Ultrasonic Terms


  • Absolute error - The difference between a measurement and itÍs true value. We must estimate the true value so this is often expressed with confidence intervals.

  • Acoustic Ohm - 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 second. Used mostly in resonator applications and not popular in NDT. Sometimes confused with acoustic impedance which is a different quantity.

  • Accuracy - A qualitative expression of closeness of the measurement to the true value.

  • Amplitude - Unit of acoustic magnitude. Expressed usually as pressure (fluids) or stress (solids). 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). In ultrasonic applications the amplitude can be much less or much more than this range.

  • Calibration - A process by which a measurement instrumentÍs 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. ASTM E-317 would be an example of calibration under this definition.

  • 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). Logrithms 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 wave - A pressure wave 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. A normal wave in helium is called first sound.



    These definitions are from a variety of sources and represent the authors personal collection.


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