The characteristic 'whoop' sound in pertussis is due to:

The characteristic "whoop" sound in pertussis occurs due to rapid inspiration against a narrowed, edematous airway following a series of intense coughing spells (paroxysms). After the cough, the glottis partially closes, creating a high negative intrathoracic pressure, which causes the distinctive high-pitched "whooping" noise during inspiration. This is a hallmark of pertussis and reflects airway obstruction rather than expiration or wheezing.

Reference: Harrison's Principles of Internal Medicine, J. Larry Jameson, 20th Edition.