Following the eruption of the Mount St. Helens volcano on May 18, 1980, atmospheric waves were recorded by a network of microbarographs located over 7000 km from the source. Analysis of these data requires the use of complex processing techniques based on a high-resolution method to extract the signals produced by the St. Helens source from spurious waves or noise in each record. This facilitates interpretation of the wave trains in terms of propagation modes. It is thus shown that Lamb mode L0 is present in the low-frequency part of all the signals, whereas acoustic modes (more probably A2) are needed to explain all the properties of the high-frequency part, which is clearly observed for a westward and a southward propagation.