  We introduce a minimal model of ultracentral heavy-ion collisions to study
the relation between the speed of sound of the produced plasma and the final
particles' energy and multiplicity. We discuss how the particles' multiplicity
$N_{\textrm{tot}}$ and average energy $E_{\textrm{tot}}/N_{\textrm{tot}}$ is
related to the speed of sound $c_s$ by $c_s^2=d \ln
(E_{\textrm{tot}}/N_{\textrm{tot}})/d\ln N_{\textrm{tot}}$ if the fluid is
inviscid, its speed of sound is constant and all final particles can be
measured. We show that finite rapidity cuts on the particles' multiplicity $N$
and energy $E$ introduce corrections between $c_s^2$ and $d \ln (E/N)/d\ln N$
that depend on the system's lifetime. We study analytically these deviations
with the Gubser hydrodynamic solution, finding that, for ultrarelativistic
bosons, they scale as the ratio of the freezeout temperature $T_{\mathrm{FO}}$
over the maximum initial temperature of the fluid $T_{0}$; the
non-thermodynamic aspect of these corrections is highlighted through their
dependence on the system's initial conditions.
