Binaural rendering of Ambisonic signals is of great interest in the fields of virtual reality, immersive media, and virtual acoustics. Typically, the spatial order of head-related impulse responses (HRIRs) is considerably higher than the order of the Ambisonic signals. The resulting order reduction of the HRIRs has a detrimental effect on the binaurally rendered signals, and perceptual evaluations indicate limited externalization, localization accuracy, and altered timbre. In this contribution, a binaural renderer, which is computed using a frequency-dependent time alignment of HRIRs followed by a minimization of the squared error subject to a diffuse-field covariance matrix constraint, is presented. The frequency-dependent time alignment retains the interaural time difference (at low frequencies) and results in a HRIR set with lower spatial complexity, while the constrained optimization controls the diffuse-field behavior. Technical evaluations in terms of sound coloration, interaural level differences, diffuse-field response, and interaural coherence, as well as findings from formal listening experiments show a significant improvement of the proposed method compared to state-of-the-art methods.

To improve the limited degree of immersion of static binaural rendering for headphones, an increased measurement effort to obtain multiple-orientation binaural room impulse responses (MOBRIRs) is reasonable and enables dynamic variable-orientation rendering. We investigate the perceptual characteristics of dynamic rendering from MOBRIRs and test for the required angular resolution. Our first listening experiment shows that a resolution between ${15}^{\circ }$ and ${30}^{\circ }$ is sufficient to accomplish binaural rendering of high quality, regarding timbre, spatial mapping, and continuity. A more versatile alternative considers the separation of the room-dependent (RIR) from the listener-dependent head-related (HRIR) parts, and an efficient implementation thereof involves the measurement of a first-order Ambisonic RIR (ARIR) with a tetrahedral microphone. A resolution-enhanced ARIR can be obtained by an Ambisonic spatial decomposition method (ASDM) utilizing instantaneous direction of arrival estimation. ASDM permits dynamic rendering in higher-order Ambisonics, with the flexibility to render either using dummy-head or individualized HRIRs. Our comparative second listening experiment shows that 5th-order ASDM outperforms the MOBRIR rendering with resolutions coarser than ${30}^{\circ }$ for all tested perceptual aspects. Both listening experiments are based on BRIRs and ARIRs measured in a studio environment.

The quote from Pierre Boulez, given as an epigraph to this article, inspired French researchers to start developing technology for spherical loudspeaker arrays in the 1990s. The hope was to retain the naturalness of sound sources. Now, a few decades later, one might be able to show that even more can be done: In electroacoustic music, using the icosahedral loudspeaker array called IKO seems to enable spatial gestures that enrich alien sounds with a tangible acoustic naturalness. After a brief discussion of directivity-based composition in computer music, the first part of the article describes the technical background of the IKO, its usage in a digital audio workstation, and psychoacoustic evidence regarding the auditory objects the IKO produces. The second part deals with acoustic equations of spherical beamforming, how the IKO's loudspeakers are controlled correspondingly, how we deal with excursion limits, and the resulting beam patterns generated by the IKO.

Can auralization of a highly directional source in a room succeed if it employs a room impulse response (RIR) measurement or simulation relying on a first-order directional source, only? This contribution presents model and evaluation of a source-and-receiver-directional Ambisonics RIR capture and processing approach (SRD ARIR) based on a small set of responses from a first-order source to a first-order receiver. To enhance the directional resolution, we extend the Ambisonic spatial decomposition method (ASDM) to upscale the first-order resolution of both source and receiver to higher orders. To evaluate the method, a listening experiment was conducted based on first-order SRD-ARIR measurements, into which the higher-order directivity of icosahedral loudspeaker’s (IKO) was inserted as directional source of well-studied perceptual effects. The results show how the proposed method performs and compares to alternative rendering methods based on measurements taken in the same acoustic environment, e.g., multiple-orientation binaural room impulse responses (MOBRIRs) from the physical IKO to the KU-100 dummy head, or higher-order SRD ARIRs from IKO to em32 Eigenmike. For optimal externalization, our experiments exploit the benefits of virtual reality, using a highly realistic visualization on head-mounted-display, and a user interface to report localization by placing interactive visual objects in the virtual space.

Virtual acoustic environments benefit from involving the source and receiver directivity to become fully interactive. In this contribution we discuss virtualization of a room based on measured source-and-receiver-directional room impulse responses (SRD RIR), captured by first-order-directional source and receiver arrays. Low order arrays may in some ways overcome a directional versus temporal resolution trade off one encounters in directional measurements. This is done by employing the spatial decomposition method of room impulse responses on both the first-order source and receiver side. We discuss the result of these resolution-enhanced and efficiently measured SRD RIR based on a comparative listening experiment involving the variable-directivity icosahedral loudspeaker (IKO), a spherical beamforming loudspeaker array, whose perceptual sculptural effects have been studied in a room. For the virtualization of the IKO, high resolution directivity measurement of the IKO are inserted, and for its rendering, a high-resolution set of head related impulse responses (HRIRs).