In this paper a closed‐chamber technique for measuring the complex free‐field sensitivity and directivity patterns of long underwater acoustic arrays is described. Performing these free‐field calibrations normally requires a large body of water to ensure that the reflections from the sides, surface, and bottom have a negligible effect on the measurement. The chamber contains acoustic projectors and monitoring hydrophones located along its length. The projectors are driven to produce an acoustic pressure field in the chamber that is equivalent to a free‐field plane acoustic wave propagating from any angle of incidence. This is accomplished by first measuring the transfer matrix for the calibration system. The transfer matrix relates the sound pressures at the hydrophone locations to the electrical drives to the projectors. The required projector drives, both amplitude and phase, are then determined by inverting the transfer matrix. A prototype calibration system has been developed. The system can operate over the frequency range from 10 to 600 Hz and is capable of simulating depths to 410 m. Directivity patterns obtained using the prototype for a test line array are in good agreement with theoretically predicted patterns.