Virtual acoustic environments for the evaluation of hearing devices

Theoretical thesis."A thesis submitted in fulfilment of the requirements for the degree of Doctor of Philosophy at the National Acoustic Laboratories & Department of Linguistics, Faculty of Human Sciences" -- title page.Bibliography: pages 157-169.1. Introduction -- 2. The theory of higher-order ambisonics -- 3. Measurement of a full 3D set of HRTFs for in-ear and hearing aid microphones on a head and torso simulator (HATS) -- 4. Objective analysis of ambisonics for hearing aid applications : effect of listener's head, room reverberation, and directional microphones -- 5. Effect of higher-order ambisonics on evaluating beamformer benefit in realistic acoustic environments -- 6. An objective and subjective validation of applied loudspeaker-based virtual sound environments used for directional hearing and testing -- 7. Concluding discussion.Listening tests are essential for optimising and evaluating novel signal processing concepts for hearing devices. When they are performed inside the laboratory they are highly controlled, but typically lack ecological validity. On the other hand, field-studies provide high ecological validity, but are difficult to control, time-consuming and costly. Even though field-studies may ultimately be required, there still is a necessity for more realistic laboratory-based listening tests.This thesis focuses on creating and validating realistic virtual sound environments (VSEs), primarily based on the method of higher-order Ambisonics (HOA), for testing hearing aids (HAs). Particular emphasis has been given on how the VSE limitations affect the output of multi-microphone directional HAs. To systematically and objectively analyse the errors incurred by such VSEs, we developed and verified a framework that consists of: (1) the simulation of a reverberant acoustic scene; (2) the coding of that scene using HOA; (3) the reconstruction of the VSE via a loudspeaker array; and (4) the evaluation of the SNR benefit and output pressure of directional HAs worn on an acoustic manikin placed inside the VSE. A listening experiment was additionally designed to test the simulation results. For that purpose, a real-room 'cocktail-party' acoustic scene was created and acoustically modelled. Ambisonics was then applied to reproduce that scene inside a loudspeaker array. Listening tests involving hearing-impaired subjects, fitted with directional HAs, were next conducted inside the real and the virtual sound environments. The intelligibility and acceptable-noise-level benefits of the directional HAs were thereby estimated.The above evaluation illustrated that most of the real-environment properties can be faithfully reconstructed by the considered VSEs. Moreover, directional HA algorithms were shown to behave and adapt in a similar way as in real environments. However, sound-field reconstruction deviations were noticeable. For the case of highly directional schemes, such as the considered bilateral beamformer, such deviations may result in a reduced benefit estimation. Hence, we can conclude that although the described VSEs illustrated a reduced sensitivity in demonstrating real-life benefits, they are nonetheless suitable for evaluating directional HA processing schemes.xxii, 169 pages colour illustrations 30 c

Validation of realistic acoustic environments for listening tests using directional hearing aids

Assessing the real-life benefit of hearing devices inside realistic acoustic environments (RAEs) has long been desired. However, no verified methods exist in literature. This study compares the short-time SNRbenefit provided by two multi-microphone directional hearing aid strategies (fixed cardioid & binaural beamformer) operating inside: (1) a real room; (2) the real room environment generated using room acoustics modelling; (3) the real room environment reconstructed using a 4/7 Mixed-Order Ambisonics system. The modelled RAE (2), compared to the real room (1), incurs small median SNRbenefit errors (<;2 dB), although the short-time errors are much larger (up to ±10 dB). The Ambisonics-coded RAE (3), compared to the modelled RAE (2), incurs smaller median errors (<;1 dB), while also preserving the short-time values remarkably well. The encouraging results require experimental verification with subjective measures (e.g., speech reception threshold benefit).5 page(s

Measurement of a full 3D set of HRTFs for in-ear and hearing aid microphones on a Head and Torso Simulator (HATS)

