to excite the forming fabric 12 directly without the use of membrane 14 this may result in undesired drainage through forming fabric 12 several methods are available to excite membrane 14 as shown conceptually in fig1 b a roll 22 of irregular diameter with protrusions 24 can be oriented across a full width of membrane 14 perpendicular to the direction of travel of forming fabric 12 and made to contact membrane 14 and rotated such that protrusions 24 i e the irregularities in the profile of roll 22 cause membrane 14 to be excited vertically as roll 22 extends across the entire width of membrane 14 and has a constant profile thereacross the excitation resulting from the rotation of roll 22 is evenly distributed across the apparatus 10 of the papermaking machine similarly as the protrusions 24 are symmetric and regularly spaced circumferentially about the rotational axis of roll 22 frequency of excitation is easily controlled by the rotational speed of roll 22 amplitude of excitation depends on the extent of the irregularity of roll 22 alternately viewed as the amplitude of protrusions 24 and position of the centerline of the roll 22 relative to the membrane 14 while the roll 22 conceptually illustrated in fig1 b has four lobes the technique is applicable to any number of lobes fig2 illustrates an embodiment where roll 22 journaled for rotation about horizontal axis 23 has many lobes or protrusions 24 and on which lobes or protrusions 24 are designed such that the displacement of membrane 14 is controlled by the profiles of lobes or protrusions 24 the vertical position of horizontal axis 23 of roll 22 is adjustable so as to allow variation in the amplitude of vibration imparted from roll 22 to membrane 14 such as.