·  Acoustic Ohm -  A unit of acoustic resistance. An acoustic impedance is one ohm (cgs) when a sound pressure of 1 microbar produces a volume velocity of 1 cubic cm per sec. This is used mostly in resonator applications and not popular in NDT. Sometimes confused with acoustic  impedance which is a different quantity.

·  Accuracy – Qualitative expression of closeness of the measurement to the true value.

·  Amplitude - Unit of acoustic magnitude. Expressed usually as pressure or stress . Typical units are dynes per square cm, Pascals (Newtons per square meter) or bars (1 atmosphere). Typical values for audio sounds in air are 10e-10 bars (threshold of hearing) to 1 bar(pain).

·  Calibration - A process by which a measurement instrument’s accuracy a precision are qualitatively determined. Calibration is not the process by which an ultrasonic instrument is standardized before and after NDT measurements although it is possible to do a calibration most standard only redquire reference standards and do not define accuracy or precision

·  Confidence interval - An interval within which a true value of a measurand lies with a given probability. May be expressed a absolute or relative interval.

·  Dead zone - The range through which a stimulus can be varied without producing a change in the response of the measuring instrument. An ultrasonic example is the inability to make a measurement for some period after the main bang, or in some cases after a high amplitude signal that saturates the amplifier.

·  Decibel - A unit expressing the intensity or amplitude of a sound wave as a ratio. Decibels for two power level, I1 and I2 is 10log(I1/I2), while for two amplitudes A1 and A2 it is 20 log(A1/A2). Logarithms are to the base 10. It is meaningless to specify a dB level without a reference level, I2 or A2.

·  Defect - A condition in a part that would render it unsuitable for use.

·  Descrimination threshold - The smallest change in a stimulus that produces a change in the response of measuring instrument.

·  Drift - A slow variation with time at an output of a measuring that is independent of a stimulus.

·  Elementry Error - Uncertainty of a measurement due to a single source of inaccuracy.

·  Error - See measurement error.

·  Fiducial error - A ratio of limit of absolute error of an instrument and a value specified for that instrument. The specified value is called the fiducial value. Examples are percent of range, or percent of a particular value. e.g. 1% of full scale, 0.2% at 1 volt etc.

·  Flaw - A condition or change that indicates a non-normal condition in a part. A flaw is not necessarily a defect since only flaws that are unacceptable are defects. Most if not all parts have flaws.

·  Indication - A condition or change that indicates a non-normal condition in a part. Also used as an measurement that may be a flaw. The term indication is sometimes thought to be less judgemental than a flaw but they are synonyms. Some have argued that indication is an uninterpreted measurement but I have yet to encounter a convincing example.

·  Intensity - Unit of acoustic power. Usually expressed as Watts per square meter or Watts per square centimeter.

·  Longitudinal waves - Pressure waves in fluids. In a solid the wave is a stress wave with the stress in the same direction as wave propogation, in some anisotropic materials this view is an over simplification.

·  Measurand - A value of physical quantity to be measured.

·  Measurement - The process of finding a value of a quantity experimentally with the help of special technical means (measuring instruments).

·  Measurement Error - Deviation of a measurement from the true value, expressed in absolute or relative form.

·  Mode conversion - The conversion of one wave mode into another. Examples are the conversion of a longitudinal wave into a shear wave when sound strikes a solid at an angle. Another is the generation of a surface wave at the critical angle.

·  Neper - Ratio of two amplitudes expressed as a natural (Nasperian) logrithm. 1 Np = 8.687 dB. Nepers are sometimes used to express attenuation as in Nepers/cm.

·  Result of measurement - A value of quantity expressed in the units adopted for it and obtained with a measuring instrument.

·  True value - Value of a measured quantity, which, being known, would ideally reflect the corresponding property of the object. Usually not known.

·  Quantity - A property of phenomena, bodies, or substances that can be defined or expressed quantitatively. Also call physical quantities.

·  Shear wave - A wave whose particle displacement is usually at right angle to the wave propogation vector. Can only exist in solids.

·  Second sound - A wave that propogates only in superfluid helium below the Lambda point (4.2K). Second sound is a temperature wave, that is the particle motion is random with respect to the wave propogation vector.