  We provide a measurement of the deficit in the Sunyaev-Zel'dovich Compton-$y$
signal towards cosmic voids, by stacking a catalogue of 97,090 voids
constructed with BOSS-DR12 data, on the $y$ maps built on data from the Atacama
Cosmology Telescope (ACT) DR4 and the Planck satellite. We detect the void
signal with a significance of $7.3\,\sigma$ with ACT and $9.7\,\sigma$ with
Planck, obtaining agreements in the associated void radial $y$ profiles
extracted from both maps. The inner-void profile (for angular separations
within the void angular radius) is reconstructed with significances of
$4.7\sigma$ and $6.1\sigma$ with ACT and Planck, respectively; we model such
profile using a simple model that assumes uniform gas (under)density and
temperature, which enables us to place constraints on the product
$(-\delta_{\rm v}T_{\rm e})$ of the void density contrast (negative) and the
electron temperature. The best-fit values from the two data sets are
$(-\delta_{\rm v}T_{\rm e})=(6.5\pm 2.3)\times 10^{5}\,\text{K}$ for ACT and
$(8.6 \pm 2.1)\times 10^{5}\,\text{K}$ for Planck ($68\%$ C.L.), which are in
good agreement under uncertainty. The data allow us to place lower limits on
the expected void electron temperature at $2.7\times10^5\,\text{K}$ with ACT
and $5.1\times10^5\,\text{K}$ with Planck ($95\%$ C.L.); these results can
transform into upper limits for the ratio between the void electron density and
the cosmic mean as $n^{\rm v}_{\rm e}/\bar{n}_{\rm e}\leqslant 0.73$ and $0.49$
($95\%$ C.L.), respectively. Our findings prove the feasibility of using tSZ
observations to constrain the gas properties inside cosmic voids, and confirm
that voids are under-pressured regions compared to their surroundings.
