Acoustic pressure is dynamic, so you are measuring a pressure perturbation (variation). Think of a DC signal vs an AC signal. The acoustic pressure would be a (usually) very small AC pressure signal. It would be measured on top of any existing static or dynamic pressure variation present in the fluid. Acoustic pressure is the variation in pressure wrt time. Human hearing range is generally accepted to be betwen 20 and 20,000 hz (cycles per second).....so if you have pressure that varies with time in that range, you'll be able to hear it. Acoustic pressure values can be very small. The reference pressure for the threshold of hearing is 20 microPascals. A sound pressure level of 100 dB would have an actual pressure magitude of 2 Pa. Compare that to a typical fluid pressure in whatever application your are thinking of. A microphone is just a very sensitive dynamic pressure transducer....the microphone diaphragm measures very small displacements as acoustic pressure waves impinge on it.