An apparatus for mounting one or more transducers to a drilling pipe comprising: a sub-pipe comprising a bore-defining surface defining an axially oriented bore therethrough, a connector at a first axial end, and a connector at a second axial end, at least one of the connectors (130, 132) connectable to the drilling pipe; and a transducer-holding assembly comprising one or more transducers, the transducer-holding assembly operably inserted into the bore of the sub-pipe from one of the first and second axial ends (102A, 102B) and mounted within the bore of the sub-pipe to provide intimate contact which facilitates acoustic communication between the one or more transducers and the sub-pipe; wherein the sub-pipe is monolithically fabricated; wherein the intimate contact which facilitates acoustic communication between the one or more transducers and the sub-pipe comprises the transducer-holding assembly directly or indirectly bearing against the bore-defining surface of the monolithically fabricated sub-pipe at at least two axially spaced apart locations (104A, 104B), located at opposing first and second axial ends (104A, 104B) of the transducer-holding assembly with the one or more transducers located between the two axially spaced apart locations (104A, 104B), for providing, at each such axially spaced apart location (104A, 104B), an acoustic connection for communication of an acoustic signal into the sub-pipe, and, at at least one of the axially spaced apart locations (104A, 104B), a bearing surface portion of the bore-defining surface of the monolithically fabricated sub-pipe comprising an axially-facing, annular bearing surface; wherein the bore-defining surface of the sub-pipe comprises an axial facing annular arrest shaped to reduce a cross-sectional area of the bore in a vicinity of the arrest relative to at least one region of the bore adjacent the arrest; wherein the transducer-holding assembly is mounted within the bore of the sub-pipe and the apparatus comprises a tension collar located in the bore on a first axial side of the transducer-holding assembly to bear against the first axial end of the transducer-holding assembly; and wherein the apparatus comprises a feed-through located in the bore on a second axial side of the transducer-holding assembly to bear against the second axial end of the transducer-holding assembly. a sub-pipe comprising a bore-defining surface defining an axially oriented bore therethrough, a connector at a first axial end, and a connector at a second axial end, at least one of the connectors (130, 132) connectable to the drilling pipe; and a transducer-holding assembly comprising one or more transducers, the transducer-holding assembly operably inserted into the bore of the sub-pipe from one of the first and second axial ends (102A, 102B) and mounted within the bore of the sub-pipe to provide intimate contact which facilitates acoustic communication between the one or more transducers and the sub-pipe; wherein the sub-pipe is monolithically fabricated; wherein the intimate contact which facilitates acoustic communication between the one or more transducers and the sub-pipe comprises the transducer-holding assembly directly or indirectly bearing against the bore-defining surface of the monolithically fabricated sub-pipe at at least two axially spaced apart locations (104A, 104B), located at opposing first and second axial ends (104A, 104B) of the transducer-holding assembly with the one or more transducers located between the two axially spaced apart locations (104A, 104B), for providing, at each such axially spaced apart location (104A, 104B), an acoustic connection for communication of an acoustic signal into the sub-pipe, and, at at least one of the axially spaced apart locations (104A, 104B), a bearing surface portion of the bore-defining surface of the monolithically fabricated sub-pipe comprising an axially-facing, annular bearing surface; wherein the bore-defining surface of the sub-pipe comprises an axial facing annular arrest shaped to reduce a cross-sectional area of the bore in a vicinity of the arrest relative to at least one region of the bore adjacent the arrest; wherein the transducer-holding assembly is mounted within the bore of the sub-pipe and the apparatus comprises a tension collar located in the bore on a first axial side of the transducer-holding assembly to bear against the first axial end of the transducer-holding assembly; and wherein the apparatus comprises a feed-through located in the bore on a second axial side of the transducer-holding assembly to bear against the second axial end of the transducer-holding assembly.