The accurate reproduction of acoustic real-world environments is becoming of increasing importance in hearing device research and development. It is thereby often required to accurately predict the sound pressure at the microphones of a hearing device in a simulated or recorded acoustic environment. For that reason, an extensive set of head-related transfer functions (HRTFs) was measured in free-field with a pair of behind-the-ear (BTE) hearing aids placed on a Head and Torso Simulator (HATS). Transfer functions to the in-ear HATS microphones were also measured. A spherical head model was applied to extend the useable frequency range towards low frequencies. Special care was given to preserve the phase properties of the measurements so that the HRTFs could be widely used in phase-sensitive technical applications, including the evaluation of spatial signal processing algorithms (i.e. directional microphones, beamformers) in hearing devices and the evaluation of sound field resynthesis methods. The extended HRTF set can also be used for research in psychoacoustics. It is available for download at: http://www.nal.gov.au/download/HATS_BTE_hrirDatabase.zip.9 page(s

Evaluation of loudspeaker-based virtual sound environments for testing directional hearing aids

Background: Assessments of hearing aid (HA) benefits in the laboratory often do not accurately reflect real-life experience. This may be improved by employing loudspeaker-based virtual sound environments (VSEs) that provide more realistic acoustic scenarios. It is unclear how far the limited accuracy of these VSEs influences measures of subjective performance. Purpose: Verify two common methods for creating VSEs that are to be used for assessing HA outcomes. Research Design: A cocktail-party scene was created inside a meeting room and then reproduced with a 41-channel loudspeaker array inside an anechoic chamber. The reproduced scenes were created either by using room acoustic modeling techniques or microphone array recordings. Study Sample: Participants were 18 listeners with a symmetrical, sloping, mild-to-moderate hearing loss, aged between 66 and 78 yr (mean = 73.8 yr). Data Collection and Analysis: The accuracy of the two VSEs was assessed by comparing the subjective performance measured with two-directional HA algorithms inside all three acoustic environments. The performance was evaluated by using a speech intelligibility test and an acceptable noise level task. Results: The general behavior of the subjective performance seen in the real environment was preserved in the two VSEs for both directional HA algorithms. However, the estimated directional benefits were slightly reduced in the model-based VSE, and further reduced in the recording-based VSE. Conclusions: It can be concluded that the considered VSEs can be used for testing directional HAs, but the provided sensitivity is reduced when compared to a real environment. This can result in an underestimation of the provided directional benefit. However, this minor limitation may be easily outweighed by the high realism of the acoustic scenes that these VSEs can generate, which may result in HA outcome measures with a significantly higher ecological relevance than provided by measures commonly performed in the laboratory or clinic.16 page(s

Objective analysis of ambisonics for hearing aid applications : effect of listener's head, room reverberation, and directional microphones

Recently, an increased interest has been demonstrated in evaluating hearing aids (HAs) inside controlled, but at the same time, realistic sound environments. A promising candidate that employs loudspeakers for realizing such sound environments is the listener-centered method of higher-order ambisonics (HOA). Although the accuracy of HOA has been widely studied, it remains unclear to what extent the results can be generalized when (1) a listener wearing HAs that may feature multi-microphone directional algorithms is considered inside the reconstructed sound field and (2) reverberant scenes are recorded and reconstructed. For the purpose of objectively validating HOA for listening tests involving HAs, a framework was developed to simulate the entire path of sounds presented in a modeled room, recorded by a HOA microphone array, decoded to a loudspeaker array, and finally received at the ears and HA microphones of a dummy listener fitted with HAs. Reproduction errors at the ear signals and at the output of a cardioid HA microphone were analyzed for different anechoic and reverberant scenes. It was found that the diffuse reverberation reduces the considered time-averaged HOA reconstruction errors which, depending on the considered application, suggests that reverberation can increase the usable frequency range of a HOA system.19 page(s

Effect of higher-order ambisonics on evaluating beamformer benefit in realistic acoustic environments

