A detailed spectral analysis of tremor signals recorded at Mt. Semeru volcano, Indonesia, in October 1992 by the German‐Indonesian Volcano Expedition (GIVE'92) reveals clearly harmonic spectra containing up to 11 integer harmonics. The spectral features can be explained by any rhythmically pulsating source producing a temporarily stable source signal. As one realisation of this concept we model the source as a resonating gas volume, generating seismic signals similar to the sound generated in a recorder. Copyright 1995 by the American Geophysical Union.