This International Standard establishes procedures for specifying certain ultrasonic search unit characteristics and for measuring the associated sound field. The aim is to establish uniformity of testing techniques, to form a basis for the interpretation of results obtained by different laboratories and at different times. Note that this International Standard establishes no acceptance criteria; however, it does establish the technical basis for criteria that may be defined by user parties. IS0 2400 describes a calibration block used in weld inspection for checking the performance of the ultrasonic instrument and search unit. To further enhance the general ultrasonic inspection capability, information concerning the search unit’s time domain response, frequency response (also referred to as frequency spectrum, frequency analysis, spectrum analysis and signature analysis) and sound field must be known before reliable detection and characterization of flaws can be achieved. The search units discussed herein are straight-beam and angle-beam search units with a nominal ultrasonic frequency from 0,5 MHz to 15 MHz. Surface wave search units are not included in this standard. This standard provides procedures for specifying sound field parameters as well as methods of measurement in immersion tests. This standard does not address sound field measurement in contact tests. This International Standard describes measurement procedures for evaluating characteristics of ultrasonic search units and describes techniques used for obtaining sound field data produced by electrically pulse-excited search units used in the field or in the laboratory. The characteristic parameters discussed in this standard are the upper, lower and centre frequencies, the wavelength, the bandwidth, the time domain and frequency domain responses, the near-field length, the half-angle of beam spread for search units of different shapes in free field, echo sound measurements, beam profiles, the impedance and the relative sensitivity. Formats for specifying both flat-face and focused search units are given. To provide baseline information and to check possible performance degradation, the electrical properties of the search unit are measured independently of the ultrasonic instrument. The electrical impedance and sensitivity are measured at specific frequencies selected for each search unit. The electrical impedance corresponds to the complex input impedance of the search unit and the sensitivity of the search unit is a measure of the electroacoustic efficiency of the unit. These procedures are described in 4.6 and 4.7, respectively.