Does one’s skull vibrate when chewing biscuits?  The answer is yes,
up to a point – that’s according to a recentl experimental study performed
by the Department of Oral-Maxillofacial Surgery, Prosthodontics and
Special Dental Care Oral Physiology Group, at the University Medical
Center Utrecht, The Netherlands, along with the Department of Prosthetic
Dentistry, Faculty of Dentistry, Lutheran University of Brazil, Porto
Alegre, Brazil.

Researcher Andries van der Bilt
and colleagues used an inertial measurement system to quantify skull
vibrations in volunteers who were eating three crispy foods (biscuits)
and one non-crispy food (cake). Analysis of the results showed marked
differences.

“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.”

Practical applications:

The measurement of skull vibration, produced by the breakage of crispy food during chewing, provides information on how food characteristics gradually change during chewing. This information may help to understand the relationship between food structure and texture perception.

SKULL VIBRATION DURING CHEWING OF CRISPY FOOD is published in the Journal of Texture Studies Volume 41, Issue 6, pages 774–788, December 2010.