An exhalation valve , comprising: a valve seat comprising a first major surface , a second major surface , an orifice disposed between the first and second major surfaces of the valve seat, and a valve seat axis extending between a first end and a second end of the valve seat, wherein the orifice has a substantially circular shape in a plane defined by the first major surface of the valve seat, wherein the valve seat further comprises a seal surface and a flap retaining surface each disposed on the first major surface of the valve seat, wherein the seal surface circumscribes the orifice and comprises a substantially noncircular shape in the plane defined by the first major surface of the valve seat; and a valve flap disposed over the seal surface and the orifice and comprising a first end connected to the flap retaining surface , wherein the valve flap is adapted to be sealed against the seal surface of the valve seat when the exhalation valve is disposed in a closed configuration (FIG. 3) such that fluid is prevented from flowing through the valve seat , wherein a second end of the valve flap is adapted to be spaced apart from the seal surface when the exhalation valve is disposed in an open configuration (FIG. 4) such that fluid can flow through the valve seat , and further wherein the valve flap comprises a curved shape in a plane orthogonal to the first major surface of the valve seat when the exhalation valve is disposed in the closed configuration; wherein one or more portions of a perimeter through the orifice are coincident with the seal surface as measured along the first major surface of the valve seat . wherein no greater than 75% of the perimeter of the orifice is coincident with the seal surface as measured in the plane defined by the first major surface of the valve seat . a valve seat comprising a first major surface , a second major surface , an orifice disposed between the first and second major surfaces of the valve seat, and a valve seat axis extending between a first end and a second end of the valve seat, wherein the orifice has a substantially circular shape in a plane defined by the first major surface of the valve seat, wherein the valve seat further comprises a seal surface and a flap retaining surface each disposed on the first major surface of the valve seat, wherein the seal surface circumscribes the orifice and comprises a substantially noncircular shape in the plane defined by the first major surface of the valve seat; and a valve flap disposed over the seal surface and the orifice and comprising a first end connected to the flap retaining surface , wherein the valve flap is adapted to be sealed against the seal surface of the valve seat when the exhalation valve is disposed in a closed configuration (FIG. 3) such that fluid is prevented from flowing through the valve seat , wherein a second end of the valve flap is adapted to be spaced apart from the seal surface when the exhalation valve is disposed in an open configuration (FIG. 4) such that fluid can flow through the valve seat , and further wherein the valve flap comprises a curved shape in a plane orthogonal to the first major surface of the valve seat when the exhalation valve is disposed in the closed configuration; wherein one or more portions of a perimeter through the orifice are coincident with the seal surface as measured along the first major surface of the valve seat . wherein no greater than 75% of the perimeter of the orifice is coincident with the seal surface as measured in the plane defined by the first major surface of the valve seat . <EOS>