Understanding the Ultrasonic Near Field (More Scientific Detail)

The near field, also called the Fresnel zone, is the region directly in front of the ultrasonic transducer where sound waves interfere with each other in complex ways. This interference arises because ultrasonic transducers emit waves from their entire active surface, causing multiple wavefronts to overlap. In practical terms, this can lead to drastic variations in signal amplitude and uneven wave patterns throughout this zone.

Mathematically, the length of the near field can be estimated by the formula:

Near Field Length (N) Formula:

\[ N = \frac{D^2 \times f}{4 \times v} \]

Where:

• D is the diameter of the transducer (probe).
• f is the frequency of the ultrasound.
• v is the speed of sound in the material being tested.

Within this region, the superposition of individual wave components creates constructive and destructive interference, leading to areas of high intensity (“hot spots”) and low intensity. Because of these fluctuations in wave amplitude, measuring defects or material features within the near field can be unpredictable and less repeatable.

Once you pass the near field boundary (i.e., move beyond N), you enter the far field (or Fraunhofer zone). In this zone, the sound beam spreads out more uniformly, and its intensity distribution becomes smoother and more predictable. This is why most critical evaluations and defect sizing tasks are done in the far field, where measurement results tend to be more reliable.

In a graphical representation:

• The blue shaded area typically shows the complex interference patterns of the near field, where the beam profile can vary dramatically.
• The red dashed line indicates the approximate start of the far field, beyond which you can expect more consistent signal amplitudes.

By using this formula and visualizing the beam profile, technicians can better plan inspections and avoid the uncertainties associated with the near field, ensuring more accurate results in ultrasonic testing.