Abstract: A system and method for accurately determining the power flow within a test subject's ear canal. A calibration device (300) is used in conjunction with a probe device (400) that measures the frequency response of a cavity. The probe (400), with eartip (440), is inserted into a patient's ear and a transducer (460,470) emits a periodic signal into the ear canal. The transducer (460,470) also measures the frequency response of the ear canal and relays the data back to a digital signal processor. The same probe setup (400) is then calibrated by measuring the frequency response of a plurality of cavities (340a, 340b, 350c, 350d) having known geometries. Mean cavity length is solved for based upon the known geometries of the plurality of cavities. Consideration is also given to spreading mass wave phenomena due to point source transducer emission. Cavity temperature for calibration purposes is maintained as close as possible to body temperature thereby negating a farther source of error.