An analytical expression is derived to calculate the wave propagation velocity in a fluid-coupled membrane. Starting from the equation of motion, a dispersion relation is obtained in the form of a sixth-order polynomial. An effective, i.e., fluid-loaded, wave speed is identified, and its dependence on structural and fluid parameters is established. An experimental validation on two membranes is finally presented. The model could be useful for applications in musical or structural acoustics.

Sound zones aim to create distinct listening areas within the same physical space. Such systems have potential applications in environments where listeners may move or where the acoustic environment may change. However, most existing sound zone methods are designed for static conditions, which limits their performance in dynamic scenarios-particularly when reproducing music signals that exhibit strong temporal correlation. To address this challenge, some methods including closed-loop algorithms have been proposed to continuously adapt filters in real time, using acoustic pressure at control points. Unfortunately, these methods suffer from slow convergence speeds due to correlated input signals. In this paper, three leaky gradient descent-based adaptive filtering approaches for generating sound zones in a potentially dynamic context are proposed to solve the sound zone problem. Their results are compared to the ones obtained with an existing adaptive filtering method: the filtered-x least mean squares. The leaky filtered-x affine projection algorithm (LFx-APA) enhances convergence speed and robustness by projecting over multiple past measurements, making it well-suited for non-stationary audio signals. Simulations performed in a reverberant room demonstrate that the LFx-APA achieves higher performance in terms of contrast without increasing reproduction error when the audio content is non-stationary.

MEMS loudspeakers designed for in-ear applications have garnered increasing attention in recent years, offering the potential to surpass traditional non-MEMS speakers in terms of size, power efficiency, and cost. Among the various actuation mechanisms, the piezoelectric one is the most extensively studied, largely due to advancements in microfabrication processes that enable the integration of high-precision piezoelectric thin films with strong electromechanical coupling. This work introduces a new class of high-performance piezoelectric MEMS loudspeakers that utilize PZT-actuated thin plates, engineered with a pattern of narrow slits. These slits serve a dual purpose: they enhance the mechanical flexibility of the diaphragm and reduce the acoustic short-circuit between the front and rear of the speaker. The diaphragm design, an evolution of a previously proposed concept by the Authors, has been modified to enhance acoustic performance while minimizing piezoelectric capacitance. The developed devices deliver a Sound Pressure Level (SPL) exceeding 108 dB SPL across the full audible range at 30 V-pp in an IEC 60318-4 ear simulator, maintaining a Total Harmonic Distortion (THD) below 1% at 1 kHz and 94 dB SPL, and a compact 4.5x4 .5 mm(2) footprint. Experimental results show good agreement with predictions from a full-order Finite Element Model (FEM) and a FEM-assisted lumped element model. [2025-0112]

Personal sound zones (PSZ) allow several people sharing the same room to listen to different audio content without using headphones.Loudspeaker arrays with filtered inputs are typically used to create the PSZ.The driving filters are calculated from the transfer functions between each loudspeaker and the points sampling each listening zone.These filters are usually computed once and are no longer relevant if the loudspeakers are driven at higher levels, especially for low-cost drivers which can exhibit a strong non-linear behavior.In this paper, a state space model is used to predict the nonlinear behavior of loudspeakers.Nonlinear parameters (force factor, stiffness and inductance) as a function of the diaphragm displacement are measured for each 2" Tectonic loudspeaker using a Klippel R&D system.Then, the accuracy of the state space model is checked by comparing the predicted harmonic distortion to the measured one for several input levels.Finally, the state-space representation is used to predict the effect of non-linearities on PSZ metrics.Simulation results show that the contrast can be reduced by about 10 dB and the error can increase by a factor of about 1000 in the low frequency range when large input signals are required.

This work is grounded on the force analysis technique, an identification method that directly uses a structure's equation of motion to formulate an inverse problem, explicitly identifying the force causing the system's motion. This technique has mainly been applied on heavy objects since contact vibration measurements using accelerometers on light and thin structures are biased by the added mass and are complex to setup. Using non-contact and full-field vibration measurements (here, deflectometry), opens the possibility of applying the force analysis technique to such structures. This research aims to investigate the opportunities offered by the identification of space-time varying loadings of various types (acoustical, mechanical, and turbulent boundary layer) all these loadings being applied on a membrane. Through this study, we also strive to highlight the challenges and opportunities brought by of coupling non-contact and full-field measurements on membranes to the force analysis technique.

