ACOUSTIC IMPEDANCE - A material property defined as the product of sound velocity (c) and the material's density (d).

ACOUSTIC INTERFACE - The boundary between two materials of different acoustic impedance.

ACOUSTIC VELOCITY - The speed of sound in a test material. Different wave modes such as longitudinal and shear will have different velocities.

AMPLIFIER - An electronic device that increases the strength of a signal, using power from a source other than the input signal.

AMPLITUDE - The height of an indication on a waveform display, measured from the lowest to the highest point on the indication. This corresponds to relative signal strength and is often referred to as %FSH (percent full screen height).

ANALOG OUTPUT - A data output format consisting of a DC voltage whose amplitude corresponds to a measured parameter such as echo amplitude or depth.

ANGLE BEAM TRANSDUCER - A transducer that transmits or receives acoustic energy at an angle to the surface, to generate refracted shear waves or surface waves in the part being inspected.

A-SCAN - The most common waveform display format, plotting signal amplitude versus distance. An A-Scan shows the pulse travel time in the horizontal direction (left to right), representing the corresponding sound paths, and the echo amplitude in the vertical direction.

ATTENUATION - The loss in acoustic energy that occurs between any two points of travel. This loss may be due to absorption, reflection, scattering, or other phenomena.

BACKWALL ECHO - The echo received from the side of the test piece opposite the side to which the transducer is coupled. This echo represents the thickness of the test piece at that point.

BACKGROUND NOISE - Extraneous signals caused by sources within the ultrasonic testing system and/or the material being tested.

BEAM-INDEX POINT (BIP) - For an angle beam probe, the point where the sound leaves the wedge and enters the specimen.

COUPLANT - A material (usually a liquid or gel) used between the transducer and the test piece to eliminate air from this space and facilitate the coupling of sound waves into and out of the specimen.

CRITICAL DEFECT - Either the largest tolerable defect or the smallest intolerable defect in a test piece. The critical defect size is usually given by a specification or code.

CROSS TALK - An unwanted condition affecting dual element transducers in which acoustic energy travels from the transmitting element to the receiving element by a route other than the intended path through the test piece.

DAMPING (CONTROL) - A variable resistance across the pulser circuit output that shapes the excitation pulse. Typically it is used to change pulse characteristics to optimize either penetration (low damping) or near surface resolution (high damping).

DAMPING MATERIAL - Any epoxy or other high attenuation material used in the transducer to shorten the ringing time of the piezoelectric element.

DECIBEL (dB) - A unit which compares levels of power, commonly used to express relative sound intensities in ultrasonic testing. Two amplitude levels V1 and V2, expressed as either voltages or screen percentages, are said to differ by n decibels where:
dB = 20 log (V2 / V1)

DELAY CONTROL - Software function that introduces an adjustable time period between the excitation pulse and the start of the sweep across the data display.

DETECTABILITY - The ability of a test system (instrument and transducer) to detect or "see" a given size reflector.

DISTANCE AMPLITUDE CORRECTION (DAC) - A method of flaw sizing that plots an on-screen curve representing the echo amplitude from a known size reflector at varying distances from the transducer. This curve compensates for the loss of energy due to beam spreading and attenuation.

DISTANCE GAIN SIZING (DGS) - A sizing technique that mathematically relates the amplitude of the echo from a reflector to that of a flat bottom hold at the same depth or distance. The generated curves are derived from the calculated beam spreading pattern of a given transducer, based on its frequency and element diameter, and adjusted for material attenuation.

DUAL ELEMENT PROBE - A transducer containing two piezoelectric elements, one which transmits and one which receives.

DYNAMIC RANGE - The range of gain or signal amplitude that can be displayed on a given instrument.

EXCITATION PULSE - The pulse of electrical energy sent by the pulser to the transducer, sometimes known informally as the main bang. Both spike (broadband) and square wave (tuned) excitation pulses are commonly used.

FILTER - Receiver function that passes selected frequency components of a signal and rejects others, used to improve signal-to-noise, near surface resolution, and dynamic range. Filters may be high pass, low pass, narrowband, or broadband.

FIRST CRITICAL ANGLE - In angle beam or immersion testing, the minimum incident angle in the wedge or water path at which the refracted longitudinal wave disappears from the test piece.

FLAW - A discontinuity that may be undesirable but does not necessarily call for rejection, depending on procedural requirements.

FREQUENCY - The number of complete cycles per second undergone or produced by an oscillating as body expressed the reciprocal of the period t.
f = 1/t

GAIN - Used in electronics to reference to an increase in signal power, usually expressed as the ratio of the output power to the input power in decibels.

GATE - A software function that selects and marks a portion of the displayed range for measurement of distance or amplitude.

HERTZ (Hz) - The unit of frequency, defined as the frequency of a periodic phenomenon of which the period is one second; equal to one cycle per second. Symbol Hz.
1 kilohertz (kHz) = 103 cycles per second
1 Megahertz (MHz) = 106 cycles per second

IMMERSION TESTING - A test method in which a water bath or a water column is used to couple sound energy from the transducer to the test piece. It is commonly used in automated scanning applications.

INCIDENCE, ANGLE OF - The angle between a sound beam striking an acoustic interface and the normal (perpendicular) orientation to the surface at that point. Usually designated by the Greek symbol α (alpha).

INDICATION - A displayed signal signifying the presence of a sound wave reflector.

INDICATION (DEFECT) LEVEL - The amount of gain which must be added to an instrument setup to bring an indication (defect) echo signal to peak at a selected reference level.

LEG - In angle beam testing, the path the shear wave travels in a straight line before being reflected by the opposite surface of the material being tested.

