In acoustic applications, the signal source of interest is generally broadband and the background noise field is predominantly isotropic. In this letter, the performance of the broadband generalized sidelobe canceller, optimized with respect to an isotropic noise field, is analyzed and compared against the performance of the conventional summing beamformer. The comparison is based on an ideal continuous-time infinite-length generalized sidelobe canceller with the aim of determining the performance limits of this beamforming structure. ©2000 Acoustical Society of America.