• Applications of dynamic diffuse signal processing in sound reinforcement and reproduction.

      Moore, Jonathan B.; Hill, Adam J.; University of Derby (Institute of Acoustics, 2017-11-21)
      Electroacoustic systems are subject to position-dependent frequency responses due to coherent interference between multiple sources and/or early reflections. Diffuse signal processing (DiSP) provides a mechanism for signal decorrelation to potentially alleviate this well-known issue in sound reinforcement and reproduction applications. Previous testing has indicated that DiSP provides reduced low-frequency spatial variance across wide audience areas, but in closed acoustic spaces is less effective due to coherent early reflections. In this paper, dynamic implementation of DiSP is examined, whereby the decorrelation algorithm varies over time, thus allowing for decorrelation between surface reflections and direct sounds. Potential applications of dynamic DiSP are explored in the context of sound reinforcement (subwoofers, stage monitoring) and sound reproduction (small-room low-frequency control, loudspeaker crossovers), with preliminary experimental results presented.
    • Defining true propagation patterns of underwater noise produced by stationary vessels

      Lusted-Kosolwski, Claire; Piercy, Julius J. B.; Hill, Adam J.; University of Derby; Department for Environment, Food and Rural Affairs (Acoustical Society of America, 2016-07-10)
      The study of underwater vessel noise over the past sixty years has predominantly focused upon the increase in ambient noise caused by the propulsion mechanisms of large commercial vessels. Studies have identified that the continuous rise of ambient noise levels in open waters is linked to the increase in size and strength of anthropogenic sound sources. Few studies have investigated the noise contribution of smaller vessels or ambient noise levels present in coastal and in-shore waters. This study aimed to identify the level of noise common to non-commercial harbors by studying the noise emissions of a diesel generator on board a 70m long sailing vessel. Propagation patterns revealed an unconventional shape (specific to the precise location of the noise source on board the vessel), unlike those of standard geometric spreading models, as typically assumed when predicting vessel noise emission. Harbor attributes (including water depth, ground sediment and structural material components) caused for altered level and frequency characteristics of the recorded underwater noise, and were correlated to the sound measurements made. The measurements (taken in eight harbors around Northern Europe) were statistically analyzed to identify the primary factors influencing near-field sound propagation around a stationary vessel.
    • Disagreement between STI and STIPA measurements due to high level, discrete reflections

      Hammond, Ross; Mapp, Peter; Hill, Adam J.; University of Derby; Peter Mapp Accosiates (Audio Engineering Society, 2017-05-21)
      Objective measures of intelligibility, speech transmission index (STI) and speech transmission index for public address systems (STIPA), often form the basis for sound system verification. The reported work challenges the accuracy of both measures when encountering high level, discrete reflections. Tests were carried out in an anechoic environment with artificial reflections added between 0 and 500ms. Discrepancies were found to occur above 80ms due to synchronisation between modulation frequencies and reflection arrival times. Differences between STI and STIPA of up to 0.1 were found to occur for the same delay condition. Results suggest STIPA should be avoided in acoustic environments where high level, discrete reflections occur after 80ms and STI should only be used alongside other verification methods.
    • Dynamic diffuse signal processing for low-frequency spatial variance minimization across wide audience areas.

      Moore, Jonathan B.; Hill, Adam J.; University of Derby (Audio Engineering Society, 2017-10-08)
      Diffuse signal processing (DiSP) is a method of decorrelating coherent audio signals that is applicable to various components of sound reinforcement systems. Previous tests have indicated that DiSP can successfully decorrelate multiple low-frequency sources, leading to the reduction of comb filtering effects. However, results also show that performance is variable with source material and that effectiveness is reduced in closed acoustic spaces. In this work a dynamic variant of DiSP is examined where the decorrelation algorithm varies over time. The effectiveness of the processing is analyzed and compared to static DiSP and unprocessed systems. Results show that dynamic DiSP provides superior low-frequency spatial variance reduction to static DiSP due to improved decorrelation between direct sounds and early reflections.
    • The effect of performance stages on subwoofer polar and frequency responses

      Hill, Adam J.; Paul, Joe; University of Derby (Institute of Acoustics, 2016-11-17)
      Precise control of low-frequency energy is a common requirement at large-scale live events, whereby sound energy transmitted into performance areas and outside event grounds must be limited. Industry-standard sound system design and prediction software typically omits any acoustical effect a performance stage will have on the overall system response (both in terms of polar and frequency response).This research highlights the significant effect a stage can have on subwoofer performance, where in particularly poor cases directionality is lost and the frequency response is strongly colored by resonances. This work puts forward recommendations for subwoofer system configurations that avoid unwanted stage effects as much as possible to maintain the desired (and predicted) system performance.
    • Efficient acoustic modelling of large acoustic spaces using finite difference methods.

