A method of producing output indicative of the content of speech or mouthed speech from movement of speech articulators is described. The method may including fixing a plurality of magnets respectively to a plurality of speech articulators of a human individual. Providing a support. Providing a plurality of signal magnetic field sensors. Providing at least three reference magnetic field sensors orientated differently from one another with respect to the Earth's magnetic field. The signal and reference magnetic field sensors being fixed to the support which holds the sensors in fixed spatial relationships to one another. Producing, over a period of time, a respective signal from each signal magnetic field sensor and a respective signal from each reference magnetic field sensor. Obtaining, over the period of time, for each said signal magnetic field sensor signal, a respective correction value.