This work is grounded on the force analysis technique, an identification method that directly uses a structure's equation of motion to formulate an inverse problem, explicitly identifying the force causing the structure's motion. Prior research employing this technique has been predominantly conducted in the frequency domain and was limited to stationary, mechanical excitations using physical sensor arrays such as accelerometers. The objective of this research was mostly quantitative (amplitude, location), while the proposed approach is rather qualitative identification. Indeed and by combining the force analysis technique with full-field and non-contact vibration measurements conducted on a system, here a membrane, this communication describes a proof-of-concept for the identification of a time-space-varying sound pressure loading. A compact and tonal sound source is used to draw freehand shapes against the membrane surface, and the objective is to follow/reconstruct the trajectory followed by this source. Results are provided for different drawn shapes or letters, and the effect of mechanical or calculation parameters on the reconstructed information is studied. Finally, potential research directions are discussed and fed by preliminary measurements on a percussion instrument.

At very low frequency, the loudspeakers face technical issues (size, weight, energy consumption). An alternative to the electrodynamic loudspeaker is the Harmonic Acoustic Pneumatic Source (HAPS) demonstrated efficient in active tonal control. It comprises a high-pressure pneumatic air source, a rotating flow chopper, and an exhaust. The rotation of the flow chopper generates a pulsed flow which radiated noise out from the exhaust. An analytical model of the HAPS is presented to introduce the main challenge associated with its use at very low frequencies: having a high mean flow rate with a relatively small exhaust duct section. To overcome these challenge, a dedicated flexible tube (under light mean pressure) is added to the exhaust circuit to obtain a pulsating sphere activated by a HAPS. This configuration has been experimentally studied in a semi-anechoic room with two flexible tubes and without. The sound pressure level of the first harmonic at 1 meter range from 75 to 95 dB SPL (25 to 160 Hz, plenum pressure from 5 to 20 PSI). Thanks to fluid-structure interaction, the fundamental harmonic sound levels radiated by the compliant tube were free of jet noise. The drawback is the high harmonic distortion observed during the experiments.

Identifying acoustical and mechanical loadings on structures is a common problem in acoustics and vibration analysis and stationary loadings are mostly considered on plate-like structures. This work describes a proof of concept for reconstructing the trajectory of an acoustic source moving in front of a membrane. Compared with works focusing on precisely identifying a loading’s amplitude at a given location, the objective is to reconstruct the loading’s trajectory–Qualitative loading identification is sought rather than quantitative. The force analysis technique is used to recover a space-time varying loading on a structure, starting from time-resolved full-field non-contact vibration measurements conducted on a circular membrane. At the same time, a compact and tonal sound source is used to draw freehand shapes in front of the membrane. The loading trajectory, therefore, contains information that was “acoustically written”. Simple hand gestures that correspond to the drawing of a Greek letter (Σ), a capital letter (P), two shapes (♡, ⋆), and a 3-letter word (net) are recovered using the proposed procedure. The effect of various parameters on the reconstructed information is studied. Perspectives in terms of possible research areas and applications are finally discussed. These perspectives include, for example, the use of membranes to help reconstruct complex and space-time-varying loadings or even applications in musical acoustics on membranophones.

The measurement of the pressure field radiated by a sound source has many applications in the fields of noise control and loudspeaker system design. In this paper, the radiation mode method is used to measure the field radiated by a complex acoustic source whose surface impedance is arbitrary and does not correspond to the Neumann boundary condition used for the calculation of radiation modes. The most effective radiation modes are used as test functions to calculate a pressure expansion around the source under test, an expansion that matches the measured pressure at a limited number of points close to the source. This expansion is then used to calculate the radiated pressure at a greater distance at unmeasured locations. In a first step, numerical simulations are performed to evaluate the method's most influential parameters. Then, measurements are performed in a semi-anechoic room on two real sources of increasing complexity. Obtained results show that the radiation mode method allows an accurate evaluation of the pressure field radiated by the test object over a fairly wide frequency band (between 100 Hz and 2 kHz) even for complex sources.

Impulse responses (IRs) estimation of multi-input acoustic systems is a prerequisite for many audio applications. In this paper, an adaptive identification problem based on the Autostep algorithm is extended to the simultaneous estimation of room IRs for multiple input single output linear time invariant systems without any a priori information. To do so, the proposed algorithm is initially evaluated in a simulated room with several sound sources active at the same time. Finally, an experimental validation is proposed for the cases of a semi-anechoic chamber and an arbitrary room. Special attention is dedicated to the algorithm convergence behavior, considering different meta parameters settings. Results are eventually compared with the other normalized version of the least mean square algorithm.

