,000 mm/s
Subsituting these figures into the equation above gives us: