A study on the pressure leakage correction of pistonphones at infrasonic frequencies

Pressure leakage correction is an important part of the sound pressure measurement using pistonphones, especially at infrasonic frequencies. The existing literature on the calculation of the pressure leakage correction is ambiguous on the time constant T involved in the formulation. On one hand, it states that T=RC, which means the adiabatic time constant Ta. On the other hand, it states that the value of T is measured from the pressure step response of the pistonphone, which could be more specifically a polytropic process. Using the measured (polytropic) time constant Tm as Ta could lead to an error for pressure leakage correction. In this paper, explicit solutions to both the adiabatic and polytropic pressure step responses of a pistonphone are derived. These solutions indicate that Ta is different from Tm. Experimental validations confirm that the measured pressure response agrees well with the explicit solution of polytropic pressure step response. Substituting the optimized parameters into the formulations derived, accurate adiabatic time constant Ta may be obtained and precise sound pressure readings be acquired. The method could improve the precision of calibrating measurement microphones at infrasonic frequencies using pistonphones.