Brightness temperature delay between sidebands. Top: typical detrended brightness temperatures of the LSB (solid) and USB (dotted) for one pixel. The vertical solid and dotted lines mark the peak temperatures of the pixel for LSB and USB, respectively. Bottom: phase spectrum (2D histogram of phase-angle as a function of frequency) between ALMA Band 3 LSB and USB from the Fourier transform analysis of each individual pixel within the white box shown in Fig. 1. The slope, indicated with the black solid line, represents the wave travel time of acoustic waves. The error bars show 1σ from Gaussian fitting.