Spatial Auditory Human-Computer Interfaces

Research and development work in the field of electrical engineering and other technical sciences has always aimed to develop more powerful devices with ever-increasing processor power and responsiveness (Sodnik et al. 2014). A reduction in the size of modern devices and the extension of their use to new fields and environments has been a significant design trend in recent years. Mobile devices enabling various types of communication with and access to important sources of information in mobile environments constitute an important segment of modern electronics. Mobile devices can be general-purpose laptops, notebooks, tablets or smartphones, or can be highly-adapted purpose-built devices found in vehicles and other means of transport. In terms of the range of functionalities and the corresponding software, the majorities of advanced mobile devices are becoming very similar to traditional desktop computers, and thus allow for continuous upgrades and the addition of new applications.

Sound is a vibration which propagates as a mechanical wave through various types of compressible media (air, gas, liquids, or even solids). In most cases it is a longitudinal mechanical wave of pressure and displacement, although it can also have the form of a transverse wave in solids. The most important properties of sound are its amplitude, frequency (period), wavelength and speed of propagation (Kinsler et al. 1999). In a free field, sound is not subject to many influences and is therefore not reflected, absorbed deflected or refracted in any way. The propagation of a sound wave is commonly described by the three-dimensional acoustic wave equation for sound pressure in the Cartesian coordinate system:where x, y and z denote the three coordinates in space, t denotes time, p denotes pressure and c denotes the speed of sound. The speed of sound in dry air at 20 °C at sea level is 343 meters per second (m/s) and it increases with the density of a medium or its temperature. The speed of sound in water is approx. 1500 m/s while the speed of sound in iron is almost 5200 m/s (Bamber 1986). The speed increases for approx. 0.6 m/s if the temperature of the air increases by 1 °C.

People are very social beings and interact with their environment in many ways (O’Shaughnessy 2003). Information can be received through seeing, hearing, smelling, tasting or feeling, while, conversely, it can be sent back into the environment visually, vocally, and through body gestures. Interaction between a user and a machine is performed through a user interface that consists of various hardware and software components. Its main functionality is to transform the users’ orders and requirements into a set of machine commands and also to provide the corresponding feedback to the user, preferably in real-time. The available communication channels between a user and a machine are limited by human senses and their capabilities on the one hand and the technological restraints of input–output devices on the other. The primary communication channels for the majority of people are seeing and hearing, and consequentially visual and auditory interfaces are the dominant types of user interfaces in the HMI domain. The major paradigm of our times is undoubtedly mobility. The mobile life, such as driving a car, cycling or walking, requires a lot of visual attention. Interacting through a visual interface in these situations leads to distraction from one’s primary task and consequently to a difficult, dangerous, ineffective and potentially frustrating user experience. In this chapter we present the basic properties and advantages of auditory interfaces that offer an excellent alternative to Human-Machine Interaction (HMI) in visually-demanding mobile situations. Auditory interfaces are very flexible and they do not interfere significantly with the reception and processing of visual information.

Spatial auditory interfaces use three-dimensional sound as an additional display dimension and consist of audio items at different spatial locations. They have evolved significantly in the last couple of years and can be found in a variety of environments where visual communication is obstructed or completely blocked by other activities, such as walking, driving, flying, operating multimodal virtual displays, etc. The precise spatial position of each source can offer an additional informational cue in the interface or can simply help resolving various ambiguities in the content of simultaneously-played sources. It can also be used to increase realism in virtual worlds by imitating real environments where the majority of sounds can be localized and associated with their sources.