AC pressurisation: Fourier analysis and the effect of compressibility of air

The AC pressurisation method of measuring the airtightness of buildings is briefly described. Using a simplified model, the leakage characteristics of the enclosure may be found by Fourier analysis of the pressure signal if the effects of compressibility and flexing are neglected. These effects become small at low frequency, but the frequency required to make them negligible may be too low for the available instruments (low-frequency microphones) and the resulting pressure signal may be smaller than appropriate for building airtightness measurements. The conditions under which the other approximations in the model may be justified are not yet known.