Multi-channel loudspeaker systems have been proposed to assess the real-life benefit of devices such as hearing aids, cochlear implants, or mobile phones. This paper investigates to what extent sound fields recreated by Higher-Order Ambisonics (HOA) can be used to evaluate the performance of spatially selective multi-microphone processing schemes (beamformers) inside complex acoustic environments. Two example schemes are considered: an adaptive directional microphone (ADM) and a contralateral suppression bilateral beamformer (BBF), both implemented in the context of a hearing aid device. The acoustic scenarios consist of a single speech target (0°) competing against three speech jammers (±90° and 180°) set either in an anechoic or in a reverberant simulated classroom (T₃₀ = 0.6s). The HOA effect on the directional algorithm performance is quantified through: (a) the adaptive, frequency-dependent, algorithm gains, (b) the SNR improvement calculated in one-third octave bands, and (c) the processed target frequency response. The HOA reconstruction errors influence the beamformers in mainly two ways; first, by altering the spatial characteristics of the sound field, which in turn modifies the adaptation of the algorithms, and second, by affecting the spectral content of the sources. The results suggest that although HOA (here 7th order) does not degrade the broadband, long-term, intelligibility-weighted SNR improvement of the two beamformers, it imposes a low-pass effect on the processed target. This renders the HOA coding problematic above the system's cut-off frequency.4 page(s

Frequency dependent regularization of a mixed-order Ambisonics encoding system using psychoacoustically motivated metrics

In mixed-order Ambisonics (MOA) encoding the right choice of the regularization parameter λ is crucial to the reproduction quality of the system. The choice of λ is a trade-off between low microphone noise amplification and high directivity. Commonly used performance metrics are not suited to determine an optimum λ(f)-curve due to their limited psychoacoustical relevance. Therefore, it is proposed here to use two perceptually motivated metrics that consider the reproduced signals at the listener's ear to quantify the reproduction error and the audibility of the total microphone noise. Regularization was then optimized towards previously defined requirements on both of these metrics separately. The results showed that at frequencies below 500Hz both requirements can be met. Above this frequency one or both of the requirements have to be relaxed in order to find a suitable λ(f).8 page(s

The Ambisonic Recordings of Typical Environments (ARTE) Database

Everyday listening environments are characterized by far more complex spatial, spectral and temporal sound field distributions than the acoustic stimuli that are typically employed in controlled laboratory settings. As such, the reproduction of acoustic listening environments has become important for several research avenues related to sound perception, such as hearing loss rehabilitation, soundscapes, speech communication, auditory scene analysis, automatic scene classification, and room acoustics. However, the recordings of acoustic environments that are used as test material in these research areas are usually designed specifically for one study, or are provided in custom databases that cannot be universally adapted, beyond their original application. In this work we present the Ambisonic Recordings of Typical Environments (ARTE) database, which addresses several research needs simultaneously: realistic audio recordings that can be reproduced in 3D, 2D, or binaurally, with known acoustic properties, including absolute level and room impulse response. Multichannel higher-order ambisonic recordings of 13 realistic typical environments (e.g., office, cafè, dinner party, train station) were processed, acoustically analyzed, and subjectively evaluated to determine their perceived identity. The recordings are delivered in a generic format that may be reproduced with different hardware setups, and may also be used in binaural, or single-channel setups. Room impulse responses, as well as detailed acoustic analyses, of all environments supplement the recordings. The database is made open to the research community with the explicit intention to expand it in the future and include more scenes

Dispersive FSO Performance Estimation with Gaussian Pulses and Laplace Modeled Time Jitter

FSO communications tend to be one of most convenient, wireless, high-data-rate communications technologies of global telecom networking, and they are implemented and operated with low-cost resources. Despite their advantages, FSO systems’ performance is delimited by several physical phenomena, which act on propagating signal beams through the atmospheric path. Among other effects, chromatic dispersion and time jitter affect the shape and the detection instant of the incoming optical pulse, respectively. This results in signal fading and probable misdetections, and the signal fades along the propagation path due to power losses. Particularly, chromatic dispersion affects the width of the longitudinal information pulse, while the stochastic nature of the time jitter effect is treated with the use of a statistical model for the instantly received irradiance of the detecting pulse at the corresponding time slot. In this study, the symmetrical Laplace distribution was chosen for weak time jitter effect emulation because of its symmetry in pulse detection before or after the center of the specific timeslot. Thus, the joint influence of all three effects could considered, including all the parameters involved. Moreover, new-closed-form mathematical expressions were derived in order to accurately estimate the availability and the reliability of the FSO links under consideration. Next, using the derived mathematical forms, performance outcomes were presented for typical parameter values for realistic FSO links