A directional acoustic device (10,30,40,50,60,70,80,90) comprising: an acoustic source or an acoustic receiver (14,34,46,56,66,68,78,92); a conduit (12,40,62,94) to which the acoustic source or acoustic receiver is acoustically coupled and within which acoustic energy travels in a propagation direction from the acoustic source or to the acoustic receiver, the conduit having a finite extent at which the conduit ends; wherein the conduit has a radiating portion that has a radiating surface with leak openings that define controlled leaks through which acoustic energy radiated from the source into the conduit can leak to the outside environment or through which acoustic energy in the outside environment can leak into the conduit; wherein the only path for acoustic energy in the conduit to reach the external environment or acoustic energy in the external environment to enter the conduit is through the controlled leaks; wherein the leak openings define leaks having a first extent in the propagation direction, and also define leaks having a second extent at locations along the conduit with a constant time delay relative to the location of the source or receiver; wherein the extents of the leaks are determinative of a lowest frequency where useful directivity control is obtained, the useful directivity control corresponding to a reduction of the output or input of the directional acoustic device in a direction where radiation is unwanted by at least 3 dB when measured in the far field compared to the output or input of the acoustic source or receiver operating without the directional device; and wherein a lowest frequency of directivity control for the leaks having a first extent in the propagation direction is within 3 octaves of a lowest frequency of directivity control for the leaks having a second extent at locations along the conduit with a constant time delay relative to the location of the source or receiver, and wherein the radiating portion of the conduit lies in a plane and the source or receiver is not located in said plane of the radiating portion. an acoustic source or an acoustic receiver (14,34,46,56,66,68,78,92); a conduit (12,40,62,94) to which the acoustic source or acoustic receiver is acoustically coupled and within which acoustic energy travels in a propagation direction from the acoustic source or to the acoustic receiver, the conduit having a finite extent at which the conduit ends; wherein the conduit has a radiating portion that has a radiating surface with leak openings that define controlled leaks through which acoustic energy radiated from the source into the conduit can leak to the outside environment or through which acoustic energy in the outside environment can leak into the conduit; wherein the only path for acoustic energy in the conduit to reach the external environment or acoustic energy in the external environment to enter the conduit is through the controlled leaks; wherein the leak openings define leaks having a first extent in the propagation direction, and also define leaks having a second extent at locations along the conduit with a constant time delay relative to the location of the source or receiver; wherein the extents of the leaks are determinative of a lowest frequency where useful directivity control is obtained, the useful directivity control corresponding to a reduction of the output or input of the directional acoustic device in a direction where radiation is unwanted by at least 3 dB when measured in the far field compared to the output or input of the acoustic source or receiver operating without the directional device; and wherein a lowest frequency of directivity control for the leaks having a first extent in the propagation direction is within 3 octaves of a lowest frequency of directivity control for the leaks having a second extent at locations along the conduit with a constant time delay relative to the location of the source or receiver, and wherein the radiating portion of the conduit lies in a plane and the source or receiver is not located in said plane of the radiating portion.