Ultrasonic antifouling

Range of sound frequencies including audible and inaudible sound

Ultrasound (ultrasonic) is sound at a frequency high enough that humans can not hear it. Sound has a frequency (low to high) and an intensity (quiet to loud).

Ultrasound is used to clean jewellery, weld rubber, treat abscesses, and sonography. These applications rely on the interaction of sound with the media through which the sound travels. In maritime applications, ultrasound is the key ingredient in sonar; sonar relies on sound at frequences ranging from infrasonic to ultrasonic.

The three main stages are formation of a conditioning biofilm, microfouling and macrofouling. A biofilm is the accretion of single-celled organisms on a surface. This creates a habitat that enables other organisms to establish themselves. The conditioning film collects living and dead bacteria, creating the so-called the primary film.[2]

The two approaches to ultrasonic antifouling are:

Cavitation: Ultrasound of high enough intensity causes water to boil, creating cavitation. This physically annihilates living organizsm and the supporting biofilm. One concern is to the potential effect on the hull. Cavitation[3] can be predicted mathematically through the calculation of acoustic pressure. Where this pressure is low enough, the liquid can reach its vaporisation pressure. This results in localised vaporisation, forming small bubbles; these collapse quickly and with tremendous energy and turbulence, generating heat on the order of 5,000 K (4,730 °C; 8,540 °F) and pressures of the order of several atmospheres.[4] Such systems are more appropriate where power consumption is not a factor, and the surfaces-to-be-protected can tolerate the forces involved.

Sub-cavitation: The sound vibrates the surfaces (e.g. hull, sea chests, water coolers) to which the transducer is attached. The vibrations prevent the cyprid stage of the biofouling species from attaching themselves permanently to the substrate by disrupting the Van Der Waals Force that allow their microvilli to hold themselves to the surface .[5]

Different frequencies and intensities (or power) of ultrasonic waves have varying effects on marine life, such as barnacles,[5] mussels and algae.

Ultrasonic antifouling systems are generally only capable of maintaining a clean surface. They can't clean a surface that already has a well established and mature biofouling infestation. To this end, they are a preventative measure with the goal of an ultrasonic antifouling system being to maintain the protected surface as close to its optimum clean state as possible.

Ultrasonic systems are ineffective on wooden-hulled vessels, or vessels made from ferro-cement as these materials dampen the vibrations from the transducers. Composite hulls with a sandwich construction may also require modification to form monolithic plinths of solid material at each transducer location.