For biomedical applications (where these two processes are termed diagnosis and therapy, respectively), the issue of hazard has been studied in depth, although this has not been done to such a degree for industrial uses of ultrasound in liquids/solids (sonar, non-destructive testing, ultrasonic processing etc.). The first concerns the use of ultrasound in liquids/solids, for measurement or material processing.
It is noted that there are two very distinct regimes, in terms of wave characteristics and potential for bioeffect. These issues include the measurement and description of fields and exposures, and the ability of ultrasound to affect tissue (through microstreaming, streaming, cavitation, heating, etc.). Given the Health Protection environment which framed the original presentation, explanation and examples are given of how these complexities affect issues of practical importance. This progresses through an exposition of the characteristics of linear waves, in order to explain the propensity for, and properties of, the nonlinear propagation which occurs in many practical ultrasonic fields.
To explain the complexities responsible for this, the description of ultrasound is first built up from the fundamental properties of these local particle vibrations. In answering the question ‘what is ultrasound?’, it shows that the simple description of a wave which transports mechanical energy through the local vibration of particles at frequencies of 20 kHz or more, with no net transport of the particles themselves, can in every respect be misleading or even incorrect. This paper is based on material presented at the start of a Health Protection Agency meeting on ultrasound and infrasound.
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