Free-field Source Research Articles

Comparing subjective similarity ratings and quantitative errors for the evaluation of free-field binaural panning techniques

A free-field sound source can be accurately rendered over headphones via direct convolution with head-related impulse responses (HRIRs). Using binaural panning techniques, a free-field source can be approximated, which can be thought of as HRIR reconstruction. However, a loss of fidelity occurs related to the panning algorithm’s spatial resolution. Previously, perceptual comparisons of noise bursts were made between panned sources and direct convolution with measured HRIRs. Reconstructions were generated using five different binaural panning methods, both with and without time-alignment of the HRIRs with separate application of the interaural time delay (ITD). To further explore the listening test results, the quantitative differences between the reconstructed and original HRIRs were investigated alongside the perceptual data. Several perceptually-motivated error metrics were evaluated, including errors in both ITD and interaural level difference. The fidelity of the reconstructed HRTF magnitude response was evaluated using metrics that leveraged auditory modeling steps. Both subjective and objective results support that HRIR time-alignment reduces the number of filters needed for high perceptual accuracy. A principal component-base rendering filter set produced the best subjective accuracy with only a small number of required filters (16-25). The ability to predict perceptually detectable degradation via quantitative errors will be discussed.

Study on the factors influencing the interaction and coalescence of shock waves from multiple explosion sources in free field

Study on the factors influencing the interaction and coalescence of shock waves from multiple explosion sources in free field

Recovery of non-stationary free field based on pressure and acceleration measurements in a noisy environment

In a noisy environment, the interested sound field is interfused with the non-stationary incoming field from the back side of the measurement plane and the scattered field caused by the incoming wave falling on the surface of the target source. In order to remove the non-stationary incoming and scattered fields simultaneously, a recovery method of the non-stationary free field with the pressure and particle acceleration measurements is proposed. First, the mixed time-evolving pressure and particle acceleration are firstly measured on one measurement plane, where the particle acceleration is obtained by the finite difference approximation with the aid of an auxiliary measurement plane; Then, two physical relations are employed to deduce a forward complete recovery formulation of the target source in the noisy environment. One relation contains two impulse response functions relating the time-wavenumber pressure spectrum to particle acceleration and pressure spectra, respectively, and the other is the surface reflection coefficient of the target source relating the scattered field to the incoming field. Finally, the mixed pressure and particle acceleration are substituted into the recovery formulation, and the time-evolving pressure radiated by the target source in free-field is recovered. Thereby, the proposed method possesses a significant feature of real-time recovery of the non-stationary free field. A circular piston fixed on an infinite rigid baffle and two monopole sources are designed in a numerical simulation to test the performance of the proposed method. The simulation results attest that the proposed method not only can recover the time-evolving pressure radiated by the target source in free-field at any space points, but also recover the space distribution of the non-stationary sound field of the target source in free-field at different time instants effectively. An experiment with two speakers embedded in a planar steel plate and a speaker is further employed to verify the validity of the proposed method.

Recovery of non-stationary free field with double-layer pressure measurements

Abstract To remove the non-stationary incoming field from the back side of the measurement plane and the non-stationary scattered field caused by the incoming wave falling on the surface of the target source, a recovery method of the non-stationary free field with double-layer pressure measurements is proposed. In this method, the time-evolving mixed pressures on double-layer measurement planes are first measured and transformed into the time-wavenumber domain. Then, with the aid of these time-wavenumber pressure spectrums and the impulse response function, the non-stationary incoming and outgoing fields are separated by the conventional non-stationary sound field separation method. Subsequently, the time-wavenumber spectrum of the surface reflection coefficient of the target source is employed to establish the relationship between the non-stationary scattered and incoming fields, based on which the non-stationary scattered field is calculated. Finally, the non-stationary scattered field is removed from the outgoing field and the non-stationary sound field that would be radiated by the target source in free field is recovered. Numerical simulation with an exciting planar steel plate as the target source and a monopole source as the disturbing source is used to examine the feasibility of the proposed method. The recovered results demonstrate that the proposed method not only removes the influence of the incoming field in the time and space domains, but also clears the non-stationary scattered field, thus providing more accurate recovered results than the non-stationary sound field separation method. The experiment further proves the validity of the proposed method.

Fast and accurate analytical modeling of broadband noise for a low-speed fan.