      Durbridge, Simon E.; Hill, Adam J.; Bowers and Wilkins; University of Derby (Institute of Acoustics, 2017-11-23)
      Time domain methods for solving wave based acoustic models have been of continued interest and development since early work by key figures such as Bottledooren, as these methods can provide a simple and flexible approach for simulating a wide range of acoustic phenomena such as room modes. The nature of many time domain difference methods present significant computational resource requirements, as the size, sampling rate and inherent stability of the simulation has a distinct impact on the memory and execution time required for the simulation to give a satisfactory result. In this study the execution speed is analysed, for variations of the finite difference time domain method that may provide some increase in computation speed for large domains. It is suggested that leveraging a dynamic windowing method may reduce total computation time for some simulations, by decreasing the number of computations per time-step in the early stage of a simulation.
    • The influence of discrete arriving reflections on perceived intelligibility and speech transmission index measurements

      Hammond, Ross; Mapp, Peter; Hill, Adam J.; University of Derby; Peter Mapp Associates (Audio Engineering Society, 2016-09-20)
      The most widely used objective intelligibility measurement method, the Speech Transmission Index (STI), does not completely match the highly complex auditory perception and human hearing system. Investigations were made into the impact of discrete reflections (with varying arrival times and amplitudes) on STI scores, subjective intelligibility, and the subjective “annoyance factor.” This allows the effect of comb filtering on the modulation transfer function matrix to be displayed, as well as demonstrates how the perceptual effects of a discrete delay cause subjective “annoyance,” that is not necessarily mirrored by STI. This work provides evidence showing why STI should not be the sole verification method within public address and emergency announcement systems, where temporal properties also need thoughtful consideration.
    • The influence of discrete arriving reflections on perceived intelligibility and STI measurements

      Hammond, Ross; Mapp, Peter; Hill, Adam J.; University of Derby; Peter Mapp Associates (Audio Engineering Society, 2016-05-26)
      The most widely used objective intelligibility measurement method, the Speech Transmission Index (STI), does not completely match the highly complex auditory perception and human hearing system. Investigations were made into the impact of discrete reflections (with varying arrival times and amplitudes) on STI scores, subjective intelligibility, and the subjective annoyance factor.’ This allows the effect of comb filtering on the modulation transfer function matrix to be displayed, as well as demonstrates how the perceptual effects of a discrete delay cause subjective ‘annoyance,’ that is not necessarily mirrored by STI. This work provides evidence showing why STI should not be the sole verification method within public address and emergency announcement systems, where temporal properties also need thoughtful consideration.
    • Investigation into the relationship between standing audience density and absorption.

      Hammond, Ross; Hill, Adam J.; Mapp, Peter; University of Derby; Peter Mapp Associates (Institute of Acoustics, 2018-11)
      Predicting acoustics of occupied performance venues is problematic due to difficulties in the selection of accurate audience absorption coefficients. The absorption due to a small group of people measured in a reverberation chamber cannot be accurately transferred to larger audiences due to differences between area to edge ratio. Analysis of data from an FDTD acoustic model of a reverberation chamber with an audience modelled as columns distributed at different densities with various perimeter to area ratios, allows derived absorption coefficients to be transferable to larger audience sizes. For densely-packed audiences, diffraction results in low frequencies having linear correlation between audience size and total absorption. There is less increase in total absorption per person at higher frequencies. Comparable real-world measurements confirm these findings, allowing the verified absorption coefficients to be applied to an acoustic model of a performance venue to inspect audience effects on absorption for typical configurations.
    • Live sound loudspeaker array optimization for consistent directional coverage with diffuse radiation characteristics.

