Page 24 - HydrophoneHandbook_eReader_Ed2_20151208
P. 24

Hydrophone Handbook

         Choosing a Hydrophone

2.4 Spatial Resolution

The hydrophone sensing area needs to be chosen to achieve
adequate spatial resolution of the beam. Otherwise the spatial peaks
will be underestimated and the beamwidths will be overestimated.
For this reason, Onda’s hydrophones are offered with a range of
aperture sizes from 85 µm to 1500 µm.

The IEC recommends a method to select the correct hydrophone size
which requires a less detailed knowledge of the field. As discussed in
Section 5.1.6.1 of IEC 62127-1:
The effective radius of the element should ideally be comparable with
or smaller than one quarter of the effective wavelength, so that phase
and amplitude variations do not contribute significantly to
measurement uncertainties.
It is not possible, because of the large range of types of ultrasonic
transducers, to establish a simple relationship between the optimum
effective element size of the hydrophone and parameters such as the
ultrasonic transducer dimension, the effective wavelength and the
distance from the ultrasonic transducer. However, in the far field it is
reasonable to relax the above criterion. For unfocused circular
ultrasonic transducers, the following criterion may be used as a guide
to the determination of the maximum effective radius amax of a
hydrophone active element. amax is given by:

                  ( )amaxλ
                  =  8a   l 2 +a12 1/ 2  (6)

                       1

where:
a1 is the effective radius of the ultrasonic transducer;
l is the distance between the hydrophone and the source aperture
plane

λ is the effective wavelength corresponding to the acoustic-working
frequency.

  22

0NDA Corporation
   19   20   21   22   23   24   25   26   27   28   29