Piezoelectric micro-electro-mechanical-system (MEMS) speakers are emerging as promising implementations of loudspeakers at the microscale, as they are able to meet the ever-increasing requirements for modern audio devices to become smaller, lighter, and integrable into digital systems. In this work, we propose a finite element model (FEM)-assisted lumped-parameters equivalent circuit for a fast and accurate modeling of these types of devices. The electro-mechanical parameters are derived from a pre-stressed FEM eigenfrequency analysis, to account for arbitrarily complex geometries and for the shift of the speaker resonance frequency due to an initial non-null pre-deflected configuration. The parameters of the acoustical circuit are instead computed through analytical formulas. The acoustic short-circuit between the speaker front and rear sides is taken into account through a proper air-gaps modeling. The very good matching in terms of radiated sound pressure level among the equivalent circuit predictions, FEM simulations, and experimental data proves the ability of the proposed method to accurately simulate the speaker performance. Moreover, due to its generality, it represents a versatile tool for designing piezoelectric MEMS speakers.

Digital image correlation, deflectometry and digital holography are some of the full-field optical measurement techniques that have matured in recent years. Their use in vibroacoustic applications is gaining attention and there is a need for cataloging their performance in order to provide, to a broad community of users and potential future users, quantitative and qualitative evaluations of these three approaches. This paper presents an experimental comparison of the three optical methods in the context of vibration measurements, along with classical reference measurements provided by an accelerometer and a laser Doppler vibrometer. The study is carried out on two mechanical structures exhibiting various vibration responses when submitted to an impact.

The steering and the focusing of acoustic beams are of increasing interest for personalized-sound-zone applications. Sound zones require the generation of a strong contrast in sound pressure level between different regions of the space. A way to do so is to control and steer the acoustic field radiated from a source with use of metamaterial-based acoustic lenses. Predictive models based on numerical approaches such as the finite-element method are commonly used, but they are generally cumbersome and time-consuming. In contrast, explicit models rely on Snell's law of refraction and phase gratings but do not account for the couplings between the metamaterial atoms. In this work, a metamaterial-based acoustic lens is proposed as a solution to steer and focus acoustic sources, with use of slits loaded with periodic Helmholtz resonators. We control the radiation of a baffled duct by a metalens located at the output of the waveguide and excited at the other end by a loudspeaker. A fully explicit two-dimensional predictive model is developed on the basis of the mode-matching technique, accounting for thermoviscous losses, interslit evanescent coupling, and high-order modes in the baffled duct. The pressure radiated by the metalens outside the baffled duct is then explicitly derived. The semianalytical results show excellent agreement when compared with the solutions obtained by the finite-element method, both in the near field and in the far field over a wide frequency range. An optimization strategy is then proposed, and the beam steering and focusing are reported experimentally on a three-dimensionally-printed prototype.

A method for measuring the diffuse field sound absorption coefficient of a material using sound field synthesis is proposed. A planar loudspeaker array is first used to generate acoustic plane waves with variable incidence angle on the surface of a material under test. Using a two-microphone probe positioned closely to the sample’s surface, the angle-dependent sound absorption coefficients are then estimated. Finally, the diffuse field absorption coefficient is computed following Paris formula. Numerical simulations are used to evaluate the respective effects of the maximum incidence angle value and the number of individual incidence angles that are required for a robust calculation of the diffuse sound field absorption. Measurements are conducted on three different materials and compared with either simulation results obtained using the Johnson-Champoux-Allard theory, or with measurement results obtained using the standard reverberation chamber method. For all considered materials and over a wide frequency range, the proposed method leads to results that are in better agreement with theoretical predictions than those obtained using standardized methods.

Standardized methods for measuring sound absorption such as the impedance tube and reverberation chamber methods are limited to normal or diffuse incidence, respectively. Two research axes have been generally followed in the literature to develop alternative techniques, the first one focusing on the measurement part, that is from the two-microphone technique to the use of microphone arrays or pressure-velocity sensors. The second axis focuses on the excitation part with for instance the use of sound field synthesis techniques. Since acoustic impedance and sound absorption coefficient of materials are classically defined under normal and oblique plane wave excitation, synthesizing an “ideal” plane wave using a loudspeaker array would allow measuring these acoustics quantities using a simple microphone pair. In this article, the effect of the different parameters of a loudspeaker array on acoustic plane waves reproduction on a material’s surface is first numerically studied. Then, numerical and experimental results for the estimation of both impedance and absorption coefficients are reported. These results show that sound field synthesis allows to characterize a material for arbitrary incidence angles over a wide frequency range, thus offering an alternative method to standard techniques and an improvement over existing works.