| straight line that passes through the beam centrepoints of two planes perpendicular to the line which connects the point of maximal pulse-pressure-squared integral with the centre of the external transducer aperture
NOTE 1 – The location of the first plane is the location of the plane containing the maximum pulse-pressure-squared integral or, alternatively, is one containing a single main lobe that is in the focal zone. The location of the second plane is as far as is practicable from the first plane and parallel to the first plane with the same two orthogonal scan lines (x and y axes) used for the first plane.
NOTE 2 – In a number of cases, the term pulse-pressure-squared integral is replaced in the above definition by any linearly related quantity, e.g.:
a) in the case of a continuous wave signal the term pulse-pressure-squared integral is replaced by mean square acoustic pressure as defined in IEC 61689,
b) in cases where signal synchronisation with the scan frame is not available the term pulse-pressure-squared integral may be replaced by temporal average intensity.