Recently reported results on the origin, propagation and attenuation of pressure waves in bubbling gas-solid fluidized beds are re-valuated and the results are compared with additional experiments reported here. It is found that the measured pressure fluctuations are a result of slow and fast propagating pressure waves. Pressure waves with high propagation velocities (> 10 m/s) are unambiguously identified as compression waves, which move upwards and downwards. Upward moving compression waves originate from gas bubble formation and gas bubble coalescence. The amplitude of upward moving pressure waves is Linearly dependent on the distance to the bid surface. Downward moving compression waves are caused by gas bubble eruptions at the fluidized bed surface, bubble coalescence and by changes in bed voidage,In this case the pressure wave amplitude is independent of the distance to the bed surface. Pressure waves with propagation velocities of less than 2 m/s are caused by rising gas bubbles, These pressure waves move upwards only, with an amplitude proportional to the bubble size. The average wave propagation velocity measured in a freely bubbling bed is lower than that predicted from the pseudo-homogeneous compressible wave theory owing to the presence of slowly rising gas bubbles. The average propagation velocity of pressure waves in a gas-solid circulating fluidized bed is adequately described as a function of local voidage by pseudo-homogeneous compressible wave theory. At low voidages in the bottom of the riser, the propagation velocity is lowered by the presence of gas bubbles or large gas voids.