What is the working range of ultrasonic sound beams?
In this article:
- Working Range Defines Where Flaws Can Be Reliably Detected. In automated ultrasonic testing, the working range is the area and depth where a flaw of a given size (e.g., 6 mm) can be detected with minimal echo loss (≤ 6 dB).
- Sound Field Behaviour and Scanning Paths Shape the Range. The effective working area depends not just on probe geometry but on how the sound beam spreads and interacts with reflectors across the scanned volume.
- Normalized Diagrams and Amplitude Drop Criteria Aid Calculation. Using DGS and fixed-threshold diagrams, normalized reflector size (G), probe movement (B), and distance (Z) are combined to define the limits of the working range.
- Waygate technologies provides comprehensive UT solutions. With advanced phased array systems, DGS tools, and automated scanning solutions, Waygate supports precise, standards-based flaw detection in automated UT.
What is the working range of ultrasonic sound beams?
In automated ultrasonic testing, accuracy and efficiency must be balanced with the physical constraints of sound propagation. A central question becomes: over what area and depth can a sound beam reliably detect a flaw of a given size? This working range is not defined merely by the probe’s geometry, but by how sound energy is distributed within the field and how reflectors respond at different positions. This article explores how to define and calculate the working range using DGS diagrams, normalized variables, and amplitude drop criteria.
Influence of scanning paths and sound field behaviour
With automatic testing the test specimen is scanned along pre-selected paths although most probably not all possible flaw locations are located on these paths. For reason of economy the testing is to be done as fast as possible. One has to accept therefore, that for the evaluation, the highest possible echoes are not available as they are with the manual test. With automatic testing accordingly the same reflectors can be detected with different sensitivities depending upon their position in the sound beam.
In this connection remember that the sound field builds up and collapses in cycle with the pulse repetition frequency (chapter 7).
In order that there are no undesirable testing gaps when testing automatically the scanning grid must be adapted to those conditions called for by the sound field and the expected reflectors. It is not sufficient to establish the scanning paths simply by considering the probe size—the decisive point is the size of the portion of the sound field which is acceptable as the working range.
Defining the working range for flaw detection
This working range can be determined for flaw scanning using diagrams 46 and 47. The following example makes it easier to understand:
What is the working range when using a probe with Di, Ni taking into consideration the requirement that, within this range an (equivalent) reflector of d = 6 mm is to be indicated with an echo amplitude which is less than 6 dB below the highest possible echo indication?
This requirement means that diagrams must be used for the 6 dB drop with fixed threshold (fig. 47). The distance diagram fig. 40 (DGS) serves for evaluating the pressure on the axis of the sound beam.
Using the normalized reflector size G, one determines the normalized probe movement B for different normalized distances Z. These values are converted into absolute values with Di and Ni. In this way one obtains limiting points of the working range looked for (fig. 48).
Example: Determining the working range for a 6 mm reflector
given:
Di = 19 mm (effective diameter of the transducer)
Ni = 61 mm (effective nearfield length) d = 6 mm (diameter of the
equivalent reflector)
requirement:
∆V ≤ 6 dB evaluation:
G = d/Di = 6/19 ≈ 0,3
with fig. 40 follows for G = 0,3: Vmax = V(0) + 12 dB when Z3 = 1,15 (normalized distance)
Vmax + 6 dB = V(0) + 18 dB when Z1 = 0,70 und Z5 = 2,40
At these distances, the transverse expansion of the required working area is zero (B = 0). Diagrams with a fixed threshold between Z1 and Z5 are available for:
Z2 = 1,0 and Z4 = 2,0 (fig. 48)
z1 = Z1 ∙ Ni = 0,70 ∙ 61 mm = 43 mm
z2 = Z2 ∙ Ni = 1,00 ∙ 61 mm = 61 mm
z3 = Z3 ∙ Ni = 1,15 ∙ 61 mm = 70 mm
z4 = Z4 ∙ Ni = 2,00 ∙ 61 mm = 122 mm
z5 = Z5 ∙ Ni = 2,50 ∙ 61 mm = 146 mm
The fixed threshold -18 dB can be interpolated in fig. 47.
for G = 0,3 und Z1 = 1,0 0 it follows from fig. 47 (top):
B2 = 0,65
Für G = 0,3 und Z4 = 2,0 it follows from fig. 47 (bottom):
B4 = 0,2
from which the mm-values are calculated:
b2 = B2 ∙ Di = 0,65 ∙ 19 ≈ 12,4 mm
b4 = B4 ∙ Di = 0,2 ∙ 19 = 3,8 mm
If one draws b2 and b4 symmetrically to the central axis of the sound beam (fig. 48) (b/2 to each side) then one obtains, including points z1 and z5, an enclosed curve which limits the range in which a 6 mm reflector will be indicated by echoes whose difference in amplitude is less than 6 dB.
Width vs. volume of working range
If one does not require the working range (with reference to the volume) but only the width of the working range at a fixed reflector distance Zr then the distance law (fig. 40) does not have to be considered. The width of the range for an echo amplitude drop of 6 dB is obtained directly from the diagram for the relative thresh- old (fig. 46).
Understanding and calculating the working range of ultrasonic sound beams is essential for reliable flaw detection in automated systems. By applying normalized parameters and interpreting amplitude drop-offs using reference diagrams, inspectors can precisely define the region where flaws of a specified size will produce measurable echo signals. These calculated limits help ensure full coverage and reduce the likelihood of undetected defects—even under the time and path constraints of high-speed automatic scanning.
Further information on automatic testing:
[6] U. Schlengermann: Die automa- tische Ultraschallprüfung mit der Impuls-Echo-Methode als Infor- mationssystem (The automatic ultrasonic test using the pulse echo method as an information system), Materialprüfung 16 (1974) no 10, 326