A theory of aneurysm sounds.

Narrow-band sounds are known to be associated with some intracranial aneurysms. Previously proposed theories for the mechanism of aneurysm sounds do not satisfactorily explain the small spectral widths of the sounds. A simple theory is proposed here which gives quantitatively correct predictions of the spectral widths and which also explains other salient features of aneurysm sounds. The physical features of the aneurysm are described in terms of lumped mechanical elements, and the interaction between the aneurysm vibration and the blood flow is recognized as having the characteristic features of a nonlinear feedback system. The resulting model, with the application of the method of describing function analysis commonly used in nonlinear control theory, yields predictions of steady oscillation frequencies and predictions of the ranges of arterial flow velocities for which substantial oscillations can be excited. An analysis of radiation losses associated with peristaltic waves indicates that aneurysms, in the absence of any nonlinearity, behave as low-quality factor resonators with resonator quality factors on the order of 1-10, much lower than those that would be inferred from the observed spectral widths of aneurysm sounds. Aneurysm sounds are predicted by the present nonlinear theory to have center frequencies on the order of 400 Hz and bandwidths corresponding to quality factors on the order of 40, in good agreement with in vivo observations. It is concluded that linear resonance theories are incapable of fully describing aneurysm sounds.(ABSTRACT TRUNCATED AT 250 WORDS)

Validity of linear acoustics for prediction of waveforms caused by sonically moving laser beams.

The question is raised as to whether the analysis of the generation of sound by a laser beam moving over a water surface at the sound speed c for an interminable time period requires consideration of nonlinear effects. A principal consideration in this regard is whether the linear acoustics theory predicts a pressure waveform that is bounded in the asymptotic limit when the laser irradiation time is arbitrarily large. It is shown that a bounded asymptotic limit exists when the upper boundary condition corresponds (as is more nearly appropriate) to that of a pressure release surface, but not when it corresponds to that of a rigid surface. The asymptotic solution to the appropriate inhomogeneous wave equation is given exactly for the former case, and it is shown that the highest asymptotic amplitudes, given specified laser power and beam radius a, occur in the limit of a very small light absorption coefficient mu. In this limit, the peak amplitude is independent of mu and occurs at a depth of 0.88/mu. An approximate solution for the pressure waveform at intermediate times establishes that the characteristic time for buildup to the asymptotic limit is of the order of 2.5/(c mu 2a). If this time is substantially shorter than the time that a plane-wave pulse with the asymptotic waveform would take to develop a shock wave, then accumulative nonlinear effects are of minor importance.