Active diffusers : some prototypes and 2D measurements

Diffusing devices are used to improve room acoustics in a wide variety of applications. The dispersion generated by current diffuser technologies is often limited to mid-to-high frequencies because low-frequency diffusers are usually too large to be easily accommodated. To extend the bandwidth of diffusers to a lower frequency a new approach is proposed, that is to use active control technology. In particular, active impedance techniques have been exploited to create non-absorbing diffusers, and hybrid structures that partly absorb while dispersing any reflected sound. This paper presents results mostly from a feedforward structure. It is found that achieving active dispersion without absorption other a worthwhile bandwidth can be more difficult than achieving active absorption due to the more complex target impedance that the controller needs to learn. Measurements on polar responses provide evidence that the active diffusers can achieve wider bandwidth dispersion. Boundary element modelling has enabled the design of these structures to be examined in more application-realistic set-ups

ACTIVE NOISE CONTROL USING CARBON NANOTUBE THERMOPHONES: CASE STUDY FOR AN AUTOMOTIVE HVAC APPLICATION

The goal of this project was to reduce the overall noise levels emitted by the HVAC components in a vehicle’s cabin. More specifically, the feasibility of achieving this goal using two key technologies was investigated. The first of these technologies, Active Noise Control (ANC), is a noise attenuation technique that relies on destructive interference that “cancels” unwanted noise. Typically used in situations where physical constraints prevent passive attenuation techniques from being used, ANC is known for its high size-to-effectiveness ratio. This benefit cannot be gained without a cost however; the complexity of ANC systems is significantly higher than their passive counterparts. This is due to the signal processing and actuator designs required. These actuators often take the form of moving-coil loudspeakers which, while effective, are often bulky. Because of this they are difficult to “drop in” to an existing system. This is where the second technology comes in. Carbon Nanotube (CNT) Thermophones are solid-state speakers that operate by using rapid heat fluctuations to create sound. Called the “thermoacoustic effect,” (TE) the theory of this operating principle dates to the turn of the 20th century. Useful demonstration of TE did not occur until 2008, however, when researchers first developed the first CNT thermophones. The hallmark characteristics of these transducers are their small size and flexible nature. Compared to traditional loudspeakers they have a much smaller form factor and are more versatile in terms of where they can be placed in a cramped system. The marriage of CNT transducers to ANC technology shows promise in improving the application space and ease of installation of ANC systems. Getting these two to cooperate, however, is not without challenges. A case study for this union is presented here; the application space being the ducted environment of vehicle HVAC systems

Analysis, modeling and wide-area spatiotemporal control of low-frequency sound reproduction

This research aims to develop a low-frequency response control methodology capable of delivering a consistent spectral and temporal response over a wide listening area. Low-frequency room acoustics are naturally plagued by room-modes, a result of standing waves at frequencies with wavelengths that are integer multiples of one or more room dimension. The standing wave pattern is different for each modal frequency, causing a complicated sound field exhibiting a highly position-dependent frequency response. Enhanced systems are investigated with multiple degrees of freedom (independently-controllable sound radiating sources) to provide adequate low-frequency response control. The proposed solution, termed a chameleon subwoofer array or CSA, adopts the most advantageous aspects of existing room-mode correction methodologies while emphasizing efficiency and practicality. Multiple degrees of freedom are ideally achieved by employing what is designated a hybrid subwoofer, which provides four orthogonal degrees of freedom configured within a modest-sized enclosure. The CSA software algorithm integrates both objective and subjective measures to address listener preferences including the possibility of individual real-time control. CSAs and existing techniques are evaluated within a novel acoustical modeling system (FDTD simulation toolbox) developed to meet the requirements of this research. Extensive virtual development of CSAs has led to experimentation using a prototype hybrid subwoofer. The resulting performance is in line with the simulations, whereby variance across a wide listening area is reduced by over 50% with only four degrees of freedom. A supplemental novel correction algorithm addresses correction issues at select narrow frequency bands. These frequencies are filtered from the signal and replaced using virtual bass to maintain all aural information, a psychoacoustical effect giving the impression of low-frequency. Virtual bass is synthesized using an original hybrid approach combining two mainstream synthesis procedures while suppressing each method‟s inherent weaknesses. This algorithm is demonstrated to improve CSA output efficiency while maintaining acceptable subjective performance

