Vincent Debut

MoReeSC: a framework for the simulation and analysis of sound production in reed and brass instruments

This paper presents a free and open-source numerical framework for the simulation and the analysis of the sound production in reed and brass instruments. This tool is developed using the freely distributed Python language and libraries, making it available for acoustics student, engineers and researchers involved in musical acoustics. It relies on the modal expansion of the acoustic resonator (the bore of the instrument), the dynamics of the valve (the cane reed or the lips) and of the jet, to provide a compact continuous-time formulation of the sound production mechanism, modelling the bore as a series association of Helmholtz resonators. The computation of the self-sustained oscillations is controlled by time-varying parameters, including the mouth pressure and the players embouchure, but the reed and acoustic resonator are also able to evolve during the simulation in order to allow the investigation of transient or non-stationary phenomena. Some examples are given (code is provided within the framework) to show the main features of this tool, such as the ability to handle bifurcations, like oscillation onset or change of regime, and to simulate musical effects.

Some simulations of the effect of varying excitation parameters on the transients of reed instruments

This paper considers the simulation of self-sustained oscillations in reed and brass instruments, based on a compact continuous-time formulation of the sound production mechanism. The control parameters such as the mouth pressure and the players embouchure, but also the acoustic resonator and the reed may vary with respect to time, allowing the analysis of transient and non-stationary phenomena like changes of regime. A particular attention is first given to staccato notes, with comparison of the evolution of the instantaneous frequency in simulations to theoretical and experimental results. This shows the importance of using realistic control parameters on the onset of the oscillations. When the acoustic resonator is modelled using a modal expansion with non-stationary resonance frequencies and damping, it is also possible to simulate and study slurs and musical effects like the wah-wah, gaining some insight on the mechanisms involved.

Linear modal stability analysis of bowed-strings

Linearised models are often invoked as a starting point to study complex dynamical systems. Besides their attractive mathematical simplicity, they have a central role for determining the stability properties of static or dynamical states, and can often shed light on the influence of the control parameters on the system dynamical behaviour. While the bowed string dynamics has been thoroughly studied from a number of points of view, mainly by time-domain computer simulations, this paper proposes to explore its dynamical behaviour adopting a linear framework, linearising the friction force near an equilibrium state in steady sliding conditions, and using a modal representation of the string dynamics. Starting from the simplest idealisation of the friction force given by Coulombs law with a velocity-dependent friction coefficient, the linearised modal equations of the bowed string are presented, and the dynamical changes of the system as a function of the bowing parameters are studied using linear stability analysis. From the computed complex eigenvalues and eigenvectors, several plots of the evolution of the modal frequencies, damping values, and modeshapes with the bowing parameters are produced, as well as stability charts for each system mode. By systematically exploring the influence of the parameters, this approach appears as a preliminary numerical characterisation of the bifurcations of the bowed string dynamics, with the advantage of being very simple compared to sophisticated numerical approaches which demand the regularisation of the nonlinear interaction force. To fix the idea about the potential of the proposed approach, the classic one-degree-of-freedom friction-excited oscillator is first considered, and then the case of the bowed string. Even if the actual stick-slip behaviour is rather far from the linear description adopted here, the results show that essential musical features of bowed string vibrations can be interpreted from this simple approach, at least qualitatively. Notably, the technique provides an instructive and original picture of bowed motions, in terms of groups of well-defined unstable modes, which is physically intuitive to discuss tonal changes observed in real bowed string.

Iterative Method for the Remote Identification of Impact Forces at Multiple Clearance Supports Using Few Vibratory Measurements

Among the wear-prone components in industrial facilities, multisupported tubes with clearances are particularly critical, as flow excitation may lead to vibro-impact wear between the tubes and their supports. Following our previous studies on remote impact identification using wave-propagation and modal techniques, the approach introduced in this paper consists on an iterative constrainedinversion procedure, using a modal representation of the system, to deal with simultaneous multiple identifications of impact forces, from a limited number of response measurement transducers. Preliminary identification results, based on numerical simulations, assert the satisfactory behaviour of the method to isolate the impact forces in multi-supported systems for realistic noise levels.

Exploring the bowed string dynamical behavior using a linearized model approach

For several decades bowed-strings have captured the attention of many researchers aiming for a thorough understanding of this system. Different approaches have been adopted particularly in the time-domain numerical simulations of the self-excited nonlinear regimes. Recently, the authors have been exploring the advantages of using a linearized approach to this problem, with or without the body coupling influence. Despite the highly non-linear bow/string friction force, the problem can be linearized about the average sliding velocity, as usually done in break-squeal noise, and an eigenvalue analysis can offer interesting information. For example, this approach allowed exploring the modal dynamics of bowed-string/body coupled system, studying the prediction of modes instabilities and the possible emergence of a strongly coupled mode responsible for the wolf-note, among other features. Here, using the linearized modal dynamics of bowed-strings we look in detail to the behavior of the string modes as a functio...

