Communication apparatus comprising: a first transceiver unit; a signal processing device configured to receive a signal from the first transceiver unit and to produce a signal for transmission by the first transceiver unit, a cable connecting the first transceiver unit to the signal processing device, wherein the first transceiver unit comprises: a housing; a first pair of transducers (204R,T) located at, or adjacent, a first end of the housing; a second pair of transducers (206R,T) located at, or adjacent, an opposite end of the housing and wherein the apparatus is characterised in that it further comprises at least one further transceiver unit and at least one respective cable connecting the at least one further transceiver unit to the first transceiver unit, or to another of the at least one further transceiver units, in a daisy chain configuration and wherein the first pair of transducers of a said transceiver unit comprises a said transducer configured to transmit signals at a first frequency and another said transducer configured to receive signals at the first frequency and a second frequency, and the second pair of transducers of the transceiver unit comprises a said transducer configured to transmit signals at the second frequency and another said transducer configured to receive signals at the first frequency and the second frequency. a first transceiver unit; a signal processing device configured to receive a signal from the first transceiver unit and to produce a signal for transmission by the first transceiver unit, a cable connecting the first transceiver unit to the signal processing device, wherein the first transceiver unit comprises: a housing; a first pair of transducers (204R,T) located at, or adjacent, a first end of the housing; a second pair of transducers (206R,T) located at, or adjacent, an opposite end of the housing and wherein the apparatus is characterised in that it further comprises at least one further transceiver unit and at least one respective cable connecting the at least one further transceiver unit to the first transceiver unit, or to another of the at least one further transceiver units, in a daisy chain configuration and wherein the first pair of transducers of a said transceiver unit comprises a said transducer configured to transmit signals at a first frequency and another said transducer configured to receive signals at the first frequency and a second frequency, and the second pair of transducers of the transceiver unit comprises a said transducer configured to transmit signals at the second frequency and another said transducer configured to receive signals at the first frequency and the second frequency. a housing; a first pair of transducers (204R,T) located at, or adjacent, a first end of the housing; a second pair of transducers (206R,T) located at, or adjacent, an opposite end of the housing and wherein the apparatus is characterised in that it further comprises at least one further transceiver unit and at least one respective cable connecting the at least one further transceiver unit to the first transceiver unit, or to another of the at least one further transceiver units, in a daisy chain configuration and wherein the first pair of transducers of a said transceiver unit comprises a said transducer configured to transmit signals at a first frequency and another said transducer configured to receive signals at the first frequency and a second frequency, and the second pair of transducers of the transceiver unit comprises a said transducer configured to transmit signals at the second frequency and another said transducer configured to receive signals at the first frequency and the second frequency.