Appendix II -- Definitions

Accuracy:

The agreement between the measured value and the true value of a parameter such as thickness. The true value may be defined with the aid of appropriate reference standards.

Acoustic Impedance:

A material property defined as the product of sound velocity and the material's density.

Amplitude:

In wave motion, the maximum displacement of material particles. In electronics, the magnitude of a signal, normally expressed as a positive or negative voltage.

Attenuation:

The loss in acoustic energy which occurs between any two points in a sound path.

Backwall Echo:

The echo received from the side of the test specimen opposite the side to which the transducer is coupled. The timing to this echo corresponds to the thickness of the specimen at that point.

Delay Line:

A material (usually a polymer) placed in front of a transducer to create a time delay between the excitation pulse and the echo from the front surface of the test specimen.

Excitation Pulse:

A brief electrical pulse applied to a piezoelectric element in an ultrasonic transducer, causing it to vibrate and generate sound waves.

Frequency:

Mechanically, the number of cycles of vibration experienced by an oscillating body in a designated period of time (normally one second). Electrically, the rate at which a periodic signal (such as a sine wave) repeats during a designated period of time.

Interface Echo:

The echo reflected from the front surface of a test specimen, seen when using delay line or immersion transducers.

Phase Reversal:

An inversion (or change of algebraic sign) of the positive and negative peaks of a wave.

Resolution:

In thickness gaging, the degree to which slightly different thicknesses or time intervals can be distinguished.

Sound Velocity:

The speed at which a sound wave travels through a given material.

Sound Wave:

A coherent pattern of mechanical vibrations in a solid, liquid, or gaseous medium.

Transducer:

A device that transforms one form of energy into another. In ultrasonic testing this normally means converting electrical energy into mechanical energy or vice versa.

Waveform:

A graphic presentation of energy levels in a wave train, plotted as amplitude versus time.

Zero Offset:

A correction factor representing the difference between a measured time interval and the actual sound transit time in a test specimen, typically accounting for switching delays, cable delays, and wearplate and couplant thickness.