The potential applications for ultrasound are endless, and can be used across a range of species as has been discussed in previous articles. However, what exactly is ultrasound? It is easy to forget that, as is alluded to in the title, ultrasound is indeed a sound pressure wave, although it is at frequencies so high it is undetectable to the human ear. Humans have an upper frequency limit of 20 kilohertz (kHz), whereas ultrasound can reach frequencies of greater than 200 megahertz (MHz). Consequently, ultrasound can be considered a ‘standard’ sound, albeit not audible to humans. Interestingly however, humans can possess ultrasonic hearing if ultrasound is targeted to the cochlea via bone, bypassing the confines of the middle ear.

In contrast, a number of animals, including dogs, cats, dolphins, bats, horses and even insects are able to hear ultrasound due to their ability to hear greater upper frequency limits than humans. A dog whistle releases ultrasounds at frequencies higher than those able to be heard with the human ear that are consequently dubbed the ‘silent whistle’. Dolphins use ultrasound for navigational purposes to enable them to locate and seize their prey, similar to bats which use a range of ultrasound to locate their prey and can hear frequencies greater than 100 kHz. Finally, insects are also able to hear ultrasound aiding in the avoidance of bats and subsequent attack.

Ultrasound detection therefore highlights differences between the human and animal species. Nevertheless, humans have adapted ultrasound for a variety of purposes including; the detection of pregnancy, cleaning of items such as jewellery and dental and surgical instruments, disintegration of biological cells such as bacteria and even identification whereby tags attached to objects can release ultrasonic signals that allow the position of the tagged object to be identified. However, exposure to ultrasound should be limited, with the possibility of hearing loss following repeated exposure at levels deemed harmful as published by the UK Health Protection Agency.

Our understanding of ultrasonography has greatly advanced over the years, particularly where pregnancy establishment is concerned, with the ability of ultrasonography to deliver clear 4D scan images of the developing foetus. It is likely that with further advancements in technology, the uses for ultrasound will continue to grow in a number of fields, aiding our lives on a daily basis.