  CAPTCHAs are employed as a security measure to differentiate human users from
bots. A new sound-based CAPTCHA is proposed in this paper, which exploits the
gaps between human voice and synthetic voice rather than relays on the auditory
perception of human. The user is required to read out a given sentence, which
is selected randomly from a specified book. The generated audio file will be
analyzed automatically to judge whether the user is a human or not. In this
paper, the design of the new CAPTCHA, the analysis of the audio files, and the
choice of the audio frame window function are described in detail. And also,
some experiments are conducted to fix the critical threshold and the
coefficients of three indicators to ensure the security. The proposed audio
CAPTCHA is proved accessible to users. The user study has shown that the human
success rate reaches approximately 97% and the pass rate of attack software
using Microsoft SDK 5.1 is only 4%. The experiments also indicated that it
could be solved by most human users in less than 14 seconds and the average
time is only 7.8 seconds.