Sound field control with hemi-cylindrical loudspeaker arrays

An acoustical model for the sound field generated by hemi-cylindrical loudspeaker arrays is presented and a method for beamforming with said arrays is derived. The sound field model is obtained by introducing two independent boundary conditions for the sound field of a single impinging plane wave. The model for the radiation from a single loudspeaker in the array is then obtained from the reciprocity principle. Various beam patterns are presented and the theoretically predicted sound field is evaluated as a function of frequency. The results are discussed and an experimental array prototype is presented

Short overview of parametric loudspeakers array technology and its implications in spatialization in electronic music

UID/EAT/00472/2013In late December of 1962, a Physics Professor\from Brown University, Peter J. Westervelt,\submitted a paper called Parametric Acoustic Array considered primary waves interacting within a given volume and calculated the scattered pressure field due to the non-linearities within a small portion of this common volume in the medium. Since then, many outputs of this technology were developed and applied in contexts such as military, tomography, sonar technology, artistic installations and others. Such technology allows perfect sound directionally and therefore peculiar expressive techniques in electroacoustic music, allowing a very particular music dimension of space. For such reason, it’s here treated as a idiosyncrasy worth to discuss on its on terms. In 2010-2011 I composed the piece "A Anamnese das Constantes Ocultas", commissioned by Grupo de Música Contemporânea de Lisboa, that used a parametric loudspeakers array developed by engineer Joel Paulo. The same technology was used in the 2015 acousmatic piece “Jeux de l'Espace ” for eight loudspeakers and one parametric loudspeaker array. This paper is organized as follows. A theoretical framework of the parametric loudspeaker array is first introduced, followed by a brief description of the main theoretical aspects of such loudspeakers. Secondly, there is a description of practices that use such technology and their applications. The final section describes how I have used it in my music compositions.publishersversionpublishe

Low Frequency Simulations for Ambisonics Auralization of a Car Sound System

In this paper, a technique is described for obtaining the High Order Ambisonics (HOA) Impulse Responses (IRs) of an automotive infotainment system, relying on Finite Elements Method (FEM) simulations performed in COMSOL Multiphysics. The resulting HOA IRs are employed for auralizing the car sound system, either inside an Ambisonics listening room with a loudspeaker rig or with binaural rendering on a Head Mounted Display (HMD), benefiting from head-tracking and personalized Head Related Transfer Functions (HRTFs). This allows performing subjective tests before the prototype is built and preserving the auditory experience with a degree of realism unattainable with the static binaural approach. Measurements performed in a prototype vehicle with a spherical microphone array are compared to FEM simulations. A good agreement between numerical and experimental methods have been demonstrated

Improving hearing aids through listening tests in a virtual sound environment

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Design and implementation of a personal audio system in a car cabin

The generation of personal listening zones in a car cabin would allow the different occupants to listen to different audio programmes without the use of headphones. This would allow, for example, the driver to listen to a navigation system whilst the rear passengers watched a film. Personal audio systems have previously been implemented in mobile devices and monitors, for example, however, the investigation of the effects of an enclosure on the generation of personal listening zones has been limited. This paper presents an investigation of the effects of a car cabin sized enclosure on the generation of independent listening zones in the front and rear seats. The standard car audio loudspeaker array is used to produce independent listening zones at low frequencies, while a second array of small loudspeakers positioned at the four headrest positions is used to provide control over the rest of the audio bandwidth. The proposed arrays are implemented in a real car and the results of a real-time implementation are presented.<br/

Two-Dimensional Spatial Control using Multiple Circular Loudspeaker Arrays and Elliptical Loudspeaker Array

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FDTD/K-DWM simulation of 3D room acoustics on general purpose graphics hardware using compute unified device architecture (CUDA)

The growing demand for reliable prediction of sound fields in rooms have resulted in adaptation of various approaches for physical modeling, including the Finite Difference Time Domain (FDTD) and the Digital Waveguide Mesh (DWM). Whilst considered versatile and attractive methods, they suffer from dispersion errors that increase with frequency and vary with direction of propagation, thus imposing a high frequency calculation limit. Attempts have been made to reduce such errors by considering different mesh topologies, by spatial interpolation, or by simply oversampling the grid. As the latter approach is computationally expensive, its application to three-dimensional problems has often been avoided. In this paper, we propose an implementation of the FDTD on general purpose graphics hardware, allowing for high sampling rates whilst maintaining reasonable calculation times. Dispersion errors are consequently reduced and the high frequency limit is increased. A range of graphics processors are evaluated and compared with traditional CPUs in terms of accuracy, calculation time and memory requirements