Detta kandidatarbete om ljudlokalisering av binauralt inspelade ljud utgår från tidigare studier av b.la. Parhizkari, P (2009), Brieger, S & Göther, F (2011) samt teorier från Rumsey, F och McCormick, T bok ”Sound and recording”(2009). Arbetet besvarar två frågeställningar angående hur lyssnarens egen kroppspositionering påverkar hur väl denne kan lokalisera inspelat ljud. För att besvara frågeställningarna har två tester utformats, ett för mer direkt ljudlokalisering och ett som testar lokaliseringsförmågan i en narrativ ljudmiljö. Resultatet av studien har visat att lyssnarens kroppsposition spelar marginell roll för hur väl vi kan lokalisera ljud. Vi blir dock osäkrare när vi ligger ner vilket resulterar till det tar längre tid att lokalisera ett ljud jämfört med om vi sitter upp och lyssnar.Framtida arbeten hade bl.a. kunnat rikta in sig på att undersöka hur vi kan använda oss av ljudreferenser för att underlätta lokaliseringen av andra ljud i virtuella miljöer.

Thesis (M.Sc.Eng.) PLEASE NOTE: Boston University Libraries did not receive an Authorization To Manage form for this thesis or dissertation. It is therefore not openly accessible, though it may be available by request. If you are the author or principal advisor of this work and would like to request open access for it, please contact us at open-help@bu.edu. Thank you. / Binaural hearing studies focus on how binaural processing improves the extraction of information from one source in the presence of competing sources. The most extensively studied condition is the detection of an out-of-phase tonal signal in an interaurally identical, Gaussian masking noise, called the N0Spi condition. Recently, attention turned to the dependence of detection performance on individual waveforms in the context of random noise waveforms from trial to trial. This thesis addresses this dependence, as measured in experiments (Isabelle 1991, 1995) that estimated probabilities of detection (Pd) and false alarm (Pf) for each of 30, narrowband-noise waveforms in the N0Spi condition. In previous work, models were shown to describe average performance and much of the variation over Pd, but the variation of Pf across noise samples was not explained. The current study explores two approaches to understanding the variation of Pd and Pf with noise waveform. First, a metric based on Shannon entropy is evaluated with the entropy computed from a combination of Pd and Pf. Second, internal noise in the form of temporal jitter is incorporated into existing interaural differences models. Results show that the correlation of the variation of interaural differences with the entropy is slightly stronger than that correlation with Pd alone. Models based on variations in the interaural differences with temporal jitter included are neither better nor worse than those without temporal jitter. Overall, these results suggest that the variation with Pf as captured by the entropy can be explained by interaural difference models. / 2031-01-01

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Avec la généralisation de l'écoute au casque, la technique binaurale apparaît comme une solution privilégiée pour démocratiser l'accès à des contenus sonores spatialisés. La synthèse binaurale repose sur l'utilisation de filtres appelés HRTFs qui restituent à l'auditeur l'ensemble des indices acoustiques de localisation. Ceux-ci possèdent une composante individuelle forte et l'utilisation de filtres non-individuels entraîne des défauts de localisation ou de timbre à la restitution. L’acquisition individuelle des HRTFs requiert un dispositif de mesure complexe. Les méthodes d’individualisation visent à offrir des solutions alternatives à cette mesure individuelle et reposent généralement sur l’exploitation de bases de données de HRTFs. A cet effet, une nouvelle base de données de HRTFs à haute résolution spatiale et fréquentielle a été constituée. Le développement d’un modèle visant à prédire les directions perçues de sources virtuelles synthétisées avec des HRTFs non-individuelles constitue le cœur du travail de thèse. La métrique utilisée pour quantifier la similarité entre HRTFs fait l’objet d’une attention particulière. L’objectif ultime est d’évaluer dans quelle mesure ce modèle peut être utilisé pour sélectionner le jeu de HRTFs optimal pour un individu à partir de ses réponses dans un test de localisation de sources sonores virtuelles synthétisées avec des HRTFs non-individuelles. La mise en place de tels tests implique l’utilisation d’une méthode de report, responsable d’une part d’erreur dans les réponses. La thèse comprend une étude comparative de 3 méthodes de report et la préconisation d’une méthode mieux adaptée au contexte de l’écoute binaurale au casque. / With the spread of headphone listening, binaural