Comparison of low-frequency background noise cancellation systems for room acoustics

Low frequency background noise (below 200 Hz) is perceptible in many situations such as in room acoustics or industrials issues. Generate by numerous type of source (traffic noise, aircraft, industrial plants, electricity transformer,… ), this type of noise is under evaluate by regulations laws due to the used of the A-weighted decibel (dB(A)). In spite of that low frequency background noise can reach high levels which produce poor speech intelligibility and annoyance feeling. Unfortunately traditional acoustic treatments have little impact to decrease this kind of disturbance. New approaches have to be developed to give new efficient techniques in order to overcome the problem. Three noise cancellation systems dedicated to low frequencies are studied here. The first one is based on an electromechanical transducers loaded passively to get an optimal damping around the resonance frequency of the disposal. In the second system the passive load is substituted by an active control to enhance acoustic properties of device. The last strategy consists on a control of the first modal frequencies of a room to decrease the low frequency background noise level. This paper aims at presenting those three noise cancelation systems. Their advantages and drawbacks will also be discussed.

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

Practical active and semi-active strategies for the control of room acoustics in the low frequency range

Romain Boulandet, Hervé Lissek, Anne-Sophie Moreau

Modal acoustics is one of the main sources of annoyance in the low frequency range in rooms. Active noise control techniques can be deployed to reach sufficient acoustic benefit, but limited to a single frequency, whereas broader efficiency would require complex systems with a large number of transducers. After a short presentation of the state-of-the-art active modal control techniques, this paper will introduce the semi-active concept of “electroacoustic absorbers”, which are able to handle low-frequency room acoustics problems along significant frequency bandwidth, in the experimental context of the damping of the first modal frequencies of a test chamber. This study will focus on highlighting advantages and drawbacks of such techniques when compared to active strategies (performances, complexity,…), concluding with practical consideration (integration, cost,…).

2010

Tunable Electroacoustic Resonators through Active Impedance Control of Loudspeakers

Romain Boulandet

The current trend for multipurpose rooms requires enhanced acoustic treatments capable to meet ever more demanding specifications in terms of performance, compactness and versatility. The reason is the variety of activities to be hosted and the corresponding requirements in terms of acoustic quality which may be very different and even conflicting. In any process to improve listening comfort, the treatment of low-frequency sound is a major concern. The problem stems from the proven ineffectiveness of passive soundproofing solutions of the state of the art, or from their bulkiness that may be prohibitive. This thesis focuses on the analysis, design, realization and characterization of tunable electroacoustic resonators intended to specifically address this issue. This concept deals with loudspeakers, the acoustic impedance of which can be easily adjusted in a controlled fashion. Creating an electroacoustic resonator out of a loudspeaker is the result of an interdisciplinary effort. Such a challenging task combines conceptual tools, models, and applied solutions, drawing from the fields of audio engineering, control theory, and electrical engineering, both in the analog and digital domains. A unifying theory is introduced, covering different strategies from passive electrical shunt to active control of acoustic impedance in a single formalism. This research shows that achieving a desired acoustic impedance at the transducer diaphragm is equivalent to the implementation of a specific functional relationship between the electrical current and voltage across the transducer terminals, and vice versa. From a design perspective, the specific electrical load is tailored by using an internal model of the transducer. The result is an innovative model-based synthesis methodology where the active control of acoustic impedance is reformulated as an electrical impedance synthesis, thus removing the use of sensor. This concept opens new opportunities to improve listening spaces by providing efficient acoustic absorption at low frequencies. Experiments clearly show the benefits of the proposed methodology in a field where there is currently no competitive solution. It is believed that the technological advances resulting from the coupling of a loudspeaker with a synthetic load should pave the way to innovative techniques in noise control and, hopefully, stimulate research in related areas.

EPFL2012

Tunable Electroacoustic Resonators through Active Impedance Control of Loudspeakers

Romain Boulandet

EPFL2012