Higher Order Ambisonics Research Articles

Modal beamformer analysis of ambisonics decoders

Ambisonic recording and playback is experiencing a revival with the introduction of higher-order Ambisonics (HOA) and is included in the recent MPEG-H and ATSC 3.0 standards. Higher-Order Ambisonics processing results in an improved spatial resolution compared to the original first-order Ambisonics proposed in 1970s. From a signal processing point of view, Ambisonics is a two-step process. In the first step spherical harmonic signals are generated by an Ambisonic encoder. Based on these spherical harmonic signals an Ambisonic decoder generates the output signals in the second step. Many different decoder flavors have emerged over time. Various decoder designs are motivated by different optimization criterion for the resulting soundfield. In this presentation we will analyze the Ambisonic decoders based on the fact that the two-step Ambisonics processing is closely related to spherical harmonic modal beamforming. It is shown that the decoder output signals can be interpreted in terms of beampatterns. The resulting beampatterns for the different decoders are computed and some performance criteria are presented.

Balancing the detrimental effects of office noise annoyance and distraction on work performance

Broadband, steady-state background noise can improve open office conditions by facilitating speech privacy and reducing distraction caused by intermittent, occupancy-generated noise. The background noise is typically generated by HVAC systems, though can be added with loudspeakers to boost speech masking. However, too high background noise levels can cause annoyance, fatigue, and other noise-related symptoms. It is yet unclear whether noise annoyance or distraction impairs work performance more. This study investigated the trade-off between noise annoyance and distraction, as well as their effects on acoustic dissatisfaction and performance. Subjects performed cognitive tasks while exposed to simulated office acoustic environments reproduced using higher-order Ambisonics. At fixed time intervals, the subjects could change the acoustic environment by adjusting either the background or intermittent noise levels. Lowering background noise caused the intermittent noise to rise, and vice versa. By the end of testing, it was expected that each subject equalized their dissatisfactions of the two noise types. Annoyance and distraction were assessed with a survey at each time interval. Physiological measures, including heart rate variability and skin conductance, were collected to correlate arousal/stress levels with each acoustic environment. The results of the study may provide context to effectively utilize background noise in open-plan offices.

Applications of operator theory and symmetry arguments to sound field recording and reproduction

The problem of recording and reproducing sound fields using microphone/loudspeaker arrays is treated by exploiting its symmetries. This approach leads to well-known results, such as high-order Ambisonics corresponding to the requirement of the error field to be invariant under rotations. More importantly, it allows us to treat some difficult problems critical to the application of sound field recording and reproduction to virtual reality, where one or more users can freely move around a realistic environment. In this presentation, we consider three of these problems. First, we present a method to extrapolate extended sound field information from conventional spherical microphone array recordings by imposing symmetry constrains based on a priori knowledge of the sound source positions. Second, we apply the same symmetry constrains to the problem of sound field reproduction, allowing us to formulate a method of presenting spatial sound to moving listeners. Finally, we consider the problem of describing and reproducing the sound field due to a moving sound source.

音場再現技術における数理問題 : 波面合成・高次アンビソニックスの数理( 近年の音響信号処理における数理科学の進展)

音場再現技術における数理問題 : 波面合成・高次アンビソニックスの数理( 近年の音響信号処理における数理科学の進展)

Auralizations with loudspeaker arrays from a phased combination of the image source method and acoustical radiosity

In order to create a simulation tool that is well-suited for small rooms with low diffusion and highly absorbing ceilings, a new room acoustic simulation tool has been developed that combines a phased version of the image source with acoustical radiosity and that considers the angle dependence of the surface properties. The new tool is denoted PARISM, and here PARISM is used to create loudspeaker array-based auralizations. Different auralization techniques are compared, such as Ambisonics, vector-based panning, and the method of nearest loudspeaker. The implementations of the auralization techniques with PARISM are described and compared to implementations of auralizations with another geometrical acoustic simulation tool, i.e., ODEON and the LoRA toolbox that applies Ambisonics to ODEON simulations. In opposition to the LoRA toolbox, higher order Ambisonics are also applied to the late part of the PARISM impulse response, because more directional information is available with acoustical radiosity. Small rooms with absorbing surfaces are tested, because this is the room type that PARISM is particularly useful for.

