A method of producing output indicative of the content of speech or mouthed speech from movement of speech articulators, comprising: fixing a plurality of magnets (10, 12, 14, 16, 18, 20) respectively to a plurality of speech articulators (22, 24, 26) of a human individual; providing a support (28, 30); providing a plurality of signal magnetic field sensors (32, 34, 36), each said signal magnetic field sensor being located so as to be able to sense a respective magnetic field intensity comprising a component produced by at least one of the magnets and a component produced by the Earth's magnetic field; providing at least three reference magnetic field sensors orientated differently from one another with respect to the Earth's magnetic field, each said reference magnetic field sensor being located so as to be able to sense a respective magnetic field intensity produced at least partially by the Earth's magnetic field; the signal and reference magnetic field sensors (32, 34, 36, 38) being fixed to the support (28, 30) which holds the sensors (32, 34, 36, 38) in fixed spatial relationships to one another; producing, over a period of time, a respective signal from each signal magnetic field sensor (32, 34, 36) and a respective signal from each reference magnetic field sensor, each signal being indicative of a respective magnetic field intensity sensed by the corresponding magnetic field sensor, wherein over said period of time the signals from the signal and reference magnetic field sensors change in response to movement of the support relative to the Earth's magnetic field, and the signals from the signal magnetic field sensors, and optionally also the signals from the reference magnetic field sensors, change in response to movement of the articulators; and obtaining, over said period of time, for each said signal magnetic field sensor signal, a respective correction value that is indicative of the component that is produced by the Earth's magnetic field in the magnetic field intensity sensed by the corresponding signal magnetic field sensor (32, 34, 36) and each correction value changing over said time period as the component that is produced by the Earth's magnetic field in the magnetic field intensity sensed by the corresponding signal magnetic field sensor (32, 34, 36) changes as the support moves; wherein each correction value is calculated using the signals from the at least three reference magnetic field sensors, and also using a respective spatial transformation, each spatial transformation relating the orientations of each of said at least three reference magnetic field sensors to the orientation of the corresponding signal magnetic field sensor (32, 34, 36). fixing a plurality of magnets (10, 12, 14, 16, 18, 20) respectively to a plurality of speech articulators (22, 24, 26) of a human individual; providing a support (28, 30); providing a plurality of signal magnetic field sensors (32, 34, 36), each said signal magnetic field sensor being located so as to be able to sense a respective magnetic field intensity comprising a component produced by at least one of the magnets and a component produced by the Earth's magnetic field; providing at least three reference magnetic field sensors orientated differently from one another with respect to the Earth's magnetic field, each said reference magnetic field sensor being located so as to be able to sense a respective magnetic field intensity produced at least partially by the Earth's magnetic field; the signal and reference magnetic field sensors (32, 34, 36, 38) being fixed to the support (28, 30) which holds the sensors (32, 34, 36, 38) in fixed spatial relationships to one another; producing, over a period of time, a respective signal from each signal magnetic field sensor (32, 34, 36) and a respective signal from each reference magnetic field sensor, each signal being indicative of a respective magnetic field intensity sensed by the corresponding magnetic field sensor, wherein over said period of time the signals from the signal and reference magnetic field sensors change in response to movement of the support relative to the Earth's magnetic field, and the signals from the signal magnetic field sensors, and optionally also the signals from the reference magnetic field sensors, change in response to movement of the articulators; and obtaining, over said period of time, for each said signal magnetic field sensor signal, a respective correction value that is indicative of the component that is produced by the Earth's magnetic field in the magnetic field intensity sensed by the corresponding signal magnetic field sensor (32, 34, 36) and each correction value changing over said time period as the component that is produced by the Earth's magnetic field in the magnetic field intensity sensed by the corresponding signal magnetic field sensor (32, 34, 36) changes as the support moves; wherein each correction value is calculated using the signals from the at least three reference magnetic field sensors, and also using a respective spatial transformation, each spatial transformation relating the orientations of each of said at least three reference magnetic field sensors to the orientation of the corresponding signal magnetic field sensor (32, 34, 36).