An experiment was conducted to study the effects of force produced by active touch on vibrotactile perceptual thresholds. The task consisted in pressing the fingertip against a flat rigid surface that provided either sinusoidal or broadband vibration. Three force levels were considered, ranging from light touch to hard press. Finger contact areas were measured during the experiment, showing positive correlation with the respective applied forces. Significant effects on thresholds were found for vibration type and force level. Moreover, possibly due to the concurrent effect of large (unconstrained) finger contact areas, active pressing forces, and long duration stimuli, the measured perceptual thresholds are considerably lower than what previously reported in the literature.

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The TouchBox is a low-cost human-computer interface yielding advanced auditory and vibrotactile feedback, made available in open-source form. It offers affordances similar to small touchscreens and isometric pointing devices as it tracks the position of up to two finger-pads in contact with its top surface, measures their contact areas as well as the applied normal and lateral forces. The interface is the result of several design iterations that on the one hand optimized its sensing accuracy and output reliability, and on the other hand expanded its input capabilities so as to measure various quantities relevant to everyday finger-based interaction. Applications range from using the interface as a calibrated measurement device to advanced human-machine interaction.The present work is supported by the research project 'Haptic technology and evaluation for digital musical interfaces' (HAPTEEV), funded by the Swiss National Science Foundation.

While a standard approach is more or less established for rendering basic vibratory cues in consumer electronics, the implementation of advanced vibrotactile feedback still requires designers and engineers to solve a number of technical issues. Several off-the-shelf vibration actuators are currently available, having different characteristics and limitations that should be considered in the design process. We suggest an iterative approach to design in which vibrotactile interfaces are validated by testing their accuracy in rendering vibratory cues and in measuring input gestures. Several examples of prototype interfaces yielding audio-haptic feedback are described, ranging from open-ended devices to musical interfaces, addressing their design and the characterization of their vibratory output.

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