In a paper-making machine, a method of determining a volumetric percentage of air in a fiber suspension for delivery to a headbox. The fiber suspension is transported through a fluid conduit. A sound transmitter and a target receiver are positioned relative to the fluid conduit. A sound is transmitted through the fiber suspension from the sound transmitter to the target receiver. A speed of the transmitted sound from the sound transmitter to the target receiver is established. The volumetric percentage of air in the fiber suspension is determined, dependent upon the established speed of sound.