Whalen D H, Liberman A M
Science. 1987 Jul 10;237(4811):169-71. doi: 10.1126/science.3603014.
Some components of a speech signal, when made more intense, are heard simultaneously as speech and nonspeech--a form of duplex perception. At lower intensities, the speech alone is heard. Such intensity-dependent duplexity implies the existence of a phonetic mode of perception that takes precedence over auditory modes.