Audio captchas when visual images are unusable

Posted by T.V. Raman, Research Scientist

From time to time, our own T.V. Raman shares his tips on how to use Google from his perspective as a technologist who cannot see -- tips that sighted people, among others, may also find useful. - Ed.

Wikipedia defines 'captcha' as an acronym for "Completely Automated Public Turing test to tell Computers and Humans Apart" -- a word which is trademarked by Carnegie Mellon University. Most web users think of captchas as those hard to read distorted letters or images that one often is confronted by when websites attempt to verify that they're indeed talking to a live human. Google Accounts support captchas. Of course, bloggers (no matter which platform they use) can also use them to prevent comment spam.

Captchas were never intended to be purely visual -- however, most initial implementations used fuzzy images, and in attempting to lock out automated agents also inadvertently locked out people unable to see the image. As an alternative to these, this past spring Google Services that require verification began to provide an audio alternative -- people have the option of listening to a sequence of spoken digits that they then type into a form field to verify to the web application that there is indeed a live human at the other end.

To keep the audio captcha as challenging as the visual captcha when confronted by automated agents, we add some distortion to the spoken digits, and we're still experimenting with different distortion techniques to ease the burden on the genuine human user while locking out automated agents. We welcome feedback on the effectiveness of these techniques from you (we automatically collect feedback from those evil automated agents pretending to be human) :-).

You can easily spot the availability of audio captchas by the presence of the well-recognized "wheelchair" icon for accessibility --- the image is tagged with appropriate alt text to help blind users. Incidentally you don’t have to be visually impaired to use the audio captcha; if you are in a situation where you find it hard to view the visual captcha -- either because you're at a non-graphical display, or because the specific visual challenge we offered you turned out to be unusable in a given situation, feel free to give the audio captcha a try. We've worked hard to ensure that the audio captchas work on different hardware/software combinations, and you do not need any special hardware (or software) other than a sound card to be able to use them.