Apparatus 10 (FIG. 1(a)) for providing acoustic power spectra, all defined over a frequency band from signals received in a predetermined time interval, for individually identifiable directions comprises a pair of arrays 11, 11' of omnidirectional microphone elements 12. The time interval is divided into sampling periods and for each period the microphone signal sampled at 17, digitized at 21 and applied to array sample storage means 27 comprising corresponding shift register 28.sub.1. . . 28.sub.N for each microphone channel. Previously applied samples are clocked one stage with each new sample so each stage represents a delay of one sampling period. Register stages are connected to nodes 34 of a switching matrix 33, selected by ROM 36 to define the relative delays between samples for each beam direction and those samples read by the matrix are combined to form directed beam samples for that sampling period, stored at 40. At the end of the time interval they are extracted in a set for each beam direction, the samples taken at the succession of sampling periods in the interval being analyzed by FFT circuit 44 to give an acoustic power spectrum for the direction.