A communication system 100) comprises a first set (204, 206) of transducers and a second set (210, 212) of transducers. The system further comprises a first signal processing device (106) configured to produce a signal for transmission by at least some of the transducers in the first set to at least some of the transducers in the second set, and a second signal processing device (110) configured to process signals received from at least some of the transducers in the second set in order to select a best said signal that is used for further processing. The first set of transducers comprises a first subset (204) comprising at least one said transducer, and a second subset (206) comprising at least one said transducer. Transducers in the first subset transmit on a first channel and transducers in the second subset transmit on a second channel having a different frequency.