The accuracy of measurements of the wall pressure beneath a turbulent boundary layer is affected by the size of the pressure transducer because of spatial averaging over the sensing area of the transducer. The effect of transducer size on the wall pressure spectrum was investigated by numerically applying wavenumber filters corresponding to various size and shape transducers to a database of wall pressure generated from a direct numerical simulation of turbulent channel flow. Circular transducers with piston-type and deflection-type sensitivities were modeled along with square transducers with piston-type sensitivity. A deflection-type transducer attenuates the rms wall pressure less than a piston-type transducer of the same area. The wavenumber spectrum of the wall pressure measured using a large transducer has lobes and zeros corresponding to those in the wavenumber response function of the transducer. These lobes and zeros in the wavenumber spectrum are also evident in the frequency spectrum, although they are smeared. Using Taylor's frozen field hypothesis, an upper bound on the frequency of wall pressure fluctuations that can be measured before the zeros in the wavenumber response function affect the wall pressure spectrum is given in terms of the convection velocity and the dimensions of the transducer.