Phased Array Glossary

A-Scan: An ultrasonic waveform plotted as amplitude with respect to time. It may be either rectified or unrectified (RF).

Apodization: A computer-controlled function that applies lower excitation voltage to the outside elements of an array in order to reduce the amplitude of unwanted side lobes.

Aperture: In phased array testing, the width of the transducer element or group of elements pulsed simultaneously.

Azimuthal Scan: An alternate term for Sector scan. It is a two-dimensional view of all amplitude and time or depth data from all focal laws of a phased array probe corrected for delay and refracted angle.

B-Scan: A two-dimensional image of ultrasonic data plotted as reflector depth or distance with respect to beam position. B-scans may be either single value or cross-sectional.

B-scan, single value: A two-dimensional image based on plotting the first or largest reflector within a gate. This format is commonly used in ultrasonic flaw detectors and advanced thickness gages and it shows one reflector at each data point.

B-scan, cross-sectional: A two-dimensional image of ultrasonic data based on full waveform storage at each data, which can be plotted to show all reflectors in a cross-section rather than just the first or largest. This allows visualization of both near and far surface reflectors within the sample.

Bandwidth: The portion of the frequency response that falls within specified amplitude limits. In this context, it should be noted that typical NDT transducers do not generate sound waves at a single pure frequency, but rather over a range of frequencies centered at the nominal frequency designation. The industry standard is to specify this bandwidth at the -6dB (or half amplitude) point. As a general rule, broader bandwidth results in better near surface and axial resolution, while narrow bandwidth results in higher energy output and thus higher sensitivity.

Beam Forming: In phased array testing, generating a sound beam at a particular position, angle, and/or focus through sequential pulsing of the elements of an array transducer.

Beam spread: The angle of divergence from the centerline of a sound beam in its far field.

Beam Steering: The capability to modify the refracted angle of the sound beam generated by a phased array probe.
Calibration, wedge delay -- A procedure that electronically compensates for the different sound paths taken by different beam components in a wedge, used to normalize the measure sound path length to a reflector.

Calibration, sensitivity: A procedure that electronically equalizes amplitude response across all beam components in a phased array scan. This typically compensates for both element-to-element sensitivity variations, and the varying energy transfer at different refracted angles.

C-Scan: A two-dimensional view of ultrasonic amplitude or time/depth data displayed as a top view of the test piece.

Far Field: The portion of a sound beam beyond the last on-axis pressure maximum. Beam spreading occurs in the far field.

Focal Laws: The programmed pattern of time delays applied to pulsing and receiving from the individual elements of an array transducer in order to steer and/or focus the resulting sound beam and echo response.

Focus: In ultrasonics, the point at which a sound beam converges to minimum diameter and maximum sound pressure, and beyond which the beam diverges.

Grating Lobes: Spurious components of a sound beam diverging to the sides of the center of energy, caused by even sampling across the probe elements. Grating lobes occur only with phased array transducers and are caused by ray components associated with the regular, periodic spacing of the small individual elements. See also Side Lobes.

Huygens' Principle: A mathematical model of wave behavior that states that each point on an advancing wave front may be thought of as a point source that launches a new spherical wave, and the resulting unified wave front is the sum of those individual spherical waves.

Linear Scan: A scan in which the acoustic beam moves along the major axis of the array without any mechanical movement. A single focal law is multiplexed across groups of active elements, creating either a straight beam or a beam at a single angle that advances the length of the probe.

Near Field: The portion of a sound beam between the transducer and the last on-axis sound pressure peak. Transducers can be focused only in the near field.

Phased Array: A multi-element ultrasonic transducer (typically with 16, 32, or 64 elements) used to generate steered beams by means of phased pulsing and receiving.

Phasing: The interaction of two or more waves of the same frequency but with different time delays, which may result in either constructive or destructive interference.

Pitch: The separation between individual elements in a phased array transducer.

Plane, active: The orientation parallel to the phased array probe axis consisting of multiple elements.

Plane, passive: The orientation parallel to the individual transducer element length or probe width.

Plane, steering: The orientation in which the beam direction is varied for a phased array probe.

Pulse duration: The time interval between the point at which the leading edge of a waveform reaches a specified amplitude (typically -20 dB with respect to peak) to the point at which the trailing edge of the waveform reaches the same amplitude. Broader bandwidth typically reduces pulse duration while narrower bandwidth increases it. Pulse duration is highly dependent on pulser settings.

Resolution, angular: In phased array systems, the angular resolution is the minimum angular value between two A-scans where adjacent defects located at the same depth are individually resolvable.

Resolution, axial: The minimum depth separation between two specified reflectors that permits discrete identification of each. Higher frequency and/or higher bandwidth will generally increase axial separation.

Resolution, far surface: The minimum distance from the backwall surface at which a specified reflector has an echo amplitude at least 6 dB greater than the leading edge of the backwall echo. More generally, the closest distance from the backwall surface at which a reflector can be identified.

Resolution, lateral: In phased array systems, the minimum lateral separation between two specified reflectors that permits discrete identification of each. This is related to both the design of the array transducer and the selected focal law programming.

Resolution, near surface: The minimum distance from the sound entry surface at which a specified reflector has an echo amplitude at least 6 dB greater than the trailing edges of the excitation pulse, delay line, or wedge echo. More generally, the closest distance from the sound entry surface at which a reflector can be identified. The area above this point is known as the dead zone, and it generally increases as gain increases.

Sector Scan (S-Scan): A two-dimensional view of all amplitude and time or depth data from all focal laws of a phased array probe corrected for delay and refracted angle.

Side Lobes: Spurious components of a sound beam diverging to the sides of the center of energy, produced by acoustic pressure leaking from transducer elements at different and angles from the main lobe. Side lobes are generated by all types of ultrasonic transducers. See also Grating Lobes.

Virtual Aperture: The combined width of a group of phased array elements that are pulsed simultaneously.