The broadband noise sources are investigated on an isolated low-speed fan typical of engine cooling systems. Reynolds-averaged Navier-Stokes (RANS) simulations have been performed on a single blade passage for several flow rates at the same rotational speed. The flow structures responsible for the different noise contributions are identified by a systematic analysis of the simulation results. The aeroacoustic noise predictions are based on Amiet's model for rotating sources in free-field. The contribution of the turbulence-interaction noise and the trailing-edge noise are considered by the appropriate isolated blade response and statistical model of the turbulent sources. The flow parameters of the aeroacoustic response and the turbulent models are extracted from the RANS simulations. The radial evolution of the flow parameters for the different flow rates is analyzed and related to the three-dimensional flows in the machine. The acoustic predictions are validated with experimental spectra measured upstream of the fan in a reverberant room. The two considered mechanisms evolve differently with the flow rate. The leading-edge sources are dominant at low flow rate up to the design point and the self-noise becomes dominant at high flow rate for which the secondary flow structures are limited.

Estimates of Source Spectra of Ships from Long Term Recordings in the Baltic Sea

Estimates of the noise source spectra of ships based on long term measurements in the Baltic sea are presented. The measurement data were obtained by a hydrophone deployed near a major shipping lane south of the Island Oland. Data from over 2000 close-by passages were recorded during a five month period from August to December 2014. For each passage, ship-to-hydrophone transmission loss (TL) spectra were computed by sound propagation modelling using 1. bathymetry data from the Baltic Sea Bathymetry Database (BSBD) 2. sound speed profiles from the HIROMB oceanographic model, 3. seabed parameters obtained by acoustic inversion of data from a calibrated source and 4. AIS data providing information on each ship's position. These TL spectra were then subtracted from the received noise spectra to estimate the free field source level (SL) spectra for each passage. The SL were compared to predictions by some existing models of noise emission from ships. Input parameters to the models, including e.g. ship length, width, speed, displacement and engine mass, were obtained from AIS data and the STEAM database of the Finnish Metereological Institute (FMI).

Individual attributes describing the perception of synthesized sound fields in listening rooms

Sound field synthesis techniques such as Wave Field Synthesis (WFS) are in theory defined in an anechoic environment but real-world installations have to be placed inside listening rooms with reflective walls. The arising reflections influence the desired synthesized sound field both from a physical and a perceptual point of view. This study investigates the perceptual aspects that are relevant for WFS in reflective environments by means of the Repertory Grid Technique (RGT). By comparing a representative range of auditory scenes with real and virtual sources in free field and in different listening rooms, subjects generate individual constructs to describe perceived differences and similarities as contrastive pairs. In a second step, subjects rate the scenes on scales constructed by their own contrastive pairs. The ratings are used to reveal relations between the constructs and the scenes on an individual basis. Clusters of constructs can be found that in terms of content can be associated with a common perceptual aspect.

Transaural experiments and a revised duplex theory for the localization of low-frequency tones.

The roles of interaural time difference (ITD) and interaural level difference (ILD) were studied in free-field source localization experiments for sine tones of low frequency (250-750 Hz). Experiments combined real-source trials with virtual trials created through transaural synthesis based on real-time ear canal measurements. Experiments showed the following: (1) The naturally occurring ILD is physically large enough to exert an influence on sound localization well below 1000 Hz. (2) An ILD having the same sign as the ITD modestly enhances the perceived azimuth of tones for all values of the ITD, and it eliminates left-right confusions that otherwise occur when the interaural phase difference (IPD) passes 180°. (3) Increasing the ILD to large, implausible values can decrease the perceived laterality while also increasing front-back confusions. (4) Tone localization is more directly related to the ITD than to the IPD. (5) An ILD having a sign opposite to the ITD promotes a slipped-cycle ITD, sometimes with dramatic effects on localization. Because the role of the ITD itself is altered by the ILD, the duplex processing of ITD and ILD reflects more than mere trading; the effect of the ITD can be reversed in sign.

