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We present an easy-to-use, robust tool for monitoring the unrest of active volcanoes: the Base Level Noise Seismic Spectrum (BLNSS). The volcano's base level activity can easily be determined by continuous monitoring of the minimum amplitude of all spectral components, and thus indicates the volcanic activity which is always present on fixed-segment waveforms. The selection of comparative time intervals also eliminates transitory signals related to weather/environmental conditions, thus avoiding the misinterpretations in terms of predominant frequencies found in short term analysis. The application to data recorded at the Soufriere Hills, Llaima and Villarrica volcanoes demonstrates the methodology's ability to clearly distinguish volcanic background activity from signs of re-activation and provides information related to the behavior and dynamics of volcanic unrest.