A selected history of duct mode synthesis

Abstract

Attempts to attenuate noise propagating down a duct, with or without flow, must recognize that wall boundary conditions permit only certain spatial wave patterns (modes) to exist. The aircraft noise research community began building mode synthesizers at least thirty years ago in order to study duct propagation and radiation phenomena in a laboratory environment. Lockheed Georgia built a spinning mode synthesizer (SMS) for NASA in the 1970s. NASA used its SMS in a circular flow-duct to validate predictions of mode propagation through constrictions, lined sections, and inlets of various shapes. In the mid-1970s, Penn State created a mode synthesizer as part of a demonstration of active noise control in a circular, no-flow duct. NASA sponsored a series of studies in the 1990s aimed at maturing active control technology for ducted fan noise. Each of these active control systems was essentially a mode synthesizer coupled with a control system to cancel fan-generated noise. NASA is currently building a new mode synthesizer in a rectangular duct to study the effects of high-speed flow and curvature of the duct axis on advanced noise suppression technology. The evolution of mode-generation schemes will be discussed.