Ultrasound is a sound that is above the audible spectrum in the range from 20 to 100 kHz and therefore can not be heard by

humans. An example is the dog whistle. The dog hears from the bottom portion of the spectrum up to 35 kHz. Other animals, like the bat can hear the whole spectrum, while dolphins can hear parts of the spectrum. Ultrasound sent in low strength, as it is used in the algae fighting can not be heard over the water, because sound has difficulty going from sea to air.

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The effect of ultrasound has long been known and used in laboratories to ruining the cells of algae samples. The idea to use ultrasound to algae fighting came from a Belgian electrical engineer, who worked for a tomato nursery. In nursery tomatoes grow without soil but with roots directly in water. A problem for gardening was the huge efforts to clean the pools for algae. In 1999 the first ultrasonic transmitter to algae fighting in tomato nurseries was launched. Since then it have sold more than 14,000 pieces in the world of algae fighting in lakes, ponds, aquaculture, drinking water reservoir, swimming pool etc.

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How does it work?

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Algae have small air filled cavities, vacuoles, which keeps them floating in the water. Ultrasound at low volume works by vibrating the algae by which the vacuoles in the course of days to weeks brake into pieces, whereby the algae die and sink. Another effect is that the small vibrations will cluster the organells in the center of the cell. The effect of this is supposed to be that nutrients entering the cell by the passive transport across the cell membrane will have a much longer way to travel, and the algae thereby starve and die, (see photo below). Fish also have an air filled vacuole, the swim bladder, however, is so large that it is not affected by ultrasound. In some types of algea destruction of the cell wall is another effect of the ultrasound waves.

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Besides the effect of vibration ultrasound in water also create micro-cavitation - bubbles - of about 1 pm which immediately implode. The imploded bubbles generates micro turbulence within very short range and rapid water movements (micro-jets). In addition temperatures of at least 10,000 ° C in a few hundred picoseconds can be observed as the bubble collapse.
This energy blasts in water is used  to increase the speed of  chemical reactions (sonochemistry). By ultrasonic treatment of humans, the powerful ultrasonic transmitter, can increase the temperature of treated the tissue. In ultrasonic baths jewelery can be cleaned for dirt when the mikrojets from collapsing bubbles produce rapid water movements that clean the surface.

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