  Because they are sensitive to mechanical properties of materials and can
propagate even in opaque systems, acoustic waves provides us with a powerful
characterization tool in numerous fields. Common techniques mostly rely on
time-of-flight measurements and do not exploit the spectral content: however,
sound speed and attenuation spectra contain rich information. Such an acoustic
spectroscopy already exists and allows to retrieve subtle information on
systems of well-known physico-chemistry, but modeling becomes out of reach for
industrial systems. In this article, we use a simple empirical approach to
monitor the gelation of silica suspensions: we show that the gelation time
obtained from acoustic measurements is proportional to this determined with
more conventional rheological characterization. Such a results thus opens the
way for in-situ monitoring of time-evolving systems in industrial context with
acoustic methods only.
