François Ollivier

Near-field acoustic holography using sparse regularization and compressive sampling principles

Regularization of the inverse problem is a complex issue when using near-field acoustic holography (NAH) techniques to identify the vibrating sources. This paper shows that, for convex homogeneous plates with arbitrary boundary conditions, alternative regularization schemes can be developed based on the sparsity of the normal velocity of the plate in a well-designed basis, i.e., the possibility to approximate it as a weighted sum of few elementary basis functions. In particular, these techniques can handle discontinuities of the velocity field at the boundaries, which can be problematic with standard techniques. This comes at the cost of a higher computational complexity to solve the associated optimization problem, though it remains easily tractable with out-of-the-box software. Furthermore, this sparsity framework allows us to take advantage of the concept of compressive sampling; under some conditions on the sampling process (here, the design of a random array, which can be numerically and experimentally validated), it is possible to reconstruct the sparse signals with significantly less measurements (i.e., microphones) than classically required. After introducing the different concepts, this paper presents numerical and experimental results of NAH with two plate geometries, and compares the advantages and limitations of these sparsity-based techniques over standard Tikhonov regularization.

Compressed sensing for acoustic response reconstruction: Interpolation of the early part

The goal of this paper is to interpolate Room Impulse Responses (RIRs) within a whole volume, from a few measurements. We here focus on the early reflections, that have the key property of being sparse in the time domain: this can be exploited in a framework of model-based Compressed Sensing. Starting from a set of RIRs randomly sampled in space by a 3D microphone array, we use a modified Matching Pursuit algorithm to estimate the position of a small set of virtual sources. Then, the reconstruction of the RIRs at interpolated positions is performed using a projection onto a basis of monopoles. This approach is validated both by numerical and experimental measurements using a 120-microphone 3D array.

Room Reverberation Reconstruction: Interpolation of the Early Part Using Compressed Sensing

This paper deals with the interpolation of the Room Impulse Responses (RIRs) within a whole volume, from as few measurements as possible, and without the knowledge of the geometry of the room. We focus on the early reflections of the RIRs, that have the key property of being sparse in the time domain: this can be exploited in a framework of model-based Compressed Sensing. Starting from a set of RIRs randomly sampled in the spatial domain of interest by a 3D microphone array, we propose a modified Matching Pursuit algorithm to estimate the position of a small set of virtual sources. Then, the reconstruction of the RIRs at interpolated positions is performed using a projection onto a basis of monopoles, which correspond to the estimated virtual sources. An extension of the proposed algorithm allows the interpolation of the positions of both source and receiver, using the acquisition of four different source positions. This approach is validated both by numerical examples, and by experimental measurements using a 3D array with up to 120 microphones.

Design and implementation of a multi-octave-band audio camera for realtime diagnosis

Noise pollution investigation takes advantage of two common methods of diagnosis: measurement using a Sound Level Meter and acoustical imaging. The former enables a detailed analysis of the surrounding noise spectrum whereas the latter is rather used for source localization. Both approaches complete each other, and merging them into a unique system, working in realtime, would offer new possibilities of dynamic diagnosis. This paper describes the :design of a complete system for this purpose: imaging in realtime the acoustic field at different octave bands, with a convenient device. The acoustic field is sampled in time and space using an array of MEMS microphones. This recent technology enables a compact and fully digital design of the system. However, performing realtime imaging with resource-intensive algorithm on a large amount of measured data confronts with a technical challenge. This is overcome by executing the whole process on a Graphic Processing Unit, which has recently become an attractive device for parallel computing.

Prestress effects on the eigenfrequencies of the soundboards: experimental results on a simplified string instrument.

