The 1999 eruption of Shishaldin volcano (Alaska, USA) displayed both Strombolian and Subplinian basaltic activity. The Subplinian phase was preceded by a signal of low amplitude and constant frequency (≈2 Hz) lasting 13 h. This “humming signal” is interpreted as the coalescence of the very shallow part of a foam building up in the conduit, which produces large gas bubbles before bursting. The acoustic waveform of the hum event is modelled by a Helmholtz resonator: gas is trapped into a rigid cavity and can only escape through a tiny upper hole producing sound waves. At Shishaldin, the radius of the hole (≈5 m) is close to that of the conduit (≈6 m), the cavity has a length of ≈60 m, and gas presents only a small overpressure between (≈1.2×10−3 and 4.5×10−3 MPa). Such an overpressure is obtained by the partial coalescence of a foam formed by bubbles with a diameter from ≈2.3 mm at the beginning of the episode towards ≈0.64 mm very close to the end of the phase.

The intermittency between hum events is explained by the ripening of the foam induced by the H2O diffusion through the liquid films. The two extreme values, from 600 to 10 s, correspond to a bubble diameter from 2.2 to 0.3 mm at the beginning and end of the pre-Subplinian phase, respectively. The extremely good agreement between two independent estimates of bubble diameters in the shallow foam reinforces the validity of such an interpretation.

The total gas volume lost at the surface during the humming events is at most 5.9×106 m3. At the very end of the pre-Subplinian phase, there is a single large bubble with an overpressure of ≈0.42 MPa. The large overpressure suggests that it comes from significant depth, unlike other bubbles in the pre-Subplinian phase. This deep bubble may be responsible for the entire foam collapse, resulting in the Subplinian phase.