Identification of the Unconstrained Modes of 3D Axisymmetric Structures from Measurements under Constraining Support Conditions

An inverse method for extracting the unconstrained modal parameters of 3D axisymmetric structures from measurements performed under constrained configurations is proposed. The work was originally motivated by the need of knowing the tuning of large historical carillon bells which now lie on reinforcement fixtures, implying a quite different response of the bells from their original suspended state. In this paper, we extend to three-dimensional bodies the identification technique recently developed by the authors for a case of study consisting on a simple discrete mass-spring circular ring. Considering a modal model of a free cylinder as a first approximation of a bell, we present the identification strategy and then illustrate the efficiency of the technique by providing identification results for the original system modal frequencies from the constrained system. By operating directly on the constrained transfer functions measured at the constraint locations, the technique is not prone to modal identification nor to truncation errors, and therefore appears particularly convenient for modal testing on real structures.

Flow Conditions at the Inlet of Aspirating Pipes: Part 2—Experiments

Following the theoretical work and experimental strategy devised by Axisa [1] in the companion paper, a test rig was designed and built in order to validate the analytical analysis of Part 1. Two configurations of partly immersed articulated pipes were tested, both for normal (discharging) and for reversed (aspirating) flows. The water-loop enabled velocities up to 3 m/s in both normal and reversed flows. The experimental results presented pertain to the following pipe configurations: (a) one articulated pipe, with either a common protruding or a rounded baffled free end; and (b) two articulated pipes with equal lengths. For all flow velocities modal identifications were performed from the measured system responses. The results obtained under normal discharging flow are in good agreement with the theoretical model originally developed by Benjamin [2], which is also reviewed in Part 1. For the single articulated pipe, the Coriolis force term leads to a steady increase of damping with flow velocity, modal frequency being significantly affected only near critical damping, as expected. For pipes with two articulations, both the Coriolis and centrifugal flow terms are significant, leading to large changes in both modal frequencies and damping, which agree with the predictions from the classical model. The most interesting results from our experiments obviously are concerned with aspirating flows. Following the discussion of Part 1, it was found that the one-pipe configuration is nearly insensitive to aspirating flows, irrespectively of the pipe termination geometry, showing that the Coriolis force term is canceled exactly by the term arising from the change in momentum of the flow entering the pipe at the free end. The experimental results from the two-pipe configuration are sensitive to the aspirating flow velocity. Among the various inflow models explored in Part 1, the one which assumes an inflow velocity directed along the tube axis, but without the tangential component of the pipe motion, proved to capture many of the features displayed by the experimental results. Actually, as the aspirating velocity increases, both identified modal frequencies of the two-pipe system, as well as the modal damping of the first mode, closely follow the theoretical predictions from this basic inflow model. However, a discrepancy was observed, concerning the modal damping trend of the second mode, which decreases slowly but steadily in our tests as the velocity increases, while the basic inlet flow model predicts a nearly constant damping value. Nevertheless, such subtle but significant behavior of system damping can be related to small variations of the basic parameters which describe the inlet flow field.Copyright

A theoretical model to simulate two‐phase vibroacoustical frequency and damping effects in a pipe.

In order to avoid excessive flow‐induced vibrations in industrial components operating in two‐phase flow, the analysis and understanding of energy dissipation mechanisms are of prime importance. Several experimental studies since the classic work [L.N. Carlucci, “Damping and hydrodynamic mass of a cylinder in simulated two‐phase flow,” J. Mech. Des. 102, 597–602 (1980)] revealed the strong dependence of two‐phase damping on the characteristics of the two‐phase flow, particularly the void fraction and fluid used. All these investigations agreed in the complexity of the dissipation processes involved. To tackle such an intricate problem, we consider in this paper the fluid‐structure dynamics of a pipe filled with a bubbly liquid which interacts with two single‐degree of freedom piston terminations. Several formulations for the coupled problem are stated and compared, the two‐phase acoustics being based on a homogeneous mixture formulation [L. van Wijngaarden, “One‐dimensional of liquids containing small gas...

A theoretical dissipative analysis of optimized multimodal resonators coupled to room acoustics

Helmholtz resonators are often applied for the sound equalization of control rooms in recording studios, through adequate leveling of the low frequency acoustic modal room responses. The number of controlled acoustic modes depends on the central frequency and damping of resonators, as well as on the modal density of the controlled system within the resonator’s frequency range. In a recent paper, we proposed to improve the efficiency of such devices by, instead of using basic Helmholtz resonators, develop shape optimized multimodal resonators in order to cope with a larger number of intrusive room modes. In spite of the promising results thus obtained, further work is needed to demonstrate the feasibility of such an approach. The present paper is a further step in that direction by analyzing the acoustics of the fully coupled room/resonators system including dissapative effects. More specifically, using an acoustical substructure computational approach, we theoretically derive the coupled modes of rooms fi...

Optimization of baffle configurations to prevent aeroacoustic instabilities in heat exchangers: Preliminary experiments

Gas heat exchangers are prone to aeroacoustic instabilities, which often lead to severe noise levels, structural vibrations, and fatigue. Actually, this problem is solved by placing rigid baffles inside the container, which modify the acoustic modal fields and eventually inhibit the instability. For realistic industrial components using a restricted number of acoustical baffles, their optimal location is a challenging problem, as trial and error experimentation is often a costly and frustrating procedure. Recently, some strategies were proposed for the optimal location of a single baffle in a typical re‐heater from a power station boiler, based on simulated annealing as well as genetic algorithm approaches. In this paper and using the above‐mentioned optimization strategies, a more complex case of the problem—the optimal location of a given number (two or more) of baffles in a typical re‐heater, was addressed. Some preliminary experiments were performed and compared with the simulation results. From the d...