An Investigation into the Design of Structured Vibrotactile Messages for Non Visual Information Display
Lorna Brown

Introduction
Vibrotactile displays are becoming increasingly common in everyday devices, with mobile phones, pagers and games controllers all featuring vibration feedback. However, the vibrations used in such devices are usually basic and do not fully exploit the potential of vibration as a means of communication. In order to create more effective messages, it is necessary to systematically investigate how best to use vibration in computer interfaces. Therefore this research brings together an understanding of cutaneous perception and interaction design and investigates how to construct effective vibrotactile messages. This document presents the background to this work, the results so far, and discusses plans for future work.

Tactons
Tactons [1] are structured vibrotactile messages which can be used to communicate information non-visually. They are the tactile equivalent of audio Earcons [2] and visual icons, and could be used for communication in situations where vision is overloaded, restricted, or unavailable, such as navigation systems for blind people, or in mobile/wearable computers. This research aims to show that multidimensional information can be encoded in Tactons using parameters of vibration such as waveform, rhythm and spatial location, and that users can learn mappings between Tactons and their meanings, and retrieve information or alerts from Tactons when used in mobile or desktop computer interfaces.

Hardware
Tactons can be displayed using small vibrating transducers. Figure 1 shows the transducer used in this research: the Engineering Acoustics C2 Tactor (www.eaiinfo.com).

Figure 1: Engineering Acoustics C2 Tactor (www.eaiinfo.com).

Main Contributions of this Research
While tactile messages (both vibrotactile and electrotactile) have been proposed and used by a number of researchers [3, 4] there have been no formal studies of how best to design such messages. The novel contribution of this research is a formal evaluation of Tactons, and of the individual parameters used to construct them, in order that a set of guidelines for designing effective Tactons can be produced. In addition, the use of Tactons in mobile and desktop applications will be evaluated so that their effectiveness in real world applications can be assessed.

Work carried out so far
Identifying Parameters for Tactons
Before Tactons can be created, appropriate vibrotactile parameters in which information can be encoded must be identified. So far, two parameters, namely waveform and rhythm, have been tested and identified as successful parameters for Tactons [5]. Other parameters such as spatial location are currently being investigated. The waveforms used in this research are amplitude-modulated sinusoids and are categorised in terms of how "rough" they feel. Three values of roughness which can be distinguished from one another were identified through a forced choice experiment in which participants were presented with pairs of vibrations and asked "which feels rougher?". The second parameter which has been investigated is rhythm. Three distinguishable rhythms were created based on Brewster's guidelines for Earcon design [2] which state that a different number of notes should be used in each rhythm so that they are as different as possible.

Figure 2: A 250Hz sinusoid modulated by a 30Hz sinusoid.

Figure 3: Rhythms used to represent calls/messages

First Evaluation of Tactons
In order to create effective Tactons it is necessary that users are able to learn the mapping between Tactons and the data encoded in them. An experiment was therefore carried out to determine whether people could learn and identify Tactons which used roughness and rhythm as parameters.
Tactons were created to represent alerts which might occur when a call or a message arrived on a mobile phone. Two pieces of information were encoded into each Tacton: the type of call/message (voice call/text message/multimedia message) was encoded in the rhythm, while the priority of the call/message (low/medium/high) was encoded in the roughness (smooth/rough/very rough).
After being trained to understand the mapping between Tactons and the data encoded in them, participants were presented with Tactons and asked to identify the type and priority of the call/message. The results showed 71% average overall identification of the Tactons, with 93% average identification of type/rhythm and 80% identification of priority/roughness [5].

Conclusions and Future Work
Overall the results so far results indicate that Tactons can be an effective means of communicating information non-visually. The results compare favourably to those for their audio counterpart, Earcons, where overall recognition is also around 70% [6]. Research has shown that Earcons are able to encode information in three parameters [6], therefore future work will investigate the effect of adding a third parameter (namely spatial location) to Tactons.
Tactile perception may be affected by being engaged in another activity, such as walking. Since one potential use of Tactons is in mobile and wearable computing interfaces, future work will investigate identification of Tactons when used in a mobile application. This will significantly further understanding of Tacton perception and allow more effective vibrotactile interfaces to be created.

References
1. Brewster, S. and Brown, L.M. "Tactons: Structured Tactile Messages for Non-Visual Information Display", in Proceedings of AUIC 2004, Dunedin, New Zealand: Australian Computer Society, pp 15 - 23.
2. Brewster, S.A., "Providing a Structured Method for Integrating Non-Speech Audio into Human-Computer Interfaces", PhD Thesis, University of York, UK, 1994
3. van Erp, J.B.F. and van Veen, H.A.H.C. "Vibro-Tactile Information Presentation in Automobiles", in Proceedings of Eurohaptics 2001, Birmingham, UK, pp 99-104.
4. Eves, D.A. and Novak, M.M. "Animated tactile sensations in sensory substitution systems", in Proceedings of The First European Conference on Disability, Virtual Reality and Associated Technologies 1996, Maidenhead, UK, pp 193-199.
5. Brown, L.M., Brewster, S.A., and Purchase, H.C. "A First Investigation into the Effectiveness of Tactons", in To Appear in Proceedings of World Haptics 2005, Pisa, Italy.
6. McGookin, D.K. and Brewster, S.A., "Understanding Concurrent Earcons: Applying Auditory Scene Analysis Principles to Concurrent Earcon Recognition", ACM Transactions on Applied Perception 1(2), 2004, pp. 130-155.