Understanding Ultrasonic Testing Terms

Non-destructive testing (NDT), or ultrasonic testing, has its own vocabulary. The following list of terms includes the most frequently used definitions in the rail flaw detection industry.

A-scan display A data presentation method where signal amplitude is plotted along the y-axis versus time on the x-axis. The horizontal distance between any two signals represents the material distance between the two conditions causing the signals. In a linear system, the vertical excursion is proportional to the amplitude of the signal.
Acoustic impedance (Z) The resistance of a material to the passage of sound waves. The value is the product of the material density and sound velocity. The acoustic  impedance of a material determines how much sound is transmitted and reflected when the wave encounters a boundary with another material. The larger the difference in acoustic impedance between two materials, the larger the amount of reflected energy will be.
Amplitude 1. The maximum absolute value obtained by the disturbance of a wave or any quantity that varies periodically. 2. The vertical height of a received signal on an A scan.
Angle beam testing An ultrasound testing technique that uses an incidence wave angle other than 90 degrees to the test surface. The refracted angle of the sound energy is calculated using Snell's law.
Angle beam transducers A device used to generate sound energy, send the energy into a material at an angle other than 90 degrees to the surface, and receive reflected energy and convert it to electrical pulses.
Angle of incidence The angle between the direction of propagation of an electromagnetic or acoustic wave incident on a body and the local normal to that body.
Angle of reflection The angle between the direction of propagation of an electromagnetic or acoustic wave reflected by a body and the local normal of that body.
Angle of refraction The angle between the direction of propagation of an electromagnetic or acoustic wave refracted by an optically homogeneous body and the local normal to that body.
Array transducer A transducer made up of several individually piezoelectric elements connected so that the signals they transmit or receive may be treated separately or combined as desired.
Attenuator A device for causing or measuring attenuation, usually calibrated in decibels.
B-scan display A data presentation method applied to pulse echo techniques. It produces a two-dimensional view of a cross-sectional plane through the test object. The horizontal sweep is proportional to the distance along the test object and the vertical sweep is proportional to depth, showing the front and back surfaces and discontinuities between.
Back reflection The signal received from the far boundary or back surface of a test object.
Beam spread The divergence of the sound beam as it travels through a medium - specifically, the solid angle that contains the main lobe of the beam in the far field.
Compressional wave A wave in which the particle motion in the material is parallel to the wave propagation distance, also called a longitudinal wave.
Contact method The testing method in which the transducer face makes direct contact with the test object through a thin film of couplant.
Contact transducers An ultrasonic transducer that is designed to be used in direct contact with the surface of the test article.
Couplant A substance, usually liquid, used between the transducer and the test surface to permit or improve transmission of ultrasonic energy into the test object.
Cross talk The unwanted signal leakage (acoustical or electrical) across an intended barrier, such as leakage between the transmitting and receiving elements of a dual transducer (also called cross noise and cross coupling).
Cycle (Hertz) Comprises a complete set of recurrent values of a periodic quantity.
Decibel A logarithmic unit for expressing power relationships.
Defect A discontinuity or other imperfection causing a reduction in the quality of a material or component.
Density The mass of a substance per unit volume.
Discontinuity A break in the continuity of a medium or material.
Echo A signal indicating reflected acoustic energy.
Elasticity A term that describes how quickly molecules return to their original positions.
False indication A test indication that could be interpreted as originating from a discontinuity but which actually originates where no discontinuity exists.
Flat bottom hole A type of reflector commonly used in reference standards. The end (bottom) surface of the hole is the reflector.
Frequency The number of waves that pass a given point in a specified unit of time.
Gain control A control which varies the amplification of the ultrasonic system (also considered the sensitivity control).
Gate An electronic device for monitoring signals in a selected segment of the trace on an A-scan display. The interval along the baseline that is being monitored.
Hertz One cycle per second.
Inherent defects Discontinuities that are normal in the material at the time it originally solidifies from the molten state.
Longitudinal waves Commonly used term for compressional wave.
