Sensing systems for active control of sound transmission into cavities

Abstract

Driven by the need to reduce the sound transmitted into aircraft cabins from the power plant, this thesis investigates the active control of sound transmitted through a structure into coupled enclosures. In particular, it examines alternatives to conventional microphone and accelerometer error sensors. This study establishes a design framework for the development and analysis of an active noise control system which can be applied to any complex vibro-acoustic system. The design approach has focused on using techniques presently used in industry to enable the transfer of the active noise control technology from the research stage into practical noise control systems. The structural and acoustic sub-systems are modelled using FEA to estimate the in vacuo structural modal response of the structure and the acoustic pressure modal response (with rigid boundary conditions) of the interior cavity. The acoustic and structural systems are then coupled using modal coupling theory. Within this framework, two novel error sensors aimed at overcoming observability problems suffered by traditional microphone and accelerometer sensors are investigated: namely, acoustic energy density sensors and shaped radiation modal vibration sensors.

The principles of the measurement of energy density are discussed and the errors arising from its measurement using two and three-microphone sensor configurations are considered for a one-dimensional reactive sound field and a plane wave sound field. The error analysis encompasses finite separation effects, instrumentation errors (phase and sensitivity mismatches, and physical length errors), diffraction and interference effects, and other sources of error (mean flow and turbulence, temperature and humidity, statistical effects). Following the one-dimensional study, four 3-axis energy density sensor designs are proposed and error analysis is conducted over the same acoustic fields as for the one-dimensional study. The design and construction of the simplest arrangement of the 4 three-axis sensors is discussed with reference to design issues, performance and limitations. The strategy of using energy density control is investigated numerically for a purely acoustic system and a coupled panel-cavity system. Energy density control is shown to provide greater local and global control compared to that possible using an equivalent number of microphones. The performance of the control system is shown to be relatively insensitive to the placement of the energy density sensor. For an enclosed cavity system with high modal overlap, the zone of local control achieved by minimising energy density is found to be approximately the same as the zone of local control obtained when min-imising pressure and pressure gradient in a diffuse sound field. It is also shown that if there is only one control source used per energy density sensor, global control will be almost optimum. The addition of further control sources leads to an improvement in global control, however, the control is no longer optimal. The control system is found to be very tolerant of errors in the estimate of the energy density and thus the use of simpler energy density sensor designs is justified. Finally, an experiment is presented in which the global performance achieved by controlling a three-axis energy density sensor is compared with the performance achieved by minimising the acoustic potential energy and minimising the sum of squared pressures at a finite number of microphones. The experimental results are found to reflect the numerical results.

The active minimisation of harmonic sound transmission into an arbitrarily shaped enclosure using error signals derived from structural vibration sensors is investigated numerically and experimentally. It is shown that by considering the dynamics of the coupled system, it is possible to derive a set of "e;structural radiation"e; modes which are orthogonal with respect to the global potential energy of the coupled acoustic space and which can be sensed by structural vibration sensors. Minimisation of the amplitudes of the "e;radiation modes"e; is thus guaranteed to minimise the interior acoustic potential energy. The coupled vibro-acoustic system under investigation is modelled using Finite Element Analysis which allows systems with complex geometries to be investigated rather than limiting the analysis to simple, analytically tractable systems. Issues regarding the practical implementation of sensing the orthonormal sets of structural radiation modes are discussed. Specific examples relating to the minimisation of the total acoustic potential energy within a curved rectangular panel and a coupled cavity are given, comparing the performance offered using vibration sensing of the radiation modes on the structure with the more traditional error sensing; namely, the discrete sensing of the structural kinetic energy on the structural boundary and the acoustic potential energy in the enclosed space approximated by the mean squared pressures at several locations.