Source spectra are investigated by using regional events recorded by the LDG French short-period seismic network. Empirical relations describing the Lg coda envelope allow us to separate source, site and propagation effects. 28 events recorded by 14 stations of the LDG network are selected for this study. Source spectra and site-response are determined in the frequency-band of 1--16 Hz. A Lg-coda magnitude can be defined from these source spectra. Magnitudes obtained by this technique are strongly correlated to local magnitudes given by the LDG bulletin but the standard deviation is reduced by a factor 3. Other parameters such as site-response, corner-frequency and coda Q are determined very accurately. ¿ American Geophysical Union 1996