Comparison of Active Control Strategies for the Active Control of Buzz-saw Tones

Abstract

This paper presents the results of a feasibility study of active control to reduce the very low engine order components of the buzz-saw noise spectrum. It is shown that effective control performance can be achieved using a single ring of control actuators to minimise the sum of the squared pressures measures at a single ring of circumferentially spaced error sensors. The paper shows that the level of the evanescent modes excited by the control actuators is the factor that fundamentally limits control performance when there is no noise or extraneous modes present at the error sensors. The variation of sound power reduction versus tip speed is investigated in detail. It is shown that, following control, a new set of resonances arise due to standing waves set up by the secondary sources between the sources and the duct exhaust termination, and between the sensors and the duct inlet termination. Modal approaches to controlling the buzz-saw tones are also investigated in the paper. A comparison is made between the performance obtained by the pressure minimisation at the error sensors and a control system which minimises the spinning mode amplitudes deduced from pressure measurements made at the error sensors. It is shown that pressure minimisation and minimisation of the mode amplitudes yields identical control performance for single rings of sources and sensors. When an additional ring of sources and error sensors are introduced the sound power reduction is substantially greater in the pressure control case. However, improvements of only about 10dB are observed in the case where the spinning mode amplitudes are minimised at the error sensors.