Abstract: Polarity of transducer signals is controlled in encoding apparatus of the acoustic rod type, e.g. where key-actuated strikers impact an acoustic bar having a transducer at each end of the bar, the impact giving rise to divergent acoustic waves. The wave fronts of these acoustic waves are sensed by the transducers, the output signals of which are used to produce codes corresponding to the actuated key. By varying the shape of the bar at each point of impact, the polarity of the wave fronts can be controlled so as to distinguish the operation of adjacent strikers and thus provide correspondingly coded outputs or can be combined with elapsed time sensing systems to afford more reliable encoding even in the presence of lot-to-lot differences in the bar material's acoustic wave properties--differences aggravated by temperature changes.