Study on room modal equalization at low frequencies with electroacoustic absorbers

Nowadays, requirements for acoustic treatments in rooms are ever more demanding in terms of performance and compactness. A recurring issue in closed spaces is the occurrence of standing waves and acoustic interferences at low frequencies. These unwanted phenomena are likely to affect the frequency response of rooms such as offices, concert halls or home theaters. Unfortunately, state-of-the-art soundproofing solutions cannot efficiently dissipate sound energy, or their embodiments are so bulky that they become almost impracticable. To that purpose, electroacoustic absorbers employing direct-radiator loudspeaker systems can be used to improve listening quality and meet the specifications. Electrodynamic transducers are obvious candidate for this type of noise control application. They are controlled by the resistance around the mechanical resonance, typically of the order of a few tens of Hertz, which also lies in the frequency range of acoustic modes to be damped. This results in a strong interaction between both dynamic systems from which part of the incident acoustic energy of the sound field can be dissipated. A specific electrical load is also connected across the transducer terminals in order to further improve the passive dissipation through the internal damping within the loudspeaker. This paper investigates an optimized placement of electroacoustic absorbers in a reverberant room in order to damp the lowest acoustic resonances.

On the damping of room resonances with electroacoustic absorbers in the low frequency range

Romain Boulandet, Hervé Lissek, Iris Rigas, Etienne Thierry Jean-Luc Rivet

In closed spaces, standing waves occur at low frequencies, thus creating annoying acoustic resonances. Such unwanted phenomena are likely to affect the annoyance of noise sources in the low frequency range. Up to date, low-frwquency treatments are mainly based on bulky devices, such as heavy enclosures. To alleviate this problem, electroacoustic absorbers, namely loudspeakers with shunt synthetic electric loads, can be used to damp room modes and then meet noise reduction specifications. Electrodynamic loudspeakers are good candidates for this type of noise control applications. Their mechanical resonance, typically of the order of tens of Hertz, is within the frequency range where acoustic modes to be controlled. The interactions between these dynamic systems are significant, and a consequent amount of the acoustic energy in the room can be passively dissipated through internal losses in the loudspeaker. This paper investigates the design techniques and experimental validation of electroacoustic absorbers with a view to damp the low-frequency acoustic resonances, in the context of the low-frequency noise reduction in inhabitations.

2012

Optimization of electroacoustic resonators for semi-active room equalization in the low-frequency range

Hervé Lissek, Etienne Thierry Jean-Luc Rivet

At low frequencies, standing waves within the room cause large frequency-response variations in the listening environment, such as audio rooms or recording studios. This unwanted phenomenon has a significant impact on the sound quality of an audio system. Unfortunately, state-of-the-art soundprooﬁng solutions cannot eﬃciently handle such low-frequency sound energy. To alleviate this problem, electroacoustic resonators can be used to damp room modes. This concept is based on the connection of direct-radiator loudspeakers to synthetic electrical loads allowing the passive dissipation of a certain part of the incoming acoustic energy of the sound field. Through judicious control of acoustic impedance depending of the placement of the electroacoustic resonators in the room, a significant damping of the dominant natural resonances can be achieved in order to meet the specifications of audio reproduction. This paper investigates the optimization and the spatial arrangement of electroacoustic resonators with a view to damp the low-frequency acoustic resonances in enclosed spaces.

2013

Cartographie et spectroscopie des propriétés mécaniques à l'échelle du nanomètre par spectrométrie acoustique locale à fréquence variable

This work aims at completing the development of local mechanical spectroscopy as initiated by F. Oulevey during his thesis. More precisely, the measurement of the amplitude and phase lag of the strain as a function of the applied stress frequency bas to be settled. Parallel to the development of the technical aspects making such an acquisition possible, the model used to analyze the measured quantities is modified in order to describe the studied system more realistically. The local mechanical spectrometer is based on an Atomic Force Microscope (AFM) using an optical method to detect the deflection of the lever. The stress is applied locally on the sample by a transducer fixed at the base of the cantilever. The bandwidth of the microscope' s electronics does not allow one to detect the cantilever movement at frequencies higher than the MHz. Two different approaches are followed to overcome this limit. The first approach exploits the nonlinearity of the contact to down-convert, via mechanical mixing, the high-frequency signals into low-frequency ones. This work demonstrates the feasibility of this technique, as well as its limits, mainly due to the difficulty to analyze confidently the obtained results. The second approach takes advantage of the stroboscopic detection of the movement of the cantilever. The intensity of the laser used to detect the deflection of the cantilever is modulated at a frequency close to the applied stress frequency. The amplitude of the signal recorded at the difference frequency corresponds then to the high-frequency amplitude of the cantilever movement. The frequency range attainable with this method is only limited by the bandwidth of the transducer providing the excitation. The model connecting the measured quantities (strain amplitude and phase lag) to the desired mechanical properties (elasticity and damping) consists in a beam clamped at one end, with a tip attached at some point along its length. The tip itself is connected to the sample via a spring and a dashpot in parallel, representing the normal interaction, and a spring and a dashpot in parallel representing the interaction in the plane of the sample. Combined with the stroboscopic detection, this model enables one to quantitatively determine the elastic modulus of stiff samples. Many results obtained on a wide range of samples exhibiting very diverse mechanical properties are presented. They demonstrate the validity of the stroboscopic approach, which combines simplicity of use and efficiency.

EPFL2000

Cartographie et spectroscopie des propriétés mécaniques à l'échelle du nanomètre par spectrométrie acoustique locale à fréquence variable

EPFL2000