“Clear skull vibrations were observed while the subjects started to chew the biscuits. The skull vibrations gradually decreased upon further chewing down to the level of skull vibrations produced by chewing on noncrispy cake.”
And furthermore, it was found that the vibrations tailed-off completely when
“…the biscuits were chewed long enough to form a well-moistened food bolus that was ready for swallowing.”
is published in the Journal of Texture Studies Volume 41, Issue 6, pages 774–788, December 2010.
Further crispness research:
Massimiliano Zampini of the University of Trento, Italy and Charles Spence of Oxford University, UK, jointly claimed the 2008 Ig Nobel prize for Nutrition for electronically modifying the sound of a potato chip to make the person chewing the chip believe it to be crisper and fresher than it really is.