Read about Shipsonic ultrasound

Ultrasound is sound that is above the audible spectrum in the range of 20 - 100 kHz, and therefore cannot be heard by humans. An example is the dog whistle. Dogs hear from the lower part of the spectrum up to 35 kHz. Other animals, such as bats, can hear the entire spectrum, while dolphins can hear parts of the spectrum. Ultrasound that is sent at low power, which is what is used in algae control, cannot be heard over water, as sound has difficulty passing from water to air.

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The effect of ultrasound has long been known and is used in laboratories to destroy cells in algae samples. The idea to use ultrasound to combat algae came from a Belgian electrical engineer who worked for a tomato nursery. In the nursery, the tomatoes grow without soil, but with their roots directly in water. One problem for the nurseries was the enormous work of cleaning the pools of algae. In 1999, the first ultrasound transmitter for combating algae in tomato nurseries was launched. Since then, more than 14,000 units have been sold worldwide for combating algae in lakes, ponds, fish farms, drinking water reservoirs, swimming pools, etc.

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How does it work?
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Algae have small air-filled cavities, vacuoles, which keep them floating in the water. Ultrasound at low power works by causing the algae to vibrate and the vacuoles to break apart over days to weeks, causing the algae to sink and die. Another effect is that the microscopic vibrations cause the cell contents to accumulate in the center of the cell. The effect of this is supposed to be that the passive diffusive nutrient transport across the cell wall takes a much longer route, and the algae thereby starves and dies, see photo below. Fish also have a vacuole, the swim bladder, which, however, is so large that it is not affected by ultrasound. Destruction of the cell wall is an effect, which can be seen in some types of algae.

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