This paper presents an experimental study of the effects of prestresses on the vibration behavior of string instruments. These prestresses are created by gluing ribs (crowning) and tensioning string (downbearing). The effects of these prestresses were previously studied numerically for a piano soundboard by Mamou-Mani et al. [J. Acoust. Soc. Am. 123, 2401-2406 (2008)] and analytically for simplified models by Mamou-Mani et al. [Acta Acust. Unit. Acust. 95, 915-926 (2009)]. In the present study, a specified test bench is designed, including a simplified soundboard (a rectangular plate), a bridge, and a single string. The plate is subjected to in-plane and transverse loads. Vibrational eigenmodes are identified using nearfield acoustical holography (NAH) measurement. The evolution of eigenfrequencies with these specific prestresses is studied. The results show the effectiveness of NAH for this purpose and a very good qualitative concordance with previous numerical and analytical calculus.

Modal proportional and derivative state active control applied to a simplified string instrument

This study proposes an application of modal active control to musical string instruments. Its aim is to control the modal parameters of the soundboard in order to modify the sound of the instrument. Using both state and derivative state modal control, a method is given, from the modeling of the active structure through to the design of the control system. Issues such as the identification of the structure’s characteristics or the stability of the control system are dealt with for this original control method. Then, this technique is applied to a model of a simplified string instrument soundboard. Time simulations are conducted to study its effect on the instrument vibration. They show that, thanks to soundboard modal active control, it is possible to modify the amplitude of the sound harmonics to change the timbre as well as the sound level of the instrument.

Acoustic sources joint localization and characterization using compressed sensing

In this work, a Compressed Sensing (CS) strategy is developed in order to jointly achieve two complementary tasks regarding sound sources: localization and identification. Here, the sources are assumed sparse in the spatial domain, and greedy techniques are used for their localization. The case of coherent sources located in a plane is studied both numerically and experimentally at different frequencies. Results show that, in this framework, CS source localization is reliable using a significantly smaller number of microphones than classical techniques (standard or high resolution beamforming techniques), while overcoming some of their pitfalls. We then use a similar technique for the identification of the source nature, i.e. its radiation pattern, and here the sparsity domain is extended to a basis of elementary radiating functions. We present simulation and experimental results using calibrated sources and measurements performed with a 3D array of 80 randomly distributed microphones. This study investig...

A HIGH RESOLUTION BATHYMETRIC SIDESCAN SONAR USING DYNAMIC FOCUSING AND BEAM STEERING

Investigating the sea-floor may be done through several different means: — The simplest and oldest one is the sidescan sonar system which is made of two lateral transducers set on a fish towed by the surveying ship. These transducers deliver lateral beams whose shape is open in the vertical across-track plane and narrow in the axial direction, thus illuminating a narrow strip of the sea-floor. The reverberation information, properly decoded in distance from the echographic time of flight and the altitude of navigation, permits to obtain good images of the sea-floor when the local relief perturbations are well visualized due to the shadowing effect.

A high resolution sidescan sonar

A new sidescan sonar system has been conceived for the exploration of the continental shelf. High resolution imaging and bathymetry information are obtained by steering and focusing the ultrasonic beam with three arrays on each side of the vehicle, the erratic motions of which are compensated.<<ETX>>

Blind source localization in a room based on wavefield separation

Narrowband localization of sources in a room is a challenging problem because of the multiple reflections off the walls. Recently, we have been developing methods to localize monochromatic sources within an array of a few tens of microphones, without any knowledge on the physical properties of the room. To that end, we use a wavefield dereverberation approach in which the diffuse part is canceled, thanks to a projection operator built from a plane wave basis. However, this basis requires that no heterogeneities are present in the space of interest between the microphones. To overcome this limitation, in the case of an heterogeneous space, we show that a new projection operator can be experimentally built from a set of measurements of the responses between the microphones and some sources. This projection operator removes the reverberation, and can be used as preprocessing to locate the sources despite the heterogeneities, for instance by using classical beamforming processing. The method is first validated with numerical simulations. Then, experiments are performed in a large room, with an array composed of 100 microphones. A source, emitting at 500 Hz, can be located close to a strong reflector with an accuracy of about 10 cm.