Page 30 - HydrophoneHandbook_eReader_Ed2_20151208
P. 30

Hydrophone Handbook

         Choosing a Hydrophone

For high frequencies and small angles the (1+cos(θ)) term
becomes negligible and the formula becomes:

 ,   =  21( sin  )                               (1b)
                ( sin  )

Furthermore, for small angles as well as low frequency Eq. (1b)
may be approximated as:

 ,   =  21(2⁄)                               (1c)

From (1c) it can be seen that for small angles and low frequency
the directivity will scale with θ f where f is the frequency. That
means that under such conditions you can take the directivity
pattern acquired at one frequency f1, and estimate the
directivity pattern at another frequency f2 from the same curve,
replacing theta with theta * f1/ f2. For example, if at 5 MHz the
directivity is 0.9 at 10 degrees, at 2.5 MHz it will be 0.9 at 20
degrees. Onda provides typical directivity patterns measured at
5 MHz for this purpose.

The user should be aware, however, that Eq. (1c) loses accuracy
at very low frequencies, as can be seen from Eq (1), which is the
more exact formula.

                                                  Pulsed Wave



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