Second generation solid freeforming devices will have the capability to render both shape and the spatial arrangement of composition directly from a computer file. In order to build three-dimensional functional gradients in selective laser sintering it is necessary to have a computer-controlled mixing and dispensing system. We report such a system based on the horizontal acoustic vibration of vertical capillary tubes that provides both switching and flow rate control. An orchestra of such tubes can be constructed to deposit a multicomponent system onto a building platform. Our concern is the initiation of flow. It requires an "attack" waveform to break the domes that provide flow arrest and release the potential energy of powder above the dome. The intensity of attack also influences the mass that flows out before a new dome forms, i.e., the response time of the valve. Neither the extension to vibration caused by ringing nor the wave amplitude account for the over-run of the valve. Much better correlation is obtained with acceleration and with calculated kinetic energy of horizontally vibrating particles. (C) 2004 Elsevier Ltd. All rights reserved.