Loss of back reflection Absence or significant reduction of an indication from the back surface of the test object.
Noise Any undesired signal that obscures the signal of interest. It might be electrical noise or a signal from specimen dimensional or property variations.
Nondestructive testing (NDT) Testing to detect defects in materials using techniques that do not damage or destroy the items being tested.
Orientation The angular relationship of a surface, plane, discontinuity or axis to a reference plane or surface.
Phase array A mosaic of transducer elements in which the timing of the elements' excitation can be individually controlled to produce certain desired effects, such as steering the beam axis or focusing the beam.
Piezoelectric effect The ability of certain materials to convert electrical energy into mechanical energy and vice versa.
Propagation Advancement of a wave through a medium.
Pulse A transient signal or ultrasonic signal.
Pulse echo method An ultrasonic test method in which discontinuities are detected by return echoes from the transmitted pulses.
Pulse-echo test A test that can determine the location of a discontinuity by measuring the time required for a short ultrasonic pulse to travel through the material.
Pulse method Use of ultrasonic equipment that generated a series of pulses that are separated from each other by a constant period of time, i.e.. energy is not sent out continuously.
Pulse rate Number of pulses that are transmitted in a unit of time (also called pulse repetition rate).
Pulser-Receiver Used with a transducer and oscilloscope for flaw detection and thickness gauging.
Range The maximum ultrasonic path length is displayed.
Refracted beam A beam that occurs in the second medium when an ultrasonic beam is incident at an acute angle on the interface between two media having different sound velocities.
Refraction Ability to clearly distinguish signals obtained from two reflective surfaces with a minimum difference in depth. Near surface resolution is the ability to clearly distinguish a signal from a reflector under the near surface without interference from the initial pulse signal. Far surface resolution is the ability to clearly distinguish from the back surface when the sound beam is normal to that back surface.
Scanning Movement of the transducer over the surface of the test object in a controlled manner so as to achieve complete coverage. May either be contact or immersion method.
Search unit An assembly comprising a piezoelectric element, backing material (damping), wear plate or wedge (optional) and leads enclosed in a housing (also called transducer or probe).
Sensitivity A measure of the ability to detect small signals. Limited by the signal-to-noise ratio.
Shear waves Waves that move perpendicular to the direction the wave propagates.
Shear wave transducer An angle beam transducer designed to cause converted shear waves to propagate at a nominal angle in a specified test medium.
Shoe A device used to adapt a straight beam transducer for use in a specific type of testing, including angle beam or surface wave tests and tests on curved surfaces.
Sound Mechanical vibrations transmitted by an elastic medium.
Test frequency The frequency or vibration of the ultrasonic transducer employed for ultrasonic testing.
Test surface The surface of the test object at which the ultrasonic energy enters or leaves.
Time of flight The time for an acoustic wave to travel between two points, for example, the time required for a pulse to travel from the transmitter to the receiver via diffraction at a discontinuity edge or along the surface of the test object.
Transducer An electro-acoustic or magneto-acoustic device containing an element for converting electrical energy into acoustical energy and vice versa. See "search unit."
Ultrasonic A term referring to acoustic vibration frequencies greater than about 20,000 hertz.
Ultrasonic testing The transmission of high-frequency sound waves into a material to detect imperfections or to locate changes in material properties.
Ultrasonic vibrations Vibrational waves of a frequency above the hearing range of the normal human ear are referred to as ultrasonic, and the term includes all waves of a frequency of more than approximately 20,000 cycles per second. Also known as ultrasonic waves.
Ultrasonic waves Sound waves too high in frequency for humans to hear.
Ultrasonically sound material A material having no discontinuities that cause discernible ultrasonic indications at the required test sensitivity level.
Velocity Distance traveled per unit of time.
Vibration A rapid back-and-forth motion of a particle or solid.
Wavelength The distance needed in the propagation direction for a wave to go through a complete cycle.
Wedge A device used to direct ultrasonic energy into a test object at an acute angle. See also "shoe".