Acoustic cavitation bubbles under ultrasonic horn in water emit acoustic cavitation noise, which consists of spherical shockwaves. This study theoretically derived the spatial coherence of acoustic cavitation noise or, more precisely, the spectral degree of coherence. The acoustic cavitation noise was found to have spatial coherence characteristics similar to the "thermal light" in optics, unlike ultrasound generated by general transducers, which are analogous to "laser" with high coherence. The experiments validated the derived theory and showed that the spectral degree of coherence of the acoustic cavitation noise depends on the product between the distribution width of the shockwave origin, proportional to the horn diameter, and the angle between the hydrophones viewed from the horn. The lower the product gives, the higher the spectral degree of coherence at a higher frequency range.
