Acoustic impedance | EPFL Graph Search

Acoustic impedance and specific acoustic impedance are measures of the opposition that a system presents to the acoustic flow resulting from an acoustic pressure applied to the system. The SI unit of acoustic impedance is the pascal-second per cubic metre (), or in the MKS system the rayl per square metre (), while that of specific acoustic impedance is the pascal-second per metre (), or in the MKS system the rayl. There is a close analogy with electrical impedance, which measures the opposition that a system presents to the electric current resulting from a voltage applied to the system. Mathematical definitions Acoustic impedance For a linear time-invariant system, the relationship between the acoustic pressure applied to the system and the resulting acoustic volume flow rate through a surface perpendicular to the direction of that pressure at its point of application is given by: : p(t) = R * Q, or equivalently by : Q(t) = G * p,

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

Distributed analyses using distinct classes of Brillouin scatterings in optical fibres

Desmond Chow Ming Chia

Brillouin scattering is an interaction between light and sound in a material. This inelastic light scattering had been observed in the first optical fibres that were used in cross-continent communication. Via Brillouin scattering, the guided light wave in an optical fibre is back-scattered by the thermally generated high frequency acoustic waves and undergo Doppler frequency shift. Owing to the sensitivity of acoustic velocity to the change of temperature and strain, Brillouin scattering has been widely used in distributed fibre sensors and its resolution, distance range and speed of measurements have been tremendously improved. However, all reported distributed Brillouin fibre sensors incorporate the axial propagating acoustic wave and thus limited to detecting physical parameters inside the fibre core. In this thesis, distinct classes of Brillouin scatterings in optical fibres are explored and analysed in distributed fashion. Apart from the axial propagating acoustic wave, optical fibre supports a huge variety of elastic vibrations. In the first part, surface and hybrid Brillouin scatterings are studied by using the tapered optical fibre. As the diameter of optical fibre approaches subwavelength range (diameter ~1um), the material properties contrast at the boundary of microfibre (the central part of the tapered optical fibre) gives rise to Rayleigh wave which propagates on the surface. This type of Brillouin scattering as well as the other acoustic resonances along the transition region of the tapered fibre are measured using a distributed Brillouin analysis technique based on phase correlation. In the second part, forward stimulated Brillouin scattering (FSBS) in standard single-mode fibres (SMF) is explored as a new type of sensor that allows materials surrounding the optical fibre to affect the phase of the guided light. FSBS is an interaction between the guided light and the transverse acoustic waves which are stimulated through electrostriction and resonate within the fibre cross-section due to the acoustic reflection at its boundary. The variation of the acoustic impedance mismatch between the fibre bulk and the external material affects the boundary reflectivity and modify the acoustic decay rate. An experiment has been performed to demonstrate the feasibility to measure acoustic impedance of several liquids through FSBS. As the scattered light of FSBS is in forward direction, obtaining its distributed profile along a fibre is a challenge due to the absence of time-of-flight information. In order to overcome this problem, the local response of FSBS is recovered through measuring the sidebands progression of a light pulse using the backward Brillouin scattering.

EPFL2018

PID-like active impedance control for electroacoustic resonators to design tunable single-degree-of-freedom sound absorbers

Xinxin Guo, Hervé Lissek, Maxime Volery

Sound absorption at low frequencies still remains a challenge in both scientific research and engineering practice. Natural porous materials are ineffective in this frequency range, as well as acoustic resonators which present too narrow bandwidth of absorption, thus requiring alternative solutions based on active absorption techniques. In the present work, we propose an active control framework applied on a closed-box loudspeaker to enable the adjustment of the acoustic impedance at the loudspeaker diaphragm. More specifically, based on the proportionality between the pressure inside the enclosure and the axial displacement of the loudspeaker diaphragm at low frequencies, we demonstrate both analytically and experimentally that a PID-like feedback control approach allows tuning independently the compliance, the resistance and the moving mass of the closed-box loudspeaker to implement a prescribed impedance of a single-degree-of-freedom resonator. By considering different control combinations to tailor the resonator characteristics, a perfect absorption (with absorption coefficient equal to 1) is achievable at the target resonance frequency, while enlarging the effective absorption bandwidth. Moreover, the proposed feedback control strategy shows an excellent control accuracy, especially compared to the feedforward-based control formerly reported in the literature. The mismatches between the performance of experimental prototype and the model, likely to result from the control time delay and the inaccuracy in estimating the loudspeaker parameters, can be compensated directly by tuning the control parameters in the control platform. The active resonators implemented through the reported control scheme can be used to build more complex acoustic devices/structures to enable high-efficiency broadband sound absorption or other types of acoustic phenomena such as wavefront shaping.

2022