Using binaural and spherical microphone arrays to assess the quality of synthetic spatial sound fields

Recently, we completed the Collaborative-Research Augmented Immersive Virtual Environment Laboratory (CRAIVE-Lab) with a usable floor area of 12 × 10 m2 at Rensselaer. The CRAIVE-Lab project addresses the need for a specialized virtual-reality (VR) system for the study and enabling of communication-driven tasks with groups of users immersed in a high-fidelity multi-modal environment located in the same physical space. For the acoustic domain, a 134-loudspeaker-channel system has been installed for Wave Field Synthesis (WFS) with the support of Higher-Order-Ambisonic (HoA) sound projection to render inhomogeneous acoustic fields. An integrated 16-channel spherical microphone array makes the CRAIVE-Lab an ideal test bed to study different spatial rendering techniques such as Wave-Field Synthesis, Higher-Order Ambisonics and Virtual Microphone Control (ViMiC). In this talk, sound-field measurements taken with a traditional binaural manikin will be compared to spherical microphone recordings to assess the quality of the different rendering techniques for large-scale labs. A focus will hereby be set on assessing the sweet spot area for different rendering techniques. [Work supported by NSF 1229391, NSF 1631674, and the Cognitive and Immersive Systems Laboratory (CISL).]

Considerations regarding the equalization for spatial playback from spherical microphone array recordings

The encoding of spatial sound fields captured by spherical microphone arrays into Eigenbeams (HOA signals) provides a compact representation of the sound field that facilitates spatially realistic audio playback in Higher-Order Ambisonics (HOA) applications. For spatial audio reproduction, a typical Ambisonics decoder generates the loudspeaker signals from the HOA signals by applying only simple scalar weights. However, since the HOA signals derived from physical microphone arrays are frequency dependent, such a frequency independent scalar weighting scheme will not result in the desired loudspeaker sound field reproduction. An analysis of this frequency dependency and some equalization schemes to mitigate detrimental effects resulting from frequency independent loudspeaker decoding will be discussed.

Evaluation of techniques for navigation of higher-order ambisonics

Metrics are presented that assess spectral coloration and localization errors incurred by navigational techniques for higher-order ambisonics. Previous studies on the coloration induced by such navigational techniques have been largely qualitative, and the accuracy of previously-used localization models in this context is unclear. The presented metrics are applied in numerical simulations of navigation over a range of translation distances. Coloration is predicted using an auditory filter bank to compute the spectral energy differences between the test and reference signals in critical bands, and localization is predicted using a precedence-effect-based localization model. Coloration and localization errors are also measured through corresponding binaural-synthesis-based listening tests, wherein subjects are first asked to rate the induced coloration relative to reference and low-pass-filtered “anchor“ signals, and subsequently judge source position. Relationships are drawn between the metrics and the results of the listening tests in order to validate the predictive capabilities of the metrics.

Plane-wave decomposition and ambisonics output from spherical and nonspherical microphone arrays

Any acoustic sensor embedded in a baffle disturbs the spatial acoustic field to a certain extent, and the recorded field is different from a field that would have existed if the baffle were absent. Recovery of the original (incident) field is a fundamental task in spatial audio. For some sensor baffle geometries, such as the sphere, the disturbance of the field by the sensor can be characterized analytically and its influence can be undone to recover the incident field. However, for arbitrary shaped baffles, numerical methods have to be employed. In the current work, the baffle influence on the field is characterized using boundary element methods, and a framework to recover the incident field from measurements from sensors embedded on the baffle in the plane-wave function basis is developed. Field recovery also allows generation of high-order ambisonics representations of the spatial audio scene. Experimental results using both complex and spherical scatterers will be presented.

Reproduction of Nearby Sound Sources Using a Modified Distance Coding in Higher Order Ambisonics

Reproduction of Nearby Sound Sources Using a Modified Distance Coding in Higher Order Ambisonics

