Page 29 - HydrophoneHandbook_eReader_Ed2_20151208
P. 29

Choosing a Hydrophone

Estimating Hydrophone Directivity
Onda’s hydrophones are calibrated with respect to a plane-
wave. The hydrophone is positioned so that its active surface is
parallel with the planar wavefront, i.e., the direction of
propagation of the wave is normal to the active surface of the
hydrophone. For example, in the case of needle or capsule
hydrophones, the active surface is at the tip, so the normal is
the major axis of the needle or capsule.
The hydrophone’s sensitivity decreases as it is rotated with
respect to the normal position. For needle and capsule
hydrophones the sensitivity approximately follows formula 1:

 ,   =  1+cos()  ×  21( sin  )  (1)
                   2          ( sin  )

where:
   D = directivity function (voltage out of the hydrophone
   relative to voltage when an incoming wave is normal to the
   tip)
   θ = the angle relative to the normal
   f = frequency
   J1 = Bessell function of the first kind
   k = wave number (2 π/ wavelength) = 2 π f /c where c is the
   wave speed = 1500 m/s in water
   a = radius of the hydrophone (0.75 mm approximately for an
   HNC-1500, for example) HGL

                                                      Mount

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