Impact of AMX-A1 military aircraft operations on the acoustical environment close to a Brazilian airbase

Military aircraft operating on airbases usually have a considerable impact on the neighborhood. While the impact of civil aircraft operations can be modeled by commercially available software, the same is hardly possible for military aircraft as no EPNL data are available for such aircraft and flight path are not restricted to those used by civilian operations. Therefore, in this work, the noise impact of AMX-A1 aircraft operating at a Brazilian airbase was evaluated from measurements originally intended for calibration of the noise map of an university that is in the vicinity. From the data, it was possible to obtain LAeq10min with and without jet noise to see how much does AMX operations influence in the total measurement. Sound exposure levels (SEL) were also calculated. It was found that depending of AMX procedure (Approach, Departure, Touch, Go, etc.), jet noise increases the LAeq10min up to 10 dB and SEL values reaches 96, 6 dBA in sensitive areas. It will be discussed if A-weighted sound power level can be estimated form the data considering the aircraft as a point source in free field.

Localizing the sources of two independent noises: Role of time varying amplitude differences

Listeners localized the free-field sources of either one or two simultaneous and independently generated noise bursts. Listeners' localization performance was better when localizing one rather than two sound sources. With two sound sources, localization performance was better when the listener was provided prior information about the location of one of them. Listeners also localized two simultaneous noise bursts that had sinusoidal amplitude modulation (AM) applied, in which the modulation envelope was in-phase across the two source locations or was 180° out-of-phase. The AM was employed to investigate a hypothesis as to what process listeners might use to localize multiple sound sources. The results supported the hypothesis that localization of two sound sources might be based on temporal-spectral regions of the combined waveform in which the sound from one source was more intense than that from the other source. The interaural information extracted from such temporal-spectral regions might provide reliable estimates of the sound source location that produced the more intense sound in that temporal-spectral region.

Platform for virtual auditory environment real time rendering system

By dynamically synthesizing binaural signals in free-field and reflective environment, a virtual auditory environment (VAE) real time rendering system recreates realistic auditory events or perceptions for listeners. VAE systems have been applied in various fields, such as the research of binaural hearing, multimedia and virtual reality, among others. In present work, a PC and C++ language-based VAE system is designed and implemented. Schemes for improving the performances of the system, including multiple virtual source synthesis, auditory distance perception, dynamic information simulation for multiple degrees of freedom of listener, as well as dynamic characters of the system, are proposed. The results from measurement indicate that the system is capable of synthesizing 280 virtual sound sources (including free-field sources and image sources for reflections) simultaneously in conventional working mode, or 4500 virtual sources in proposed PCA (principal components analysis) working mode. The update rate is 120 Hz, and the system latency is 25.4ms. A set of psychoacoustic experiments also validate the performance of the system. The function extension of VAE can serve as a flexible and powerful platform for binaural and virtual reality research.

Spatial release from masking in a free-field source identification task by gray treefrogs

Humans and other animals often communicate acoustically in noisy social groups, in which the background noise generated by other individuals can mask signals of interest. When listening to speech in the presence of speech-like noise, humans experience a release from auditory masking when target and masker are spatially separated. We investigated spatial release from masking (SRM) in a free-field call recognition task in Cope’s gray treefrog (Hyla chrysoscelis). In this species, reproduction requires that females successfully detect, recognize, and localize a conspecific male in the noisy social environment of a breeding chorus. Using no-choice phonotaxis assays, we measured females’ signal recognition thresholds in response to a target signal (an advertisement call) in the presence and absence of chorus-shaped noise. Females experienced about 3 dB of masking release, compared with a co-localized condition, when the masker was displaced 90° in azimuth from the target. The magnitude of masking release was independent of the spectral composition of the target (carriers of 1.3 kHz, 2.6 kHz, or both). Our results indicate that frogs experience a modest degree of spatial unmasking when performing a call recognition task in the free-field, and suggest that variation in signal spectral content has small effects on both source identification and spatial unmasking. We discuss these results in the context of spatial unmasking in vertebrates and call recognition in frogs.

On Low-Complexity Simulation of Multichannel Room Impulse Responses

In this letter, we present a method for low-complexity simulation of multichannel room impulse responses (RIRs). Low-complexity RIR methods will become inevitable in next generation communication systems having massive amounts of microphones/loudspeakers. For a room with rigid boundaries, we show that proper sampling of the free-field plenacoustic spectrum results in the solution of the wave equation at any position in the room. We show that the spatial aliasing introduced by spectral sampling represents the wall reflections. These wall reflections are usually modelled, at least in low-complexity simulation algorithms, by the creation of virtual free-field sources outside the room, an image source model commonly referred to as the image method. The image method requires <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">O</i> ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> ) operations per receiver position, whereas the newly proposed method requires only <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">O</i> ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> log <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> ) operations per receiver position.