A Musical Journey towards Permanent High-Density Loudspeaker Arrays

Abstract Historically, most acousmatic works were composed in stereo and performed, or “diffused”, over loudspeaker orchestras. These systems furnished performers and composers with a wealth of opportunities to enhance the spatial contrast, motion, and musical articulations latent in the music. Although loudspeaker orchestras, stereo diffusion, and—more recently—hybrid performance techniques remain alive, especially in Europe, there is a trend towards fixed installation, high-density loudspeaker arrays (HDLAs), which I will call permanent HDLAs to differentiate them from loudspeaker-orchestra HDLAs. Permanent HDLAs (P-HDLAs) stimulate alternative approaches to musical space and pose challenges in both spatial composition and performance, encompassing both technology and aesthetics. This article consolidates many of the technical and aesthetic approaches that I have found specific to P-HDLAs with an emphasis on the application of higher-order Ambisonics (HOA) at fourth-order 3-D and above—what I will call V-HOA (“very high-order” Ambisonics) as distinguished from lower orders, or L-HOA (lower than fourth-order HOA). These approaches are guided by spatial-audio research, and then developed in a musical context. The study is divided into three sections: a description of musical and technical approaches that I have found to function over different P-HDLA installations; reflections on the compromises of lower-order monitoring during the compositional process; and presentation of the “Virtualmonium”: an instrument for the performance of stereo works that benefits from the growing popularity of P-HDLA installations.

Holophonic Sound in IRCAM's Concert Hall: Technological and Aesthetic Practices

This article presents a report on technological and aesthetic practices in the variable-acoustics performance hall, Espace de Projection, at the Institut de Recherche et Coordination Acoustique/Musique. The hall is surrounded by a 350-loudspeaker array for sound-field reproduction using holophonic approaches such as wave-field synthesis and higher-order Ambisonics. First we present the design and implementation of the audio system and discuss the challenges of both hardware and software architectures. This is followed by a discussion of spatial composition techniques, aesthetic approaches, and methodologies for composing computer music for high-density loudspeaker arrays, explored through the paradigmatic examples of pieces produced by two artist-in-research residencies.

Scene-based Audio Implemented with Higher Order Ambisonics

Scene-based audio uses a sound-field technology called 'higher-order ambisonics' (HOA) to create holistic descriptions of both live-captured and artistically created sound scenes that are independe ...

A mixed-order ambisonic scheme to improve performance for sound sources on the horizontal plane

This paper proposes a mixed-order ambisonic scheme called #P#L that improves performance in terms of estimation of the coefficient of spherical harmonics expansion, reconstruction of sound fields, and spatial resolution for sound sources located on the horizontal plane. Mixed-order ambisonics (MOA) selects appropriate components of spherical harmonics expansion instead of truncating up to a certain order as in higher-order ambisonics (HOA). Several schemes of MOA including the proposed scheme and HOA are compared each other in several ways. The results show that the proposed scheme has several advantages compared with the other MOA schemes. This scheme can be useful to further improve performance of microphone arrays if the number of the microphones is greater than what is required for HOA schemes or a non-uniform layout of microphones is used.

Multi-channel loudspeaker reproduction and virtual acoustic environments in the context of hearing aid evaluation

Virtual acoustic environments reproduced via multiple loudspeakers are increasingly used for evaluating hearing aids in complex acoustic conditions. To further establish the feasibility of this approach, this study investigated the interaction between spatial resolution of different reproduction methods and technical and perceptual hearing aid performance measures using computer simulations. Three spatial audio reproduction methods—discrete speakers, vector base amplitude panning and higher order ambisonics—were compared in regular circular loudspeaker arrays with 4 to 72 channels. The influence of reproduction method and array size on performance measures of multi-microphone hearing aid algorithm classes with spatially distributed microphones and a representative single channel noise-reduction algorithm was analyzed. In addition to the analysis of reproduction methods, algorithm performance was tested in a number of different virtual acoustic environments in order to assess the underlying factors of decr...

Head-related transfer function range extrapolation and near-field compensated high-order Ambisonics

Head-related transfer function range extrapolation (HRTF-RE) predicts HRTFs at various source distances from the known HRTFs at a fixed source distance and discrete directional grid. Near-field-compensated higher order Ambisonics (NFC-HOA) recreates virtual source at various distances by using a set of loudspeakers arranged on a spherical surface with fixed radius. HRTF-RE and NFC-HOA seem to be two different fields and were studied individually in previous works. The present work analyzes the relationship between HRTF-RE and NFC-HOA. Based on the spherical harmonics-Bessel expansion of sound field, it is proved that HRTF-RE and NFC-HOA are mathematically equivalent and therefore share some common characters and limitations. Based on the equivalence, the interchanging some of the methods used by the two fields is discussed. [This work was supported by the Guangzhou Science and Technology Plan Projects under No. 2014Y2-00021.]

