A single first-order dipole microphone placed close to a reflecting plane results in a unidirectional second-order differential microphone. In practice an infinite reflecting plane is not attainable and it is therefore of interest to examine the effects of using a finite reflecting surface on second-order image-derived microphones. This talk will present results obtained in the investigation of a finite reflecting surface size by using the solution to the wave equation for scattering and diffraction from an oblate spheroid. The particular solution that is investigated is for the case of an infinitesimally thin oblate spheroid, the disk. Results show that the response of a first-order bidirectional sensor over the disk is sensitive to sensor location as well as to disk size relative to the acoustic wavelength. For a centrally located image-derived second-order microphone, the differential sensor response approaches a second-order response at a frequency where the reflecting baffle size is at least one-half the acoustic wavelength. Experimental results for both circular and rectangular baffle geometries will also be shown.