Phase effects on the perceived elevation of complex tones

Free-field source localization experiments with 30 source locations, symmetrically distributed in azimuth, elevation, and front-back location, were performed with periodic tones having different phase relationships among their components. Although the amplitude spectra were the same for these different kinds of stimuli, the tones with certain phase relationships were successfully localized while the tones with other phases led to large elevation errors and front-back reversals, normally growing with stimulus level. The results show that it is not enough to have a smooth, broadband, long-term signal spectrum for successful sagittal-plane localization. Instead, temporal factors are important. A model calculation investigates the idea that the tonotopic details that mediate localization need to be simultaneously, or almost simultaneously, accessible in the auditory system in order to achieve normal elevation perception. A qualitative model based on lateral inhibition seems capable in principle of accounting for both the phase effects and level effects.

High-Resolution Imaging of Sound Sources in Free Field Using a Numerical Time-Reversal Sink

Numerous practical applications ― such as non destructive evaluation of industrial structures, acoustic characterization of musical instruments, and acoustic mapping of sound sources in a known propagation medium ― involve source detection and characterization. In the past, this problem has been investigated using different beamforming and backpropagation methods. In this work, a new technique, based on the time reversal sink concept, is used to detect active sound sources with a limited number of measurement points. The theory and application of super-resolution focusing of sound and vibration using a time-reversal sink (TRS) have been studied, both in ultrasonic regime and in audible range. A high-resolution imaging technique based on a numerical time reversal sink has recently been developed by the authors for vibrational imaging of active sources in a dispersive medium. In this paper, the numerical time reversal sink imaging technique is adapted to the case of high-resolution acoustic imaging of active sound sources in a three-dimensional free field. This technique allows high-resolution imaging and provides a new method of characterization and detection of sound sources. All results show the high resolution imaging capabilities of this new technique when compared with classical time-reversal (TR) backpropagation. More than simply detecting the position of the acoustic source, this technique allows to detect the size of the active sources. This technique provides an alternative to other imaging and source detection techniques, such as three-dimensional acoustic holography and beamforming.

Proprioceptive cues modulate further processing of spatially congruent auditory information. A high-density EEG study

Multisensory integration and interaction occur when bimodal stimuli are presented as either spatially congruent or incongruent, but temporally coincident. We investigated whether proprioceptive cues interact with auditory attention to one of two sound sources in free-field. The participant's task was to attend to either the left or right speaker and to respond to occasional increased-bandwidth targets via a footswitch. We recorded high-density EEG in three experimental conditions: the participants either held the speakers in their hands (Hold), reached out close to them (Reach), or had their hands in their lap (Lap). In the last two conditions, the auditory event-related potentials (ERPs) revealed a prominent negativity around 200 ms post-stimulus (N2 wave) over fronto-central areas, which is a reliable index of further processing of spatial stimulus features in free-field. The N2 wave was markedly attenuated in the ‘Hold’ condition, which suggests that proprioceptive cues apparently solidify spatial information computed by the auditory system, in so doing alleviating the need for further processing of spatial coordinates solely based on auditory information.

On Average Properties of Inhomogeneous Fluids in General Relativity: Perfect Fluid Cosmologies

For general relativistic spacetimes filled with an irrotational perfect fluid a generalized form of Friedmann's equations governing the expansion factor of spatially averaged portions of inhomogeneous cosmologies is derived. The averaging problem for scalar quantities is condensed into the problem of finding an "effective equation of state" including kinematical as well as dynamical "backreaction" terms that measure the departure from a standard FLRW cosmology. Applications of the averaged models are outlined including radiation-dominated and scalar field cosmologies (inflationary and dilaton/string cosmologies). In particular, the averaged equations show that the averaged scalar curvature must generically change in the course of structure formation, that an averaged inhomogeneous radiation cosmos does not follow the evolution of the standard homogeneous-isotropic model, and that an averaged inhomogeneous perfect fluid features kinematical "backreaction" terms that, in some cases, act like a free scalar field source. The free scalar field (dilaton) itself, modelled by a "stiff" fluid, is singled out as a special inhomogeneous case where the averaged equations assume a simple form.

