Correction of volume for the offset of piston in a calculable pistonphone with small chamber

When the high sound pressure at high frequencies is produced in the calculable pistonphone, for instance, in order to generate a sound field with a higher frequency such as not lower than 315 Hz and a sound pressure level reaching 174 dB Ref. 20 $\mu$Pa, the chamber is always designed small enough based on the sound pressure formula of the calculable pistonphone and the output capacity limitation of vibrators. Meanwhile, a balanced hole is always designed in the chamber to balance the inside and outside static pressure, which results in the offset of the piston equilibrium position and the large error of the calculated sound pressure. The mechanism of the piston offset is explained by the theory of fluid motion in circular pipes and thermal expansion. An eddy current sensor is thus introduced to measure the offset of piston of the calculable pistonphone and the primary calibration results were corrected. The experiment verified the phenomenon of the piston offset, and the effect of the correction method is confirmed in the primary calibration.