An acoustic sensor, comprising: a side wall, closed by first and second end walls (3, 4) to form a substantially cylindrical cavity for containing a fluid, wherein a radius, a, of the cavity and an axial height, h, of the cavity satisfies the inequality a/h is greater than 1.2; a transmitter, operatively associated with one of the first and second end walls (3, 4); a receiver, operatively associated with the other of the first and second end walls (3, 4); and a first stiffener plate, comprising an outer peripheral edge and an aperture which defines an inner peripheral edge, and located on an outer face of the first end wall such that the aperture overlies the axis of the cavity; wherein: the first end wall comprises at least one through-hole, located radially of the axis of the cavity between the inner and outer peripheral edges of the first stiffener plate; the first stiffener plate comprises at least one duct, which connects the at least one through-hole, of the first end wall, to at least one of the inner and outer peripheral edges of the first stiffener plate, thereby to provide at least one fluid passageway between the cavity and the external surroundings of the acoustic sensor via the first end wall; and in use: the transmitter causes oscillatory motion, of the one of the first and second end walls (3, 4) with which the transmitter is associated, in a direction substantially perpendicular to the plane of that end wall (3, 4), such that axial oscillations of that end wall (3, 4) drive substantially radial oscillations of a fluid pressure in the cavity; and the substantially radial oscillations in the pressure of the fluid drive oscillatory motion of the other of the first and second end walls (3, 4), with which the receiver is associated, generating an electrical signal. a side wall, closed by first and second end walls (3, 4) to form a substantially cylindrical cavity for containing a fluid, wherein a radius, a, of the cavity and an axial height, h, of the cavity satisfies the inequality a/h is greater than 1.2; a transmitter, operatively associated with one of the first and second end walls (3, 4); a receiver, operatively associated with the other of the first and second end walls (3, 4); and a first stiffener plate, comprising an outer peripheral edge and an aperture which defines an inner peripheral edge, and located on an outer face of the first end wall such that the aperture overlies the axis of the cavity; wherein: the first end wall comprises at least one through-hole, located radially of the axis of the cavity between the inner and outer peripheral edges of the first stiffener plate; the first stiffener plate comprises at least one duct, which connects the at least one through-hole, of the first end wall, to at least one of the inner and outer peripheral edges of the first stiffener plate, thereby to provide at least one fluid passageway between the cavity and the external surroundings of the acoustic sensor via the first end wall; and in use: the transmitter causes oscillatory motion, of the one of the first and second end walls (3, 4) with which the transmitter is associated, in a direction substantially perpendicular to the plane of that end wall (3, 4), such that axial oscillations of that end wall (3, 4) drive substantially radial oscillations of a fluid pressure in the cavity; and the substantially radial oscillations in the pressure of the fluid drive oscillatory motion of the other of the first and second end walls (3, 4), with which the receiver is associated, generating an electrical signal. the first end wall comprises at least one through-hole, located radially of the axis of the cavity between the inner and outer peripheral edges of the first stiffener plate; the first stiffener plate comprises at least one duct, which connects the at least one through-hole, of the first end wall, to at least one of the inner and outer peripheral edges of the first stiffener plate, thereby to provide at least one fluid passageway between the cavity and the external surroundings of the acoustic sensor via the first end wall; and in use: the transmitter causes oscillatory motion, of the one of the first and second end walls (3, 4) with which the transmitter is associated, in a direction substantially perpendicular to the plane of that end wall (3, 4), such that axial oscillations of that end wall (3, 4) drive substantially radial oscillations of a fluid pressure in the cavity; and the substantially radial oscillations in the pressure of the fluid drive oscillatory motion of the other of the first and second end walls (3, 4), with which the receiver is associated, generating an electrical signal. the transmitter causes oscillatory motion, of the one of the first and second end walls (3, 4) with which the transmitter is associated, in a direction substantially perpendicular to the plane of that end wall (3, 4), such that axial oscillations of that end wall (3, 4) drive substantially radial oscillations of a fluid pressure in the cavity; and the substantially radial oscillations in the pressure of the fluid drive oscillatory motion of the other of the first and second end walls (3, 4), with which the receiver is associated, generating an electrical signal.