We present a computer simulation and theoretical study of composite multiferroics acting as magnetic field detectors. Our set?up consists of a layer of piezoelectric sandwiched between two magnetostrictive layers. When magnetic field is applied the magnetostrictive strains and this is passed onto the piezoelectric producing a detectable voltage. We model the device using our own finite element code and calculate open circuit voltage and input impedance. Good agreement is shown with analytical formulas. We then optimise the coupling by altering the ratio of piezoelectric to magnetostrictive volume and the shape of the inner piezoelectric layer: the former effect is more important. We show that nearly equal amounts of piezoelectric and magnetostrictive give best coupling but the exact optimal ratio depends on the relative stiffnesses of the two materials. Most calculations carried out in previous literature have assumed that the magnetic field found in the device is simply the applied field. Here we show to what extent such an applied field can enter the device by simulating also the air region around the detector. The field in the device is calculated to be ?17% less than the applied field.