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Hydrophone Handbook

          12. Glossary of Terms

Term              Definition                                    Symbol  Units  Reference

               Acoustic energy flow rate in the direction
               of propagation per unit area normal to the
               direction of propagation; the
               instantaneous intensity typically varies due
               to the oscillatory nature of acoustics so it is
               common to refer to the temporal average
               intensity, in which case the instantaneous
               intensity is averaged over the repetition
               period of the acoustic signal.

   Acoustic    Note – Instantaneous intensity is the            I       W/cm²  www.electropedi
  Intensity-   product of instantaneous acoustic pressure                      a.com, 802-01-09
Instantaneous  and instantaneous particle velocity. It is
               difficult to measure intensity in the
               ultrasound frequency range. For
or Average     measurement purposes and if it is

               reasonable to assume that the
               instantaneous acoustic pressure and
               instantaneous particle velocity are
               substantially in phase along the beam(-
               alignment) axis for a given frequency, than
               the instantaneous intensity, I(t), is
               approximated as p(t)2/ρc where p(t) is the
               instantaneous acoustic pressure; ρ is the
               density of the medium; c is the velocity of
               sound in the medium.

Beam Area      Area in a specified plane perpendicular to       Ab      cm²    www.electropedi
   Beam        the beam axis, consisting of all points at       ---            a.com, 802-01-19
               which the pulse-pressure-squared integral is
(Alignment)    greater than a specified fraction of the                 ---    www.electropedi
    Axis       maximum value of the pulse-pressure-                            a.com, 802-01-08
               squared integral in that plane Note – Some
               commonly specified levels are 0,25 and 0,01
               for the −6 dB and −20 dB beam areas,
               respectively.
               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.

   Beam        Position determined by the 2D-centroid of        ---     ---    www.electropedi
Centrepoint    a set of pulse-pressure-squared integrals                       a.com, 802-01-07
               measured over the –6 dB beam-area in a
               specified plane.

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