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

      Hill, Adam J.; University of Essex (University of Essex, 2012-01)
      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.
    • Chameleon subwoofer arrays in live sound

      Hill, Adam J.; Hawksford, Malcolm O. J.; University of Essex (Institute of Acoustics, 2011-06)
      Live-sound subwoofer systems should deliver low-frequency sound evenly distributed throughout the audience area while simultaneously minimizing sound pressure levels on stage. Approximate solutions generally exploit cardioid subwoofers and/or steerable subwoofer clusters, yet require venue-specific manual fine tuning limited mainly by practical positioning issues. Enhanced live-sound systems are explored using a virtual three-dimensional acoustic space to model dominant venue characteristics. Specifically the Chameleon Subwoofer Array (CSA) is incorporated, already proposed as a solution to small-room low-frequency sound reproduction by extending the available degrees of freedom to control sound distribution in the target space. The CSA is adapted and scaled to match the large-scale dimensions typical of live events with 3-D simulation used to optimize and validate performance. Adaptation of existing industry-standard equipment with only minor modification is presented as a core feature.
    • Individualized low-frequency response manipulation for multiple listeners using chameleon subwoofer arrays

      Hill, Adam J.; Hawksford, Malcolm O. J.; University of Essex (IEEE, 2011-07)
      Low-frequency acoustical responses are naturally position dependent across wide listening areas. This is predominantly due to room modes in small, closed spaces. Numerous methodologies have been proposed targeting room mode compensation to give an objectively even response across all listening locations. These techniques cannot guarantee, however, that every listener receives an equally pleasing subjective response. Chameleon subwoofer arrays (CSA) were originally developed to minimize low-frequency spatiotemporal variations by addressing frequency response errors at multiple listening locations using a subwoofer system consisting of multiple degrees of freedom. The CSA system can alternatively be utilized to control listening locations independently, allowing each listener to adjust their localized low-frequency response to their liking. This alternate CSA implementation is evaluated using a bespoke finite-difference time-domain (FDTD) algorithm for small home theater applications.
    • Low-frequency temporal accuracy of small-room sound reproduction

      Hill, Adam J.; Hawksford, Malcolm O. J.; University of Derby; University of Essex (Audio Engineering Society, 2012-10)
      Small-room sound reproduction is strongly affected by room-modes in the low-frequency band. While the spectral impact of room-modes is well understood, there is less information on how modes degrade the spatiotemporal response of a sound reproduction system. This topic is investigated using a bespoke finite-difference time-domain (FDTD) simulation toolbox to virtually test common subwoofer configurations using tone bursts to judge waveform fidelity over a wide listening area. Temporal accuracy is compared to the steady-state frequency response to determine any link between the two domains. The simulated results are compared to practical measurements for validation.
    • Practical applications of chameleon subwoofer arrays

      Hill, Adam J.; Hawksford, Malcolm O. J.; University of Essex (Audio Engineering Society, 2012-04)
      Spatiotemporal variations of the low-frequency response in a closed-space are predominantly caused by room-modes. Chameleon subwoofer arrays (CSA) were developed to minimize this variance over a listening area using multiple independently-controllable source components and calibrated with one-time measurements. Although CSAs are ideally implemented using hybrid (multiple source component) subwoofers, they can alternatively be realized using conventional subwoofers. This capability is exploited in this work where various CSA configurations are tested using commercially-available subwoofers in a small-sized listening room. Spectral and temporal evaluation is performed using tone-burst and maximum length sequence (MLS) measurements. The systems are implemented with practicality in mind, keeping the number of subwoofers and calibration measurements to a minimum while maintaining correction benefits.
    • Towards a generalized theory of low-frequency sound source localization

      Hill, Adam J.; Lewis, Simon P.; Hawksford, Malcolm O. J.; University of Derby; University of Essex (Institute of Acoustics, 2012-11)
      Low-frequency sound source localization generates considerable amount of disagreement between audio/acoustics researchers, with some arguing that below a certain frequency humans cannot localize a source with others insisting that in certain cases localization is possible, even down to the lowest audible of frequencies. Nearly all previous work in this area depends on subjective evaluations to formulate theorems for low-frequency localization. This, of course, opens the argument of data reliability, a critical factor that may go some way to explain the reported ambiguities with regard to low-frequency localization. The resulting proposal stipulates that low-frequency source localization is highly dependent on room dimensions, source/listener location and absorptive properties. In some cases, a source can be accurately localized down to the lowest audible of frequencies, while in other situations it cannot. This is relevant as the standard procedure in live sound reinforcement, cinema sound and home-theater surround sound is to have a single mono channel for the low-frequency content, based on the assumption that human’s cannot determine direction in this band. This work takes the first steps towards showing that this may not be a universally valid simplification and that certain sound reproduction systems may actually benefit from directional low-frequency content.
    • Visualization and analysis tools for low-frequency propagation in a generalized 3D acoustic space

      Hill, Adam J.; Hawksford, Malcolm O. J.; University of Essex (Audio Engineering Society, 2011-05)
      A software toolbox is described that enables three-dimensional animated visualization and analysis of low-frequency wave propagation within a generalized acoustic environment. The core computation exploits a finite-difference time-domain (FDTD) algorithm selected because of its known low-frequency accuracy. Multiple sources can be configured and analyses performed at user-selected measurement locations. Arbitrary excitation sequences enable virtual measurements embracing both time-domain and spatio-frequency-domain analyses. Examples are presented for a variety of low-frequency loudspeaker placements and room geometries to illustrate the utility of the toolbox for various acoustical design challenges.
    • Wide-area psychoacoustic correction for problematic room-modes using nonlinear bass synthesis

      Hill, Adam J.; Hawksford, Malcolm O. J.; University of Essex (Audio Engineering Society, 2011-11)
      Small room acoustics are characterized by a limited number of dominant low-frequency room-modes that result in wide spatio-pressure variations that traditional room correction systems may find elusive to correct over a broad listening area. A psychoacoustic-based methodology is proposed whereby signal components coincident only with problematic modes are filtered and substituted by virtual bass components to forge an illusion of the suppressed frequencies. Although this approach can constitute a standalone correction system, the impetus for development is for use within well-established correction methodologies. A scalable and hierarchical approach is studied using subjective evaluation to confirm uniform wide-area performance. Bass synthesis exploits parallel nonlinear and phase vocoder generators with outputs blended as a function of transient and steady-state signal content.