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.