LINEARITY, VERTICAL OR AMPLITUDE - The degree to which an ultrasonic instrument responds in a proportional manner to a range of echo amplitudes produced by specified reflectors.

LINEARITY, HORIZONTAL OR DISTANCE - The degree to which an ultrasonic instrument responds in a proportional manner to a range of echo signals produced by specified reflectors at different points in time.

LOS - Acronym for Loss Of Signal.

LONGITUDINAL WAVE - Mode of wave propagation characterized by particle movement parallel to the direction of wave travel.

MAIN BANG - Informal term for excitation pulse.

MODE CONVERSION - The change of a portion of sound beam energy into a wave of a different mode due to refraction at incident angles other than zero degrees. In NDT, this usually involves conversion of longitudinal waves into shear waves or surface waves.

PEAK MEMORY - A software function that records an echo envelope as the transducer scans across a test piece by capturing and storing on the screen the peak echo amplitude at each pixel location.

PEAKING UP - Maximizing the height of a displayed indication by optimizing the position of the sound beam.

PENETRATION - The ability of the test system to overcome signal amplitude loss across a sound path due to factors like attenuation, scattering, and beam spreading.

PIEZOELECTRIC MATERIALS - A family of materials (such as quartz, lead metaniobate and lead zirconium titanate) that possess the characteristic ability to produce a voltage differential across their faces when deformed by an externally applied mechanical force, and a change in their own physical configuration (dimensions) when an external voltage is applied to them.

PITCH-CATCH - A test method which utilizes separate transmitting and receiving transducers on the same side of the test piece.

PROBE - In ultrasonic testing, an alternate name for transducer.

PULSE REPETITION FREQUENCY (PRF) - The frequency at which an instrument generates an excitation pulse and initiates a data acquisition cycle. Also known as Pulse Repetition Rate.

RANGE - The total distance represented by the horizontal display axis.

RECEIVER - The circuits a flaw detector that receive and process returning echoes (as voltages) from the transducer. Typical signal processing includes amplification, filtering, and rectification.

RECTIFICATION - Receiver function that selects display of waveforms as an RF signal with both positive and negative deflections, a full wave rectified signal in which all deflections are displayed as positive, or half wave negative and positive in which only one side of the RF signal is displayed as a positive deflection.

REFERENCE ECHO - The echo from a specified reference reflector.

REFERENCE GAIN (REFERENCE LEVEL) - A gain level established by a test procedure, commonly by setting a reference reflector echo to a specified height.

REFERENCE LINE - A designated percentage of total screen height at which reference echoes and indication echoes are compared.

REFERENCE REFLECTOR - A reflector of known size and geometry at a known distance, such as a flat bottom hole.

REFRACTION, ANGLE OF - The angle with respect to perpendicular of the sound beam in the test material, generated by refraction from the angle of incidence in the wedge.

REJECT (CONTROL) - Also known as suppression, this limits the input sensitivity of the amplifier in the receiver to display only indications greater than a selected height. It is used to reduce or eliminate "grass" or scattering noise from the display.

RESOLUTION - The ability of the test system (instrument and transducer) to distinguish between two reflectors at slightly different depths, or within a given distance of the surface of the test piece.

SCANNING GAIN - The amount of gain above the reference level added per procedure to optimize visibility of indications.

SECOND CRITICAL ANGLE - In angle beam or immersion testing, the minimum incident angle in the wedge or water path at which the refracted shear wave disappears from the test piece.

SENSITIVITY - The ability of the test system (instrument and transducer) to detect a given size reflector at a given distance.

SIGNAL-TO-NOISE RATIO - The amplitude ratio between the echo from a given indication and background indications such as grain scattering or instrument noise.

SINGLE ELEMENT PROBE - A transducer containing only one piezoelectric element, which is used to both transmit and receive sound.

SKIP DISTANCE - In angle beam testing, the surface distance which represents one V-path of sound in the material.

SOUND BEAM - The characteristic shape of the ultrasonic wave in a test piece.

SOUND PATH DISTANCE - The distance between the transducer and a reflector. In angle beam and immersion testing, this is commonly measured from the point at which sound enters the test piece. This is sometimes referred to as angular distance in angle beam testing.

STRAIGHT BEAM PROBE (Normal beam probe) - A transducer that transmits sound into the test piece perpendicular to the entry surface.

SURFACE WAVE - A mode of wave propagation characterized by an elliptical movement of the particles on the surface of the specimen as the wave front moves forward, penetrating the specimen to a depth of one wavelength.

THROUGH TRANSMISSION - A test method in which the pulses from a transmitting transducer are received by a second transducer on the opposite side of the test piece.

TIME VARIED GAIN (TVG) - A software function that adjusts instrument gain so that the echo amplitude of a given size reflector is displayed at a constant display height regardless of the distance to that given size reflector.

TRANSDUCER - A device that transforms one form of energy into another.

TRANSMITTER - The circuits of a flaw detector that sends the initial pulse voltage to the transducer.

ULTRASONIC - Of or relating to frequencies above the human audible range, above 20,000 cycles/s (20 kHz).

V-PATH - The angular distance sound travels, measured from the top surface of the test piece to the bottom and reflecting back up to the top surface.

WAVELENGTH - The distance between like points on successive wave fronts; the distance between any two successive particles of the oscillating medium that are in the same phase. It is denoted by the Greek letter λ (lambda).

ZERO OFFSET - A time offset between the moment the excitation pulse is generated and the point at which the screen display and measurement begins, typically used to subtract transit time through a wedge, delay line, or wearplate from a depth or distance measurement.