Original version
American Journal of Physics (AJP). 2022, 90 (1), 20-30, DOI: https://doi.org/10.1119/10.0006613
Abstract
Hanbury Brown and Twiss determined the angular size of a visible light source (the star Sirius) by studying how the cross-correlation in intensity fluctuation recorded by two detectors changes with the distance between the detectors. We find that this principle works equally well for sound waves from a waterfall. This is remarkable, since sound is a completely different kind of wave from the HBT case. The frequency of the waves differs by a factor ∼1012 and the wavelength as well as the angular extension of the source seen from the observer's position differ by a factor ∼107. Our analysis is based on the general properties of broadband waves. We start with broadband waves at the amplitude level (not at intensity level) and demonstrate a HBT-like effect. We follow up with an explanation and demonstrations showing how the effect also manifests itself at the intensity level, providing a bridge to the original HBT work. We use the same reasoning in our numeric and analytical treatments, as well as in the experimental work, with identical results. The presentation is simple enough to be introduced even for second year bachelor students. Computer programs (in Matlab), including software for time-resolved frequency analysis, as well as original sound files are available from the authors.