| time-average of the instantaneous intensity at a particular point in an acoustic field
NOTE 1 – Time average is taken usually over an integral number of acoustic repetition periods; if not, the averaging period should be specified.
NOTE 2 – In a plane acoustical wave in a non-absorbing fluid medium, acoustic intensity is the same at all points of the wave, when averaged over a complete cycle.
NOTE 3 – In a perfect standing wave, temporal-average intensity is zero.
NOTE 4 – (relating to ultrasonic medical diagnostic systems) In principle, the temporal-average intensity is an average over a relatively long time interval. For non-auto-scanning systems, the instantaneous intensity may be averaged over one or more pulse-repetition periods. For auto-scanning systems, the instantaneous intensity may be averaged over one or more scan-repetition periods for a specified operating mode.