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Nonlinear Acoustics Today
 Figure 8. Top: locations where the International Symposium on Nonlinear Acoustics (ISNA) has been held starting in 1968. Bottom: year, city, and number of proceedings papers published for each ISNA, including the dates and locations of the next two symposia.
viscosity converts mechanical energy in a sound wave into heat, momentum is associated with inertia and can only be conserved by mechanical means. Therefore, the time aver- age of the momentum lost by a sound wave as its energy is reduced due to acoustic absorption is replaced by the momentum corresponding to mass transport associated with steady flow of the fluid. This steady flow is referred to as acoustic streaming (Nyborg, 1965).
An example of acoustic streaming created by the absorption of a sound beam in a fluid in a closed container is depicted in Figure 7, bottom, in which the sound beam propagates from left to right. The time average of the acoustic momentum in the beam is also directed to the right, and therefore absorption of the sound generates fluid flow in the same direction. This type of streaming is called Eckart streaming. In the case of a stand- ing wave in a narrow tube, the vortices generated by viscous losses along the walls of the tube are referred to as Rayleigh streaming. The latter is more prevalent in microfluidic devices.
International Symposia on Nonlinear Acoustics
Nonlinear acoustics as an established discipline within the broader area of physical acoustics came of age in 1968 when the first International Symposium on Nonlinear Acoustics (ISNA) was held in New London, CT. Although the first ISNA was convened largely in response to the surge in research spawned by the invention of the para- metric array and its application to sonar, by the end of the 1970s, the field had expanded to include fundamental and applied research in nonlinear acoustics in all media (gases, liquids and solids), including the areas of acoustic cavitation and bubble dynamics.
As illustrated in Figure 8, ISNA continues to maintain an enduring presence in the international acoustics commu- nity by holding a symposium typically every three years, with its venue alternating between North America, Europe, and Asia (Hamilton et al., 2012). It is, in fact, extremely rare for a specialized subdiscipline of physics to exhibit such vitality for half a century, yet ISNA has surpassed this mile- stone with future symposia already scheduled for 2021 in Oxford, UK, and 2024 in Nanjing, China. Such longevity is testimony to the fundamental nature of nonlinear acoustics and its manifestations in all areas of acoustics.
The mathematical framework underlying the basic physi- cal principles discussed in this overview may be found in several textbooks on nonlinear acoustics that have been published over the years (Rudenko and Soluyan, 1977; Beyer, 1997; Naugolnykh and Ostrovsky, 1998; Hamilton and Blackstock, 2008).
We thank Jason Sagers and Aleksander Smetanka for creat- ing several of the figures.
Atchley, A. A. (2005). Not your ordinary sound experience: A nonlinear-acous- tic primer. Acoustics Today 1(1), 19-24. Bailey, M. R., Khokhlova, V. A., Sapozhnikov, O. A., Kargl, S. G., and
Crum, L. A. (2003). Physical mechanisms of the therapeutic effect of ultrasound (a review). Acoustical Physics 49(4), 369-388. https://doi. org/10.1134/1.1591291.
Bailey, M. R., McAteer, J. A., Pishchalnikov, Y. A., Hamilton, M. F., and Colonius, T. (2006). Progress in lithotripsy research. Acoustics Today 2(2), 18-29.
Beck, S. D., Nakasone, H., and Marr, K. W. (2011). Variations in recorded acoustic gunshot waveforms generated by small firearms. The Journal of the Acoustical Society of America 129(4), 1748-1759. https://doi. org/10.1121/1.3557045.
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