engleski

Measuring derived acoustic power of an ultrasound surgical device in the linear and nonlinear regime of working

The method for measurement of a derived acoustic power of an ultrasound (US) point source has been considered in the free acoustic field according to the IEC 61847 standard. The pressure magnitude spatial distribution of the ultrasound surgical device in the form of sonotrode tip has been measured in the free acoustic field conditions. In the linear regime of working, the sonotrode tip has been theoretically described as radially (ROS) and transversaly (TOS) oscillating spheres in the vicinity of pressure release boundary. The measured pressure magnitude has been fitted with theoretical curves describing pressure field of considered theoretical models. The velocity and displacement magnitudes with derived acoustic power of an equivalent theoretical sources have been found and electroacoustic efficiency factor has been calculated. When transmitter is excitated at higher electrical power level, the displacement magnitude of sonotrode tip is increased and nonlinear behaviour in loading medium appears with strong cavitation activity. In the averaged power spectrum of the recorded acoustic pressure signal, it is evident the presence of harmonics (n· f0), subharmonics (f0/q), ultraharmonics (n· f0/q) of excitation frequency (f0), and cavitation noise with continouus frequency components as a consequence of transient cavitation. It has been shown that spatial pressure magnitude distribution of discrete frequency components in this nonlinear operating mode can be measured and fitted with appropriate theoretical model. The pressure magnitude at frequency component of interest (discrete and continouus) in the field points is found calculating average power spectral density of recorded pressure waveform signal using welch method. The contribution of discrete frequency components appearing in pressure signal to derived acoustic power has been calculated. It has been assumed that linear theoretical model is valid for each discrete frequency component of secondary nonlinear sources which are assumed as radially oscillating spheres in the vicinity of sonotrode tip and pressure release boundary.

derived acoustic power; radially and transversaly oscillating sphere; free acoustic field conditions; cavitation; pressure magnitude spatial distribution; average power spectral density

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