Active low-frequency modal noise cancellation for room acoustics: an experimental study

Low frequency background noise in rooms generated by numbers of human activities - traf-fic, railway, airport and industrial noise - creates major disturbances from loss of speech in-telligibility to stress and fatigue. To overcome such situations classical solutions generally consist in deploying resonator-like absorbing materials to reduce both the acoustic level and the reverberation time. Unfortunately these traditional acoustic treatments are not efficient enough to attain A-weighted level specifications imposed by regulation laws. In the context of cancellation of low frequencies in rooms, an experiment was performed to qualify and quantify performances of a low-frequency noise cancellation active system. So as to minim-ize and increase performance in noise reduction, active solutions are actually studied to attain and fulfill high expectations in terms of integration with high efficiency in opposite to pas-sive solutions. We propose an experiment based on active sources to counteract modal low frequency noise for quasi-stationary to permanent noise emission. We present some results using a step-by-step approach to understand the global behavior of the room excited in the frequency band of its first modes. Through coherent assumptions and observations we show the efficiency of low-power active system with a priori constraints affecting modal active control.

Low frequency sound field reconstruction in a non-rectangular room using a small number of microphones

Hervé Lissek, Vu Thach Pham

An accurate knowledge of the sound field distribution inside a room is required to identify and optimally locate corrective measures for room acoustics. However, the spatial recovery of the sound field would result in an impractically high number of microphones in the room. Fortunately, at low frequencies, the possibility to rely on a sparse description of sound fields can help reduce the total number of measurement points without affecting the accuracy of the reconstruction. In this paper, the use of Greedy algorithm and Global curve-fitting techniques are proposed, in order to first recover the modal parameters of the room, and then to reconstruct the entire enclosed sound field at low frequencies, using a reasonably low set of measurements. First, numerical investigations are conducted on a non-rectangular room configuration, with different acoustic properties, in order to analyze various aspects of the reconstruction frameworks such as accuracy and robustness. The model is then validated with an experimental study in an actual reverberation chamber. The study yields promising results in which the enclosed sound field can be faithfully reconstructed using a practically feasible number of microphones, even in complex-shaped and damped rooms.

2020

Contributions aux études sur le couplage électroacoustique dans les espaces clos en vue du contrôle actif

Pierre-Jean René

The aim of this work was to analyze the global behavior of a loudspeaker exciting a room in the frequency band of its first modes, for the purpose of active control applications. First, the loudspeaker in free field was studied. The characterization of a loudspeaker on the base of equivalent circuits has been established, showing the preponderant importance of the load impedance, determined by near field measurements. Then, the steady state has been modeled and the study has been set on the characterization of the wall impedance, which determines the eigenfrequencies and the amplitude of their modes, as well as charge impedance. The latter which modifies the volume velocity compared with the one in free field, has been characterized. The previous results enable the precise computation of the pressure field in a closed space and excited by a loudspeaker in steady state, according to the wall impedance. For active control purposes, transitory sounds are particularly important, therefore the behavior of a sound field in a room excited by several parameterized sounds has been studied in the time domain. The room response to the source activation could therefore be modeled. This response consists of a free and a transitory state, both of which are qualified through coherent assumptions and observations. Through simulation and systematic experimentation, the loudspeaker-room system has been characterized for a stationary as well as for a transitory sound, revealing the constraints affecting modal active control. Many experiments showed the efficiency of modal active noise control. Observations presented the importance of low frequencies in the annoyance of airplane noise inside rooms. Directives have been given to install active noise reduction system. The main part of this work constitutes an electro acoustical knowledge basis to perform modal active noise control.

EPFL2006

Shunt loudspeakers for modal control in rooms

Romain Boulandet, Hervé Lissek, Pierre-Jean René

Engineers dealing with noise reduction in habitations close to transportation traffic or indus-trial facilities encounter several problems to decrease noise level in rooms at low frequencies. Passive materials and current building construction knowledge enable to avoid noise trans-mission in habitations at medium and high frequencies and the regulations based on the dBA scale can often be respected. But these regulations do not take into account the real an-noyance of noise for the inhabitants who are still disturbed by low frequency noise. Because of the modal behavior of rooms, air-borne and structure-borne noise generate high sound pressure level at the first modal frequencies, even with small amount of energy. In order to lower the annoyance induced by low-frequency behavior in rooms, huge compliant wooden panels can be used, though with quite low performances in terms of resonances damping. An alternative can be found with bass-traps, comprising a mechanical resonator and a closed-box, with fixed acoustic performance at specific frequency. Enhanced performances can even be obtained with closed-box shunt loudspeakers, where the acoustic impedance of the louds-peaker's diaphragm around its resonance can be modified (in terms of magnitude and also in frequency) by way of a simple passive electrical device, providing the requested complemen-tary complex acoustic impedance for perfect absorption at the first modal frequencies. The present work describes the design of a small array of low-frequency shunt loudspeakers, ded-icated to damp a certain amount of modes within a known room. Assessment in laboratory conditions (impedance tube, reverberant chamber) will then be presented and compared to simulations, leading to concluding remarks on practical issues on the applicability of the con-cept of shunt loudspeakers for low-frequency noise reduction in habitations.

2009

Contributions aux études sur le couplage électroacoustique dans les espaces clos en vue du contrôle actif

Pierre-Jean René

EPFL2006