Perceptual Evaluation of Spatial Audio: Where Next?

From the early days of reproduced sound, engineers have sought to reproduce the spatial properties of sound fields, leading to the development of a range of technologies. Two-channel stereo has been prevalent for many years; however, systems with a higher number of discrete channels (including rear and height loudspeakers) are becoming more common and, recently, there has been a move towards loudspeaker-agnostic methods using audio objects. Perceptual evaluation, and perceptually-informed objective measurement, of alternative reproduction systems can inform further development and steer future innovations. It is important, therefore, that any gaps in the field of perceptual evaluation and measurement are identified and that future work aims to fill those gaps. A standard research paradigm in the field is identification of the perceptual attributes of a stimulus set, facilitating controlled listening tests and leading to the development of predictive models. There have been numerous studies that aim to discover the perceptual attributes of reproduced spatial sound, leading to more than fifty descriptive terms. However, a literature review revealed the following key problems: (i) there is little agreement on exact definitions, nor on the relative importance of each attribute; (ii) there may be important attributes that have not yet been identified (e.g. attributes arising from differences between real and reproduced audio, or pertaining to new 3D or object-based methods); and (iii) there is no model of overall spatial quality based directly on the important attributes. Consequently, the authors contend that future research should focus on: (i) ascertaining which attributes of reproduced spatial audio are most important to listeners; (ii) identifying any important attributes currently missing; (iii) determining the relationships between the important attributes and listener preference; (iv) modelling overall spatial quality in terms of the important perceptual attributes; and (v) modelling these perceptual attributes in terms of their physical correlates.