Romain Boulandet

A Plane and Thin Panel with Representative Simply Supported Boundary Conditions for Laboratory Vibroacoustic Tests

A technique to setup a simply supported rectangular plane panel for laboratory vibroacoustic tests is described and validated. For a given panel fixed to thin vertical supports, a dimensionless parameter is proposed to size these supports following a desired frequency precision compared to theoretical eigenfrequencies of a panel with such boundary conditions. A numerical study confirms the potential of this design parameter. Detailed instructions for assembling a panel with adequate thin vertical supports on a rigid frame are then given. Finally, three laboratory cases are described which illustrate possible experimental vibroacoustic applications using a panel assembled following previous guidelines. The design parameter viability is experimentally confirmed, and all obtained results depicted good agreement with analytical solutions and numerical predictions.

Decentralized complex envelope controller for ASAC by virtual mechanical impedances

The problem of Active Structural Acoustic Control (ASAC) is to attenuate the radiated sound power by energy injection using structural actuators to modify deflection shapes. Collocated and dual actuator-sensor pairs allow the feedback problem to be formulated as the implementation of virtual mechanical impedances. The approach is based on a two-step process: (1) the virtual impedance is derived from measurements of the primary sound and transfer functions; (2) the centralized or decentralized complex envelope controller is designed to ensure stable feedback loops. Experiments are performed on a curved composite aircraft panel comprising a window. The proposed approach leads to the implementation of active virtual impedances and unstable compensators. An increased kinetic energy of the panel is observed, thus demonstrating that the control inputs needs to provide vibrational energy to achieve sound power reduction (2.7dB).

Sensorless electroacoustic absorbers through synthesized impedance control for damping low-frequency modes in cavities

This paper presents a concept of sensorless electroacoustic absorber for damping the low-frequency modes in a cavity such as a duct or a room. Taking advantage of the reciprocity of the voice coil transducer, it is shown that a synthetic electrical admittance can be designed so that the loudspeaker diaphragm is matched to a target specific acoustic impedance. This electroacoustic device provides a relatively broadband sound absorption that can be used to dampen room modes regardless of the sound field in which the loudspeaker is located. A digital filter is used to replicate the frequency response of the synthetic load, and a voltage-controlled current source is needed so that the filter is seen as an electrical admittance. Unlike previous attempts to implement the synthetic load using an electrical network, greater flexibility and accuracy can be obtained. Experimental results confirmed the validity of this sensorless electroacoustic absorber (SEA) in a 1D sound field, showing that the dynamic range of the sound pressure level in a duct can be reduced by 15 dB from 50 Hz to 300 Hz compared to a hard surface panel. A discussion on the strengths and limitations of this concept is provided, in particular with a view to employing SEAs for modal equalization in actual listening rooms.

Active Vibration Control Using Self-Sensing Actuators: An Experimental Comparison of Piezoelectric and Electromagnetic Technologies

The paper addresses the practical implementation of active vibration control using self-sensing actuators, intending to equip smart structures. The control objective is to reduce the structural vibration of a simply-supported plate subject to time-harmonic excitation. The key challenge is to use a self-sensing actuator instead of a sensor-actuator pair to reject the primary disturbance at the control point. In this study, two types of self-sensing actuators designed from a PZT patch and an electrodynamic inertial exciter are discussed, and their overall performance is compared in terms of reduction of flexural energy and power consumption. Both technologies have proven to be efficient in achieving a timeharmonic vibration control and may be used alternately, depending on the application at hand.

Design and assessment of a distributed active acoustic liner concept for application to aircraft engine noise reduction

Acoustic liners are a widespread solution to reduce turbofan noise in aircraft nacelles, due to lightweight and relatively small dimensions for integration within nacelles. Although conventional liners might be designed so as to target multiple tonal frequencies, their passive principle prevents the adaptation to varying engine speeds and therefore lowers their performance during flight, especially in the take-off and landing phases. This paper presents a novel concept of active acoustic liner based on an engineered design of microphones and loudspeakers, aiming at absorbing noise over a broad frequency bandwidth. Integration issues have been taken into account so as to fit to the targeted application to aircraft engines, yielding thickness minimization, with a view to challenging existing passive, narrow-band, liners. The sound absorption performance of the proposed active lining concept is evaluated, through commercially available finite-element software, in a configuration mimicking an aeronautical ins...

Design of Remote Quiet Zones Using Spot-Type Sound Reducers

This paper presents a local control approach to generate remote quiet zones. To deal with situations where global control can hardly be achieved, it is proposed to use an arrangement of spot-type sound reducers as originally suggested by Olson and May. Assuming that cross-coupling between control units is weak, each can be controlled independently and a decentralised feedback controller is implemented without the need for direct monitoring of the primary source. Active noise attenuation in the remote target region is achieved using a linear quadratic optimization based on prior knowledge of the transfer path of the system. The performance of a particular configuration comprising three control units is examined by numerical simulation and experimentally evaluated for a tonal noise source in a free-field environment. An average noise reduction of about 6 dB was measured in a target region of volume 0.25 ×0.25 ×0.25 m3 for a 160 Hz tonal primary source distant more than one wavelength from the secondary sources. The performance of the control system in relation to changes in the primary field is also considered with a view to extending the concept to more realistic enclosed sound field conditions in future work.

Design of a built-in electroacoustic resonator for active noise reduction

The paper focuses on the design of a built-in electroacoustic resonator for active noise reduction purposes. This concept basically encompasses a loudspeaker connected to a synthetic electrical load that enhances the ability of the transducer to dissipate a certain part of the incoming acoustic energy. The strategy is therefore to control the dynamics of boundaries in closed sound spaces (such as room, cavity, etc.) rather than targeting a global control that requires significant input of additional acoustic energy. The main attraction of the proposed methodology is its ability to achieve broadband sound absorption without the need for any sensor. The desired dynamic response of the loudspeaker for any sound disturbance is incorporated within the synthetic electrical load admittance (current/voltage transfer function). Computational results are provided to illustrate the benefits and potential of a built-in electroacoustic resonator compared to other options. Concluding remarks and discussions on foreseen future developments are then provided.

Factoring end-user expectations into the sound design process of computer keyboards

The poor acoustic comfort of usual commercial products is often perceived as annoying. Today, more and more companies have realized that sound can be an important component of product attributes and image. They are interested in having acoustic guidelines for enhancing their products sound quality. The objective of this research is to quantify the objective and subjective noise production of computer keyboards, and hence to translate the pleasantness in an objective way. Our strategy consists in developing both benchmarks and listening tests to ensure that sound design process never looses sight of the end-users expectations. The presented work proposes an original methodology to define what keystroke sound matches the image of a pleasant keyboard. We also approach what acoustic characteristics have a significant influence on the global appreciation of keystroke sounds. Good results have been obtained showing a strong correlation between perceptive cues and metrics.