      Hill, Adam J.; Hawksford, Malcolm O. J.; University of Derby; University of Essex (Institute of Acoustics, 2018-11)
      A central aim of sound reinforcement systems is to deliver consistent tonality across a wide audience area. Loudspeaker arrays are commonly used to meet this goal, where the upper and lower frequency bounds that can be spatially controlled are dictated by inter-element spacing and array width, respectively. This work focuses on the calculation of frequency-dependent complex coefficients for each array element using a modified Fourier technique to achieve a frequency-independent radiation pattern across an array’s functional region. In order to ensure efficiency, temporally diffuse impulses are utilized within the optimization procedure to avoid clustering of radiated energy at the center of an array and to provide a diffuse radiated field while maintaining the desired directional characteristics. Example applications for subwoofer arrays are presented, although the technique is applicable to any frequency range across the audible spectrum.
    • Live sound subwoofer system performance quantification.

      Hill, Adam J.; University of Derby (Audio Engineering Society, 2018-05-14)
      The general aim of live sound reinforcement is to deliver an appropriate and consistent listening experience across an audience. Achieving this in the subwoofer range (typically between 20 – 100 Hz) has been the focus of previous work, where techniques have been developed to allow for consistent sound energy distribution over a wide area. While this provides system designers with a powerful set of tools, it brings with it many potential metrics to quantify performance. This research identifies key indicators of subwoofer system performance and proposes a single weighted metric to quantify overall performance. Both centrally-distributed and left/right configurations are analyzed using the new metric to highlight functionality.
    • Modelling the performance of speaker arrays in domestic listening environments.

      Dring, Mark; Middlicott, Charlie; Wiggins, Bruce; Vilkaitis, Alex; University of Derby (Institute of Acoustics, 2017-11-23)
      The evaluation of multi-speaker arrays is often completed in ideal anechoic conditions to better enable the observation of speaker interaction at a known listening position. Real-room testing of speaker arrays is often time-consuming and has practical limitations, which can have an impact on analysis and also the repeatability of testing. This research will investigate the performance of multiple simulated speaker arrays such as 5.1, WFS and Ambisonics in modelled domestic environments. This will be implemented using acoustic modelling software and measured speaker directivity which will allow for subjective analysis using binaural auralisations; where modelled living room impulse responses will be generated for each speaker type and position to give several ‘virtual loudspeaker arrays’. Objective evaluation of each system's spatial reproduction and localisation ability will also be considered using techniques such as velocity and energy vector analysis as examples.
    • Optimization of temporally diffuse impulses for decorrelation of multiple discrete loudspeakers

      Moore, Jonathan B.; Hill, Adam J.; University of Derby (Audio Engineering Society, 2017-05-23)
      Temporally diffuse impulses (TDIs) were originally developed for large arrays of distributed mode loudspeakers to achieve even radiation patterns. This initial investigation evaluates the performance of TDIs in terms of the reduction of low frequency spatial variance across an audience area when used with conventional loudspeakers. A novel variable decay windowing method is presented, allowing users control of TDI performance and perceptibility. System performance is modelled using an anechoic and an image source acoustic model. Results in the anechoic model show a mean spatial variance reduction of 42%, with a range of source material and using the optimal TDI generation methodology. Results in the image source model are more variable, suggesting that coherence of source reflections reduces static TDI effectiveness.
    • Optimizing wide-area sound reproduction using a single subwoofer with dynamic signal decorrelation

      Hill, Adam J.; Moore, J.B.; University of Derby (Audio Engineering Society, 2019-03-10)
      A central goal in small room sound reproduction is achieving consistent sound energy distribution across a wide listening area. This is especially difficult at low-frequencies where room-modes result in highly position-dependent listening experiences. While numerous techniques for multiple-degree-of-freedom systems exist and have proven to be highly effective, this work focuses on achieving position-independent low-frequency listening experiences with a single subwoofer. The negative effects due to room-modes and comb-filtering are mitigated by applying a time-varying decorrelation method known as dynamic diffuse signal processing. Results indicate that spatial variance in magnitude response can be significantly reduced, although there is a sharp trade-off between the algorithm’s effectiveness and the resulting perceptual coloration of the audio signal.
    • Practical considerations for subwoofer arrays and clusters in live sound reinforcement.

      Hill, Adam J.; University of Derby (Audio Engineering Society, 2017-08-21)
      The central theories behind low-frequency directionality with subwoofer clusters and arrays are well-known, but there are practical considerations that are essential to understand. This paper highlights key areas such as: the acoustic center, directionality of so-called omnidirectional sources, performance stage effects, and inter-unit decorrelation methods, primarily through the use of hemi-anechoic measurements with secondary analysis via electroacoustic simulations.