technology appears as the most appropriate solution to democratize the access to spatialized audio contents. Binaural synthesis of virtual sound sources is based on the use of filters called HRTFs, which provide the listener with accurate localization cues. These cues are however highly listener-dependent and the use of non-individual HRTFs may lead to localization and timbre artefacts.Individual acquisition of HRTFs requires a complex measurement setup installed in an anechoic chamber which is incompatible with large scale deployment. Therefore, individualization methods have been devised in order to offer alternatives to this individual measurement. They are often based on the exploitation of large HRTFs databases. To this end, a new HRTFs database with high spatial and frequency resolution has been created. The development of a model that predicts the perceived direction of a virtual source synthesized with non-individual HRTFs is the core of the thesis work. The choice of the metric used for quantifying the similarity between HRTFs receives a particular attention. The ultimate goal is to evaluate how such a model can be used to select automatically the optimal HRTFs set for an individual, from the observation of his responses in a localization test of virtual sound sources synthesized with non-individual HRTFs. The implementation of such a test implies the use of a reporting method, which may introduce some bias in the responses. This thesis includes a comparative study of 3 reporting methods and the recommendation of a method more suitable in the context of binaural listening through headphones.

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Contexte : Le démasquage binaural est un processus indispensable pour la compréhension en environnement bruyant. Ce mécanisme ferait intervenir la comparaison d’indices temporels et fréquentiels tout au long des voies nerveuses auditives. Cependant, il n’existe pas de réel consensus évoquant un traitement du démasquage à un niveau sous-cortical et/ou cortical. L’objet de cette étude est d’étudier ces indices temporels et fréquentiels du démasquage par le biais d’une étude perceptive, puis d’une étude électroencéphalographique (EEG). Matériels et méthodes : Une population normoentendante a été évaluée lors d’une étude perceptive visant à estimer l’importance du démasquage en fonction de 1) la largeur fréquentielle du bruit controlatéral (de 1 octave, 3 octaves ou à large bande), 2) la cohérence temporelle des bruits bilatéraux (corrélation égale à 0 ou 1) et 3) la fréquence des stimuli cibles (0,5, 1, 2 et 4 kHz). Puis, le démasquage a été évalué en EEG par l’étude 1) des latences précoces (<10 ms, PEA-P), 2) des latences tardives (<50 ms, PEA-T) et 3) de l’onde de discordance (PEA-MMN). Pour ces trois études EEG, l’influence de la cohérence temporelle des bruits bilatéraux a été explorée.Résultats : L’étude perceptive traduit un démasquage croissant lorsque la largeur fréquentielle du bruit controlatéral augmente. L’ajout du bruit controlatéral non corrélé (corrélation=0) se traduit par une amélioration de détection de 1,28 dB, quelle que soit la fréquence des stimuli cibles (antimasquage), alors que l’ajout d’un bruit controlatéral corrélé (corrélation=1) évoque une amélioration de détection lorsque la fréquence des stimuli cibles diminue (démasquage) : 0,97 dB à 4 kHz et 9,25 dB à 0,5 kHz. En PEA-P, les latences des ondes III et V se raccourcissent lorsqu’un bruit controlatéral corrélé ou non corrélé est ajouté (≈0,1 ms). En PEA-T, les amplitudes des ondes P1, N1 et des complexes P1N1 et N1P2 augmentent lorsqu’un bruit controlatéral corrélé ou non corrélé est ajouté. Enfin, l’amplitude de la MMN est plus conséquente lorsque le bruit controlatéral ajouté est corrélé (versus non corrélé). Conclusion : L’étude perceptive explicite l’importance des indices spectraux (antimasquage) et temporels (démasquage), pour améliorer la perception d’un signal initialement masqué. L’étude EEG suggère, quant à elle, un traitement sous-cortical influencé uniquement par les indices spectraux (antimasquage) et un traitement plus cortical influencé par les indices temporels (démasquage). / Background: Binaural unmasking is an essential process for understanding in noisy environments. This mechanism would involve the comparison of time and frequency cues throughout the hearing nerve pathways. However, there is no real consensus evoking a treatment of a binaural masking release at a subcortical and/or a cortical level. The purpose of this study is to investigate the time and frequency cues of the binaural unmasking through a perceptual study, and then through an electroencephalographic study (EEG).Materials and Methods: Normal hearing people were evaluated with a perceptive study