An earth formation tool measures pressure from within a borehole. A portion of the borehole wall is isolated from surrounding borehole fluids by placing a chamber of a borehole tool against the borehole wall. The chamber comprises a recess in an exterior surface of a borehole tool. Mudcake present on the isolated portion of the borehole wall is disintegrated by an ultrasonic transducer within the chamber. In this manner, there is no resistance to fluid flow from the chamber to the formation. A pressure gauge measures pressure of the chamber to indicate pressure of the earth formation. This approach can be used to make moving or stationary measurements of earth formation pressure.