Subjective attributes of realistic sound fields: What makes acoustic environments complex?

Real-life acoustic environments are commonly considered to be complex, because they contain multiple sound sources in different locations, room reverberation, and movement—all at continuously varying levels. In comparison, most laboratory-generated sound fields in hearing research are much simpler. However, the continuum between simple and complex fields is likely to be a function of multiple instrumental as well as perceptual factors. Although most of the applied knowledge has been gathered about simple, controlled, and synthetic acoustic environments, inferring listeners’ performance and perception from that to realistic scenarios is non-trivial. In order to clarify some of these relationships, subjective responses to twelve virtualized acoustic environments were examined using exploratory questionnaires with both descriptive and rating questions. These were specifically designed for the purpose of identifying attributes that may correlate with perceived acoustic complexity. Environments were recorded and processed using higher-order ambisonics, and reproduced in a 41-loudspeaker array in an anechoic chamber. Eleven environments spanned from a quiet library to a loud food court and diffuse noise was used as a control environment. Attributes included questions about busyness, variability, reverberance, annoyance, and others. The results enable us to break down the perceived complexity to its constituent factors.

Synthesis of wave surface by frame loudspeaker array with limited spatial frequency band

We have studied a frame loudspeaker array for sound reproduction of three-dimensional television aiming at realizing clear localization and better sound depth control. Because such array has no secondary sources in its center region, the reproduced wave surface is sometimes deteriorated even if the least squares optimization will be used to reshape the surface. Over the past few decades, a considerable number of studies have been made on the sound field reproduction by loudspeaker array. Recently, higher order ambisonics (HOA) and near-field compensated HOA (NFC-HOA) attract attention of the researchers. It is well known that these methods can limit a spatial frequency band by controlling the degree of spherical function. If such limitation is introduced to the frame loudspeaker array, the deterioration of wave surface will be alleviated because the optimization will be easy to progress by matching only the low spatial frequency components of the reproduced wave surface with those of the reference. The co...

Three-dimensional audio immersion: Development and human perception testing of a large scale, 128 loudspeaker surround sound system

Large-scale loudspeaker arrays for immersive audio are beginning to see increased interest in the research and entertainment industries. There are many algorithms to produce 3D sound fields. Although these algorithms are well developed, there is little published work that quantitatively compares their performance in terms of human perception for various numbers of speakers and speaker arrangements. In this talk, we will present listener perception test results using a 3D, 128 loudspeaker array. This loudspeaker array is part of the CUBE theatre at Virginia Tech. The CUBE is a 5-storey 50 ft. x 50 ft. black-box theatre that uses Dante™ audio-over-ethernet to send signals to the 128 loudspeakers. Combined with a 3D motion tracking system and an immersive video vision system. The CUBE provides a very high-resolution test bed for human perception studies. Using speech intelligibility tests with stimulus phrases and various interferers and noise fields, listener tests will be performed for surround sound algorithms such as Higher Order Ambisonics, Vector Based Amplitude Panning, Wave Field Synthesis, and Higher Order HARPEX (High Angular Resolution Plane Wave Expansion). The Higher Order HARPEX decoder is a novel improvement to First Order HARPEX. Also, we will present technical details of the development of a 16-element soundfield microphone.

Multichannel Acoustic Echo Cancellation in the Wave Domain With Increased Robustness to Nonuniqueness

Acoustic echo cancellation (AEC) is significantly more challenging in the case of multichannel reproduction compared to the single-channel case. This is mainly due to the so-called nonuniqueness problem, which becomes especially severe for massive multichannel reproduction systems like, e.g., wave field synthesis systems or higher-order ambisonics, where the different loudspeaker signals are often strongly correlated. In this paper, a multichannel wave-domain AEC structure is proposed that maintains robustness without altering the loudspeaker signals, as is commonly required to preserve the reproduction quality. The proposed method exploits the dominance of certain couplings in the wave-domain representation of the loudspeaker-enclosure-microphone system. In this representation, preferring a strong weight of the dominant couplings in the identified system improves the system identification performance. A modification of the well-known generalized frequency-domain adaptive filtering algorithm is then presented to implement the proposed approach, including a self-contained description of all necessary components. An experimental evaluation compares the proposed method to competing approaches and shows that the proposed method compares well in terms of convergence behavior while avoiding the performance degradation, which is intrinsic to other approaches.