Pressure data sampled at sufficiently high frequency (typically 20 Hz or higher) can yield much information about the hydrodynamic state of a fluidized bed. Since part of the pressure waves traveling through large (industrial) fluidized beds is only detectable in a limited area of the bed, pressure measurements need to be performed at several positions to cover the whole bed. The local pressure waves (caused by, e.g., passing bubbles or coalescing bubbles) in a 0.80 m diam. bubbling fluidized bed of Geldart B particles are examd. Expts. and simulations are performed to det. the intensity decrease as local pressure waves propagate from their origin. A new spectral method is applied to det. the degree of coherence for pressure signals measured at two different positions in a fluidized bed. For a superficial gas velocity of 5umf, local pressure waves can be detected up to a radial distance of .apprx. 0.5 m from their origin; this distance is somewhat lower for lower gas velocities. This means that the radial spacing of pressure probes should not exceed 1 m. For large diam. beds with a bed height <1.5 m, a set of probes at a single level and at several radial positions is sufficient to observe or monitor the dynamic state of the complete bed; the probes should preferably be placed at a height of 30-40% of the total bed height.