Directivity

Figure 7 Comparison of directional response at 1,3,5 and 10MHz with PVDF needle and membrane hydrophones [4]

Figure 8 Comparison of effective radii as a function of frequency. All devices show the effective radii initially falling off with increasing frequency, leveling out to a value close to the physical radius of the active element - such behaviour is characteristic of circular receiver whose sensitive region does not fall sharply to zero at the perimeter. For the PVDF membrane hydrophones this may be due to a gradual decrease in piezoelectric sensitivity around the edges of the spot poled sensitive region. For the needle hydrophones such apodisation may occur due to variations in the thickness and flatness of the PVDF element near the perimeter and for the optical fibre hydrophones as a result of the Gaussian profile of the illuminating beam. [4]