Two techniques have been used to analyze the densification of silica sand by horizontal sinusoidal vibrations of frequency f = 50 Hz for relative accelerations between 0 and 6: the quantitative analysis of motion observed through the transparent wall, and the altitude map of the free top surface of the sample. The first technique was used to analyze the transient regime: during the fust 10 s, slight densification occurs at the bottom of the powder bed, while the upper part enters convective notion and the intermediate part reaches densities higher than 66%. The second technique allowed to quantify the evolution of the overall density vs. acceleration f during the steady regime (dynamic density) and after the vibrations (relaxed density): a maximum density is observed in both cases for an optimal acceleration which depends on the initial height of the powder bed. These results are analyzed and discussed. (C) 2010 Elsevier B.V. All rights reserved.