A sensor comprising: a sensor housing having an interior surface defining a sensor axis and an axial interior sensor housing cavity, the sensor housing comprising a first axial sensor housing portion having a first cross-sectional area perpendicular to the sensor axis, a second axial sensor housing portion arranged adjacent to the first axial sensor housing portion along the sensor axis and having a second cross-sectional area larger than the first cross-sectional area perpendicular to the sensor axis, and a face extending from the interior surface of the first axial housing portion to the interior surface of the second axial housing portion; the sensor further comprising: a buffer rod having a first axial end and a second axial end opposite the first axial end, the buffer rod comprising a first axial buffer portion arranged within the first housing portion and comprising the first axial end, and a second axial buffer portion arranged within the second housing portion and abutting the face, and comprising the second axial end; wherein the sensor further comprising an acoustic transceiver element acoustically mated to the first axial end; characterized in that the buffer rod further comprises a third axial buffer portion, extending axially between the first axial buffer portion and the second axial buffer portion, and having a third cross-sectional area, smaller than the first cross-sectional area, perpendicular to the sensor axis; wherein a cavity is defined between the interior surface and the third axial buffer portion, wherein the cavity is either a) evacuated or b) filled with a fluid or a solid, for example, air, having an acoustic impedance that is sufficiently different from the acoustic impedance of the buffer rod to reflect an acoustic echo when struck by an acoustic wave. a sensor housing having an interior surface defining a sensor axis and an axial interior sensor housing cavity, the sensor housing comprising a first axial sensor housing portion having a first cross-sectional area perpendicular to the sensor axis, a second axial sensor housing portion arranged adjacent to the first axial sensor housing portion along the sensor axis and having a second cross-sectional area larger than the first cross-sectional area perpendicular to the sensor axis, and a face extending from the interior surface of the first axial housing portion to the interior surface of the second axial housing portion; a first axial sensor housing portion having a first cross-sectional area perpendicular to the sensor axis, a second axial sensor housing portion arranged adjacent to the first axial sensor housing portion along the sensor axis and having a second cross-sectional area larger than the first cross-sectional area perpendicular to the sensor axis, and a face extending from the interior surface of the first axial housing portion to the interior surface of the second axial housing portion; the sensor further comprising: a buffer rod having a first axial end and a second axial end opposite the first axial end, the buffer rod comprising a first axial buffer portion arranged within the first housing portion and comprising the first axial end, and a second axial buffer portion arranged within the second housing portion and abutting the face, and comprising the second axial end; a first axial buffer portion arranged within the first housing portion and comprising the first axial end, and a second axial buffer portion arranged within the second housing portion and abutting the face, and comprising the second axial end; wherein the sensor further comprising an acoustic transceiver element acoustically mated to the first axial end; characterized in that the buffer rod further comprises a third axial buffer portion, extending axially between the first axial buffer portion and the second axial buffer portion, and having a third cross-sectional area, smaller than the first cross-sectional area, perpendicular to the sensor axis; wherein a cavity is defined between the interior surface and the third axial buffer portion, wherein the cavity is either a) evacuated or b) filled with a fluid or a solid, for example, air, having an acoustic impedance that is sufficiently different from the acoustic impedance of the buffer rod to reflect an acoustic echo when struck by an acoustic wave.