An acoustic model is developed to predict the acoustic pressure waves generated by rotary positive displacement gas meters. The meter is approximated, in the long wavelength limit, to a small, rigid and circular piston, pulsating with harmonic time dependence inside an infinite baffle. The acoustic source volume velocity is first derived from the geometry of the measuring part of the meter. Rayleigh's formulation is then applied to derive the acoustic pressure fields of the meter for both the unducted and the ducted situations. The effects of the self-induced pressure waves on the registration of the meter are then considered. The performance of the meter is dependent on the configuration of the pipe network to which the meter is attached. The meter calibration curve can be seriously distorted by the self-induced acoustic pressure waves. The effectiveness of concentric Helmholtz type resonators to suppress undesired sound pressure waves in meter systems is discussed. Experimental verification shows that the acoustic model developed is adequate to predict the "a.c" response of rotary positive displacement gas meters for most of the common pipe elements. Linear plane wave theory is used throughout and the effects of flow and friction are assumed to be negligible.