A pressure wave generator configured to generate reciprocating pressure waves of operating gas for driving one or more cryogenic refrigerator systems, comprising: a housing with one or more inlet/outlet ports (57, 58, 81, 82, 83, 84, 170, 171, 172, 173) which the generated reciprocating pressure waves of operating gas pass through to drive a cryogenic refrigerator system or systems connected to the inlet/outlet port(s); at least one pair of opposed diaphragms (41, 42, 71, 72, 74, 75) located in the housing that are moveable in a reciprocating motion within the housing, each diaphragm comprising a driving side (41a, 42a, 71a, 72a, 74a, 75a), each pair of opposed diaphragms (41, 42, 71, 72, 74, 75) being driven in a reciprocating motion by a respective reciprocating drive part (50, 73, 76) and the diaphragms of each pair of opposed diaphragms being secured between the housing (46, 47, 48, 49) and at or toward a respective opposite end (63, 64, 89, 90, 91, 92) of the reciprocating drive part (50, 73, 76) such that the opposed diaphragms (41, 42, 71, 72, 74, 75) are operatively coupled to each other so that they move back and forth together, a gas space (55, 56, 85, 86, 87, 88) associated with each diaphragm (41, 42, 71, 72, 74, 75) and wherein the driving side (41a, 42a, 71a, 72a, 74a, 75a) of each diaphragm (41, 42, 71, 72, 74, 75) is arranged to move in a reciprocating motion within its respective gas space (55, 56, 85, 86, 87, 88) to generate reciprocating pressure waves and wherein at least one of the gas spaces (55, 56, 85, 86) has an associated inlet/outlet port (57, 58, 81, 82, 83, 84, 170, 171, 172, 173) of the housing through which the generated pressure waves pass, the two gas spaces (55, 56, 85, 86, 87, 88) associated with each pair of opposed diaphragms being connected to each other, independently from the inlet/outlet ports of the housing, by a respective connection pipe comprising an orifice configured to reduce gas flow between the two gas spaces to negligible levels that are sufficient to balance the average gas forces on the opposed diaphragms associated with the two gas spaces (55, 56, 85, 86, 87, 88); and a drive system comprising one or more operable actuators (51, 52, 77, 78, 79) that are arranged to drive the reciprocating drive part(s) (50, 73, 76) in a reciprocating motion to move each pair of diaphragms (41, 42, 71, 72, 74, 75) in a reciprocating motion back and forth within the housing to generate the reciprocating pressure waves to drive one or more cryogenic refrigerator systems connected to the inlet/outlet port(s) of the housing, and wherein the gas spaces (55, 56, 85, 86, 87, 88) are sealed from the moving actuators (51, 52, 77, 78, 79) of the drive system. a housing with one or more inlet/outlet ports (57, 58, 81, 82, 83, 84, 170, 171, 172, 173) which the generated reciprocating pressure waves of operating gas pass through to drive a cryogenic refrigerator system or systems connected to the inlet/outlet port(s); at least one pair of opposed diaphragms (41, 42, 71, 72, 74, 75) located in the housing that are moveable in a reciprocating motion within the housing, each diaphragm comprising a driving side (41a, 42a, 71a, 72a, 74a, 75a), each pair of opposed diaphragms (41, 42, 71, 72, 74, 75) being driven in a reciprocating motion by a respective reciprocating drive part (50, 73, 76) and the diaphragms of each pair of opposed diaphragms being secured between the housing (46, 47, 48, 49) and at or toward a respective opposite end (63, 64, 89, 90, 91, 92) of the reciprocating drive part (50, 73, 76) such that the opposed diaphragms (41, 42, 71, 72, 74, 75) are operatively coupled to each other so that they move back and forth together, a gas space (55, 56, 85, 86, 87, 88) associated with each diaphragm (41, 42, 71, 72, 74, 75) and wherein the driving side (41a, 42a, 71a, 72a, 74a, 75a) of each diaphragm (41, 42, 71, 72, 74, 75) is arranged to move in a reciprocating motion within its respective gas space (55, 56, 85, 86, 87, 88) to generate reciprocating pressure waves and wherein at least one of the gas spaces (55, 56, 85, 86) has an associated inlet/outlet port (57, 58, 81, 82, 83, 84, 170, 171, 172, 173) of the housing through which the generated pressure waves pass, the two gas spaces (55, 56, 85, 86, 87, 88) associated with each pair of opposed diaphragms being connected to each other, independently from the inlet/outlet ports of the housing, by a respective connection pipe comprising an orifice configured to reduce gas flow between the two gas spaces to negligible levels that are sufficient to balance the average gas forces on the opposed diaphragms associated with the two gas spaces (55, 56, 85, 86, 87, 88); and a drive system comprising one or more operable actuators (51, 52, 77, 78, 79) that are arranged to drive the reciprocating drive part(s) (50, 73, 76) in a reciprocating motion to move each pair of diaphragms (41, 42, 71, 72, 74, 75) in a reciprocating motion back and forth within the housing to generate the reciprocating pressure waves to drive one or more cryogenic refrigerator systems connected to the inlet/outlet port(s) of the housing, and wherein the gas spaces (55, 56, 85, 86, 87, 88) are sealed from the moving actuators (51, 52, 77, 78, 79) of the drive system.