 At the posterior end of this spermaceti complex is the frontal sac , which covers the concave surface of the cranium . The posterior wall of the frontal sac is covered with fluid – filled knobs , which are about 4 – 13 mm in diameter and separated by narrow grooves . The anterior wall is smooth . The knobbly surface reflects sound waves that come through the spermaceti organ from the phonic lips . The grooves between the knobs trap a film of air that is consistent whatever the orientation or depth of the whale , making it an excellent sound mirror . 
