The active control of the transmission of sound

Thomas, Dean Robert (1992) The active control of the transmission of sound. University of Southampton, Doctoral Thesis.

Abstract

This thesis considers the use of secondary force inputs to a structure in order to control the transmission of sound through that structure. The particular emphasis is on the possible use of this approach to control the transmission of propeller noise through an aircraft fuselage. A generalised model of a vibrating rectangular plate is used to explore the mechanisms of control involved with this approach. For both a freely mounted rigid plate and a clamped elastic plate large attentuations in the transmission of an acoustic plane wave are shown to be possible by the use of a number of secondary force inputs. Two mechanisms of control are demonstrated for the elastic plate: `modal suppression' where the dominantly radiating modes are attenuated and `modal restructuring' where amplitudes and phases of the plate modes are altered by the introduction of control forces in such a way as to produce a vibration distribution of low radiation efficiency. The results of an experimental investigation into the active control of sound transmission through a clamped rectangular steel plate and a freely mounted aluminium honeycomb panel are presented with control being applied, in both cases through three secondary force inputs to the plates. For the steel plate significant global reductions were obtained at most frequencies although reductions were limited, principally by the existence of a flanking transmission path. The phenomenon of control `spill-over' was observed to occur for the steel plate with the vibration of the plate increasing at some frequencies. For the aluminium honeycomb panel large global reductions in the transmitted sound were obtained. A practical means of applying active control of sound transmission in an aircraft is suggested. This involves the use of secondary forces acting between the fuselage and a stiff, lightweight internal trim. An experimental invetigation of this approach is undertaken in which secondary forces are applied between a clamped rectangular steel plate and either one or four aluminium honeycomb panels. Active control applied in this way is shown to produce large attenuations in sound transmission. The use of secondary forces to minimise the vibration of the fuselage of a propeller driven aircraft is investigated using a simplistic analytical cylinder model developed by Bullmore et al (A.J. Bullmore. PhD thesis. University of Southampton, 1987). Significant global reductions in vibration energy of the cylinder model are shown to be unobtainable with practical secondary force arrangements. This results from the large number of structural modes significant in the fuselage vibration and the limits imposed by the possibility of control `spill-over' into high order modes. The cylinder model is then used to show that the use of a cost function related to the internal sound field ensures that only a small number of secondary forces are required to produce large global reductions in the internal sound field. This is accounted for by the selective nature of the coupling between structural modes and acoustic modes in the cylinder model. The `spill-over' of vibrational energy into high order modes, however, is shown to sometimes cause large increases in the vibrational energy of the cylinder.

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