to estimate the importance of the binaural unmasking according to 1) the frequency width of the contralateral noise (1 octave, 3 octaves or broadband), 2) the temporal coherence of bilateral noises (correlation equal to 0 or 1) and 3) the frequency of the target stimuli (0.5, 1, 2 and 4 kHz). Binaural unmasking was then evaluated with EEG by studying 1) early latencies (<10 ms, PEA-P), 2) late latencies (<50 ms, PEA-T) and 3), the mismatch wave (PEA- MMN). For these three EEG studies, the influence of the temporal coherence of bilateral noise was investigated.Results: The study shows a growing perceptive binaural unmasking when the frequency width of the contralateral noise increases. The addition of an uncorrelated contralateral noise (correlation = 0) results in a 1.28 dB detection enhancement, regardless of the frequency of the target stimuli (antimasking), while adding a contralateral correlated noise (correlation = 1) refers to a detection enhancement when the frequency of the target stimuli decreases (unmasking): 0.97 dB at 4 kHz and 9.25 dB at 0.5 kHz. The latencies of waves III and V are shortened when a contralateral correlated or uncorrelated noise is added (≈0,1 ms) in the PEA-P. The amplitudes of P1, N1 waves and P1N1 and N1P2 complex increase when contralateral correlated or uncorrelated noise is added in PEA-T. Finally, the amplitude of the MMN is higher when a contralateral correlated noise is added (versus an uncorrelated one).Conclusion: The perceptual study shows the significance of spectral cues (antimasking) and temporal cues (unmasking), to improve the perception of an initially masked signal. The EEG study suggests a subcortical treatment which is only influenced by spectral cues (antimasking) and a cortical processing, influenced by temporal cues (unmasking).

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This thesis describes a system that can be used for the decoding of a three dimensional audio recording over headphones or two, or more, speakers. A literature review of psychoacoustics and a review (both historical and current) of surround sound systems is carried out. The need for a system which is platform independent is discussed, and the proposal for a system based on an amalgamation of Ambisonics, binaural and transaural reproduction schemes is given. In order for this system to function optimally, each of the three systems rely on providing the listener with the relevant psychoacoustic cues. The conversion from a five speaker ITU array to binaural decode is well documented but pair-wise panning algorithms will not produce the correct lateralisation parameters at the ears of a centrally seated listener. Although Ambisonics has been well researched, no one has, as yet, produced a psychoacoustically optimised decoder for the standard irregular five speaker array as specified by the ITU as the original theory, as proposed by Gerzon and Barton (1992) was produced (known as a Vienna decoder), and example solutions given, before the standard had been decided on. In this work, the original work by Gerzon and Barton (1992) is analysed, and shown to be suboptimal, showing a high/low frequency decoder mismatch due to the method of solving the set of non-linear simultaneous equations. A method, based on the Tabu search algorithm, is applied to the Vienna decoder problem and is shown to provide superior results to those shown by Gerzon and Barton (1992) and is capable of producing multiple solutions to the Vienna decoder problem. During the write up of this report Craven (2003) has shown how 4th order circular harmonics (as used in Ambisonics) can be used to create a frequency independent panning law for the five speaker ITU array, and this report also shows how the Tabu search algorithm can be used to optimise these decoders further. A new method is then demonstrated using the Tabu search algorithm coupled with lateralisation parameters extracted from a binaural simulation of the Ambisonic system to be optimised (as these are the parameters that the Vienna system is approximating). This method can then be altered to take into account head rotations directly which have been shown as an important psychoacoustic parameter in the localisation of a sound source (Spikofski et al., 2001) and is also shown to be useful in differentiating between decoders optimised using the Tabu search form of the Vienna optimisations as no objective measure had been suggested. Optimisations for both Binaural and Transaural reproductions are then discussed so as to maximise the performance of generic HRTF data (i.e. not individualised) using inverse filtering methods, and a technique is shown that minimises the amount of frequency dependant regularisation needed when calculating cross-talk cancellation filters.