A respiratory treatment device having an inlet configured to receive exhaled air into the device and an outlet configured to permit exhaled air to exit the device. A blocking member is moveable between a closed position where the flow of air through the device is restricted, and an open position where the flow of air through the device is less restricted. A biasing member is configured to bias the blocking member toward the closed position, wherein a level of bias decreases as the blocking member moves from the closed position to the open position.
