An apparatus for the transmission of data-bearing acoustic signals through a solid rigid substrate comprising: a first acoustic transducer for converting data-bearing electrical signals into data-bearing acoustic signals, the transducer being mounted on a first face of a solid rigid substrate to transmit the acoustic signals into the substrate; a second acoustic transducer mounted on a second face of the substrate so as to receive the acoustic signals emitted by the first acoustic transducer and to convert the acoustic signals into data-bearing electrical signals; characterized in that: at least one of the transducers (115, 120) is mounted on its respective face of the substrate via a wedge-shaped intermediate element (125, 130), the transducer (115, 120) being bonded by a first bonding layer to the intermediate element (125, 130) and the intermediate element (125, 130) being bonded to the substrate by a second bonding layer (135, 140), the intermediate element being configured such that the transducers (115, 120) are positioned relative to each other and/or to the substrate such that multiple-transit acoustic signals propagating between said transducers (115, 120) are effectively suppressed, by either the transducers (115, 120) having boresights inclined to each other at an angle such that a said multiple-transit signal incident on the second transducer has a phase variation across the transducer such that electrical signals induced in the transducer by said signal are attenuated by mutual interference or the spacing and/or the angular inclination of the boresights relative to each other being such that a multiple-transit signal falls outside a field of view of the second transducer. a first acoustic transducer for converting data-bearing electrical signals into data-bearing acoustic signals, the transducer being mounted on a first face of a solid rigid substrate to transmit the acoustic signals into the substrate; a second acoustic transducer mounted on a second face of the substrate so as to receive the acoustic signals emitted by the first acoustic transducer and to convert the acoustic signals into data-bearing electrical signals; characterized in that: at least one of the transducers (115, 120) is mounted on its respective face of the substrate via a wedge-shaped intermediate element (125, 130), the transducer (115, 120) being bonded by a first bonding layer to the intermediate element (125, 130) and the intermediate element (125, 130) being bonded to the substrate by a second bonding layer (135, 140), the intermediate element being configured such that the transducers (115, 120) are positioned relative to each other and/or to the substrate such that multiple-transit acoustic signals propagating between said transducers (115, 120) are effectively suppressed, by either the transducers (115, 120) having boresights inclined to each other at an angle such that a said multiple-transit signal incident on the second transducer has a phase variation across the transducer such that electrical signals induced in the transducer by said signal are attenuated by mutual interference or the spacing and/or the angular inclination of the boresights relative to each other being such that a multiple-transit signal falls outside a field of view of the second transducer. at least one of the transducers (115, 120) is mounted on its respective face of the substrate via a wedge-shaped intermediate element (125, 130), the transducer (115, 120) being bonded by a first bonding layer to the intermediate element (125, 130) and the intermediate element (125, 130) being bonded to the substrate by a second bonding layer (135, 140), the intermediate element being configured such that the transducers (115, 120) are positioned relative to each other and/or to the substrate such that multiple-transit acoustic signals propagating between said transducers (115, 120) are effectively suppressed, by either the transducers (115, 120) having boresights inclined to each other at an angle such that a said multiple-transit signal incident on the second transducer has a phase variation across the transducer such that electrical signals induced in the transducer by said signal are attenuated by mutual interference or the spacing and/or the angular inclination of the boresights relative to each other being such that a multiple-transit signal falls outside a field of view of the second transducer.