Alternating positive and negative intensity

The origin of sonochemical effects in liquids is acoustic cavitation. Ultrasound is transmitted through a medium via pressure waves by inducing vibrational motions of molecules, which alternately compress and stretch the molecular structure of the medium due to a time-varying pressure. Molecules start to oscillate around their mean position, and provided that the strength of the acoustic field is sufficiently intense, cavities are created in liquids. This will happen if the negative pressure exceeds the local tensile strength of the liquid. 

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