# A Review of Recent Literature on Audio-Based Pseudo-Haptics
## 1 Introduction
### 1.2. Haptic Devices  
A haptic device can provide users with a sense of touch [16]. More specifically, a haptic device provides touch feedback that is perceived by applying tactile and/or kinesthetic stimuli to sense and/or manipulate objects with a user input device, allowing the user to interact with virtual or tangible objects [30]. Devices can engage both kinesthetic and/or cutaneous feedback and provide users the feeling of touch [18]. [PERSON] et al. [29] divide haptic devices into three major categories: (i) graspable, (ii) wearable, and (iii) touchable. Graspable haptic devices typically rely on kinesthetic (force) feedback, are grounded (e.g., attached to a desk/table) and allow the user to push on them using an end effector held by the user. Wearable haptic devices typically employ tactile (cutaneous) feedback and are mounted on some part of the body (e.g., hands in the form of a haptic data glove) and provide feedback directly to the skin via mechanoreceptors. Wearable devices are attractive for mobile applications where the user is required to move within their environment [29]. With touchable haptic devices, the user is able to manipulate the entire surface (e.g., touch-based monitor/display). As [PERSON] et al. [22] point out, compared to the conventional (graspable/grounded) haptic devices that are cumbersome, lightweight, miniaturized haptic feedback interfaces that include portable, wearable, or even skin-integrated formats, are more suitable for the next generation of human-machine interfaces (including those used in virtual and augmented reality) and electronics. Wearable haptic devices, particularly those that are skin-integrated, are thoroughly reviewed by [PERSON] et al. [22]. Existing haptic solutions can be cumbersome and complex, requiring robotic mechanisms comprised of several actuators, sensors, and processing units [31]. Having such complex systems leads to cost-prohibitive solutions, particularly when high fidelity is required [32]. Simple haptic feedback is widely available at the consumer level through devices that include actuators that provide simple vibration feedback (e.g., video game controller \"rumble packs\" or mobile phone vibrations). As consumer interest in haptics grows, the application of lower-fidelity consumer-level haptic devices capable of providing force feedback beyond simple vibrations will become more widespread, given their decreasing cost [33]. However, the currently available consumer-level haptic devices cannot provide the higher level of fidelity and the range of motion required to simulate many tasks realistically [34]. Nevertheless, these lower-end devices are proving to be effective in entertainment, design, and educational applications, where higher levels of fidelity may not necessarily be required. In the absence of adequate haptic devices, haptic sensations can still be invoked by leveraging cross-modal illusions to invoke haptic sensations [33]. This emerging field is known as pseudo-haptics and is grounded in the work of [PERSON] [35], who showed that the visual representation of a virtual object can affect the integration of the touch-based feedback. Pseudo-haptics has been employed to convey various touch-based properties, including friction [36], stiffness [37], mass [38], and texture [39]. Most of the pseudo-haptics work has focused on visual cues rather than auditory (see [40; 41]). This requires the use of a display to present the visual information and therefore will not work with interfaces that do not include a display or with visually impaired users [42]. However, sound (and a combination of sound and visual cues) can also be used to induce the illusion of touch. [PERSON] and [PERSON] [33] provided an overview of the literature in pseudo-haptics with an emphasis on cross-modal integration and perception across the three domains most common to virtual environments, and more specifically, visuals, sound, and haptics [33].