# Guest Editors' Note--Special Issue on Spatial Audio
###### Abstract  
The aim of this paper is to provide a new approach to the design of sound sound-based systems. The proposed approach is to use a novel approach to the design of sound-based systems. The perception of sound localization has been studied over a decade, but the localization of elevated sound sources still remains a challenging problem. [PERSON] and [PERSON] [4] study the influence of the interchannel time and level differences between two loudspeaker layers of different heights on the localization threshold. The results show the dependence of the localization threshold on the interchannel time difference. The required directivity of recording microphone in height direction is also discussed based on the identified localization threshold. Objective evaluation of the reproduced sound field is another important issue. Mean squared error (MSE) has been popularly employed as a measure of similarity between target and reproduced sound fields. However, in their work [5], [PERSON] and [PERSON] propose to use beamforming powers derived from given sound fields as a new measure. The primary reasoning behind the proposed measure is the weakness of MSE against room reflections and for 2.5-D reproduction techniques, such as wave field synthesis, that inevitably exhibit amplitude bias along the distance. The beam-power measure is expected to provide a more robust means to evaluate the directional cue or the direct component of a sound field. The study by [PERSON] and [PERSON] [6] deals with the objective evaluation problem for the pairwise panning-based upmix algorithm. To access the sound quality of upmix algorithms without subjective evaluations, they propose detailed procedures and measures regarding the direction of a virtual sound source, the amount of residual direct sound, loudness, and correlations in the frontal and surround channels. The localization of sound is not only involved with the directional cue. [PERSON], [PERSON], [PERSON], and [PERSON] [7] investigated the way to control the perceived distance through the variation of source directivities. The influence of the auralized room, source-listener distance, signal, and single-channel reverberation are considered together to build a model predicting the perceived distance. They tested various third-order beam patterns in a real room, which demonstrate that the distance perception caused by the source directivity is coupled with the sense of apparent source width. For spatial audio, there are many auditory impressions to be carefully controlled along with the localization cues. Auditory sceneries deliver various spatial impressions such as stage width and ambience. The synthesis of late reverberation is studied in the paper written by [PERSON], [PERSON], [PERSON], and [PERSON] [8]. They segmented the late part of a room impulse response and approximated the segments as filtered velvet noises that are very sparse in time but sound smoother. It is demonstrated that filtering with velvet noises greatly reduces the computational cost, only resulting in minor subjective differences for transient sounds. The paper authored by [PERSON], [PERSON], [PERSON], [PERSON], and [PERSON] [9] proposes a general strategy to tackle the inverse problem that is often encountered in solving the source identification and separation problems for the spatial audio signal processing. Various inverse problem solvers, for both underdetermined and overdetermined problems, are investigated and compared in terms of PESQ and segSNR. Guidelines for choosing the right algorithm and regularization parameter are provided, with detailed examples of sound field analysis and synthesis problems. Another inverse problem discussed by [PERSON], [PERSON], and [PERSON] [10], is for the interactive auralization of sound fields in a low-frequency region. They utilized the finite element method to simulate a sound field in a room and then transformed the result using a plane wave expansion technique.