This work is mainly focused on the frequency-based analyses of non-invasive vibration and pressure fluctuations signals to characterize main hydrodynamic events inside a liquid-solid fluidized bed. The power spectra of bed shell vibration and local pressure fluctuations in a frequency domain were calculated over a wide range of liquid velocities to cover the pathway of bed behavior in between three main hydrodynamic events, namely: minimum fluidization, cluster-circulating-to-individual particle motion (termed as the solid regime transition), and solid entrainment points. First rise in the mean power spectral density function (PSDFm) of vibration fluctuations predicted the minimum fluidization accurately while a sharp level off in pressure signals showed the minimum fluidization velocity. Global maxima in the PSDFm of vibrations as well as second minima in the PSDFm of pressure fluctuations corresponded to the solid regime transition points. Finally, a plateau in the PSDFm of vibration signatures indicated a complete solid entrainment. Moreover, energy-based wavelet transform coupled with Hurst exponent analysis provided a detailed pathway of bed structure evolution, which led to the successful and accurate predictions of minimum fluidization as well as the solid regime transition inside the bed. (C) 2012 Elsevier B.V. All rights reserved.