An infrasound fish scaring system is shown alongside.
Fish can be a problem during underwater operations. They are attracted
to lights and activity. This can result in camera vision being seriously
obscured and the swim bladders present spurious echoes to acoustic
Fish can be scared by very low frequency sounds at high intensity.
Typically the frequencies are lower than 10 Hz. These low frequencies
do not interfere with the much higher frequencies used in acoustic
imaging and profiling sysyems.
The frequencies need to be taken into account when designing camera
supports. Camera support brackets need to be reasonably stiff to
ensure that the natural resonant frequency is at least 20 Hz. If
the stiffness is insufficient and the natural frequency is low then
the effect will be similar to the low frequency resonance famously
encountered on the Millenium bridge in London.
The frequencies used in these systems are close to the natural
resonant frequencies of the human respiratory system and therefore
are potentially dangerous to divers in the near vicinity and must
be used with caution.