Evaluating Existing Audio CAPTCHAs and an Interface Optimized for Non-Visual Use  

Audio CAPTCHAs were introduced as an accessible alternative for those unable to use the more common visual CAPTCHAs, but anecdotal accounts have suggested that they may be more difficult to solve. This paper demonstrates in a large study of more than 150 participants that existing audio CAPTCHAs are clearly more difficult and time-consuming to complete as compared to visual CAPTCHAs for both blind and sighted users. In order to address this concern, we developed and evaluated a new interface for solving CAPTCHAs optimized for non-visual use that can be added in-place to existing audio CAPTCHAs. In a subsequent study, the optimized interface increased the success rate of blind participants by 59% on audio CAPTCHAs, illustrating a broadly applicable principle of accessible design: the most usable audio interfaces are often not direct translations of existing visual interfaces.

Bigham, Jeffrey P. and Anna C. Cavender. University of Washington-Seattle (2008). Articles>Accessibility>Security>Web Design