A signal processing device comprising: a filter bank configured to receive at least two audio signals, to apply weights to the audio signals and to provide the weighted versions of the audio signals to at least two speakers wherein the filter bank comprises a plurality of band-pass filters configured to segment the audio signals into sub-bands; a functional constraint derivation unit configured to determine a first constraint that limits the weights applied to the sub-band signals to be provided to a first of the at least two speakers, to determine an attenuation factor for stereo balancing of a second of the at least two speakers that will affect how a user will perceive audio signals output by the second speaker relative to audio signals output by the first speaker, and to determine a second constraint based on the determined attenuation factor and the first constraint; and a functional multi-constraint optimisation unit configured to determine the weights so as to minimise a difference between an actual sound stage balance of each signal that is expected to be heard by the user when the weighted audio signals are output by the speakers and a target sound stage balance, wherein the weights to be applied to the sub-band signal to be provided to the first speaker are further determined in dependence on the first constraint and the weights to be applied to the sub-band signals to be provided to the second speaker are further determined in dependence on the second constraint, wherein the functional multi-constraint optimisation unit is further configured to derive the weights by determining the target sound stage balance so as to simulate speakers that are symmetrically arranged with respect to the user and are further apart than the at least two speakers. a filter bank configured to receive at least two audio signals, to apply weights to the audio signals and to provide the weighted versions of the audio signals to at least two speakers wherein the filter bank comprises a plurality of band-pass filters configured to segment the audio signals into sub-bands; a functional constraint derivation unit configured to determine a first constraint that limits the weights applied to the sub-band signals to be provided to a first of the at least two speakers, to determine an attenuation factor for stereo balancing of a second of the at least two speakers that will affect how a user will perceive audio signals output by the second speaker relative to audio signals output by the first speaker, and to determine a second constraint based on the determined attenuation factor and the first constraint; and a functional multi-constraint optimisation unit configured to determine the weights so as to minimise a difference between an actual sound stage balance of each signal that is expected to be heard by the user when the weighted audio signals are output by the speakers and a target sound stage balance, wherein the weights to be applied to the sub-band signal to be provided to the first speaker are further determined in dependence on the first constraint and the weights to be applied to the sub-band signals to be provided to the second speaker are further determined in dependence on the second constraint, wherein the functional multi-constraint optimisation unit is further configured to derive the weights by determining the target sound stage balance so as to simulate speakers that are symmetrically arranged with respect to the user and are further apart than the at least two speakers.