An active interleave linear array beamformer is proposed. Based on the concept of effective aperture and the frequency-response masking technique, the effective beampattern with a sharp transition band and low sidelobes can be synthesized with fewer sensor elements. Compared with other linear sparse active arrays, sidelobe level and grating lobes can be easily controlled by properly designed aperture functions. Computer simulations demonstrate the effectiveness of this beamformer.