The interaction between a fluidized bed and its air-supply system was studied in a cold one-ninth-scale model of a 12-MWth circulating-fluidized-bed (CFB) boiler and in a 0.7 x 0.12 x 8.5 m cold unit. Simultaneous measurements were carried out of pressure fluctuations in the bed and in the air plenum; of flow fluctuations in the air-supply ducts; and in the scale model, of bubble dynamics by an optical probe. It is shown that the fluid dynamics of the bed, represented by power spectra of pressure fluctuations in the bed and in the air plenum, depend significantly on the configuration of the air-supply system. In the scale model, three modes of bubble flow were identified: (1) the single-bubble regime, with the presence of one bubble at a time in the bed and with an interaction between the bed and the air-supply system at the bubble frequency; (2) a regime in which single bubbles still dominate the spectrum but another type of structure becomes significant at a frequency of its own (so-called exploding bubbles); (3) a regime in which exploding bubbles, acting as irregular voids stretching from the air distributor to the bed surface, dominate the spectrum and the bed is almost decoupled from the air-supply system. The cold CFB unit was operated at low velocity, either in the single-bubble regime or in a regime with numerous irregular bubbles, or at high velocity, in the exploding-bubble regime, with the same dominant frequency as for the single-bubble regime (the interaction with the air-supply system remains at that frequency). Pressure waves, resulting from the formation and eruption of bubbles, are identified as the main cause for the interaction between the bed and the air-supply system. These waves are recognized as the coherent part of the cross power spectra of pressure and flow fluctuations measured in the bed and the air plenum.