  The Atacama Large Millimeter/sub-millimeter Array (ALMA) has provided us with
an excellent diagnostic tool for studies of the dynamics of the Solar
chromosphere, albeit through a single receiver band at one time presently. Each
ALMA band consists of four sub-bands that are comprised of several spectral
channels. To date, however, the spectral domain has been neglected in favour of
ensuring optimal imaging, so that time-series observations have been mostly
limited to full-band data products, thereby limiting studies to a single
chromospheric layer. Here, we report the first observations of a dynamical
event (i.e. wave propagation) for which the ALMA Band 3 data (centred at 3\,mm;
100\,GHz) is split into a lower and an upper sideband. In principle, this
approach is aimed at mapping slightly different layers in the Solar atmosphere.
The side-band data were reduced together with the Solar ALMA Pipeline (SoAP),
resulting in time series of brightness-temperature maps for each side-band.
Through a phase analysis of a magnetically quiet region, where purely acoustic
waves are expected to dominate, the average height difference between the two
side-bands is estimated as $73\pm16$~km. Furthermore, we examined the
propagation of transverse waves in small-scale bright structures by means of
wavelet phase analysis between oscillations at the two atmospheric heights. We
find 6\% of the waves to be standing, while 54\% and 46\% of the remaining
waves are propagating upwards and downwards, respectively, with absolute
propagating speeds on the order of $\approx96$~km/s, resulting in a mean energy
flux of $3800$\,W/m$^2$.