Head-related transfer functions of the Rhesus monkey

Head-related transfer functions (HRTFs) are direction-specific acoustic filters formed by the head, the pinnae and the ear canals. They can be used to assess acoustical cues available for sound localization and to construct virtual auditory environments. We measured the HRTFs of three anesthetized Rhesus monkeys ( Macaca mulatta) from 591 locations in the frontal hemisphere ranging from −90° (left) to 90° (right) in azimuth and −60° (down) to 90° (up) in elevation for frequencies between 0.5 and 15 kHz. Acoustic validation of the HRTFs shows good agreement between free field and virtual sound sources. Monaural spectra exhibit deep notches at frequencies above 9 kHz, providing putative cues for elevation discrimination. Interaural level differences (ILDs) and interaural time differences (ITDs) generally vary monotonically with azimuth between 0.5 and 8 kHz, suggesting that these two cues can be used to discriminate azimuthal position. Comparison with published subsets of HRTFs from squirrel monkeys ( Saimiri sciureus) shows good agreement. Comparison with published human HRTFs from the frontal hemisphere demonstrates overall similarity in the patterns of ILD and ITD, suggesting that the Rhesus monkey is a good acoustic model for these two sound localization cues in humans. Finally, the measured ITDs in the horizontal plane agree well between −40° and 40° in azimuth with those calculated from a spherical head model with a radius of 52 mm, one-half the interaural distance of the monkey.

Receptive fields and binaural interactions for virtual-space stimuli in the cat inferior colliculus.

Sound localization depends on multiple acoustic cues such as interaural differences in time (ITD) and level (ILD) and spectral features introduced by the pinnae. Although many neurons in the inferior colliculus (IC) are sensitive to the direction of sound sources in free field, the acoustic cues underlying this sensitivity are unknown. To approach this question, we recorded the responses of IC cells in anesthetized cats to virtual space (VS) stimuli synthesized by filtering noise through head-related transfer functions measured in one cat. These stimuli not only possess natural combinations of ITD, ILD, and spectral cues as in free field but also allow precise control over each cue. VS receptive fields were measured in the horizontal and median vertical planes. The vast majority of cells were sensitive to the azimuth of VS stimuli in the horizontal plane for low to moderate stimulus levels. Two-thirds showed a "contra-preference" receptive field, with a vigorous response on the contralateral side of an edge azimuth. The other third of receptive fields were tuned around a best azimuth. Although edge azimuths of contra-preference cells had a broad distribution, best azimuths of tuned cells were near the midline. About half the cells tested were sensitive to the elevation of VS stimuli along the median sagittal plane by showing either a peak or a trough at a particular elevation. In general receptive fields for VS stimuli were similar to those found in free-field studies of IC neurons, suggesting that VS stimulation provided the essential cues for sound localization. Binaural interactions for VS stimuli were studied by comparing responses to binaural stimulation with responses to monaural stimulation of the contralateral ear. A majority of cells showed either purely inhibitory (BI) or mixed facilitatory/inhibitory (BF&I) interactions. Others showed purely facilitatory (BF) or no interactions (monaural). Binaural interactions were correlated with azimuth sensitivity: most contra-preference cells had either BI or BF&I interactions, whereas tuned cells were usually BF. These correlations demonstrate the importance of binaural interactions for azimuth sensitivity. Nevertheless most monaural cells were azimuth-sensitive, suggesting that monaural cues also play a role. These results suggest that the azimuth of a high-frequency sound source is coded primarily by edges in azimuth receptive fields of a population of ILD-sensitive cells.

Two cases of generalized auditory agnosia: electrophysiological distinction and neurobehavioral correlates

Multisensory integration and interaction occur when bimodal stimuli are presented as either spatially congruent or incongruent, but temporally coincident. We investigated whether proprioceptive cues interact with auditory attention to one of two sound sources in free-field. The participant's task was to attend to either the left or right speaker and to respond to occasional increased-bandwidth targets via a footswitch. We recorded high-density EEG in three experimental conditions: the participants either held the speakers in their hands (Hold), reached out close to them (Reach), or had their hands in their lap (Lap). In the last two conditions, the auditory event-related potentials (ERPs) revealed a prominent negativity around 200 ms post-stimulus (N2 wave) over fronto-central areas, which is a reliable index of further processing of spatial stimulus features in free-field. The N2 wave was markedly attenuated in the ‘Hold’ condition, which suggests that proprioceptive cues apparently solidify spatial information computed by the auditory system, in so doing alleviating the need for further processing of spatial coordinates solely based on auditory information.