With an ever broadening use of composite materials manufacturers are in high demand of efficient curing cycles to reduce costs and speed up production cycles. One method to to archive this goal is active cure monitoring to determine the exact time of curing needed. This article provides a novel method to measure the cure inside of closed tools by using ultrasonic spectroscopy. For this a simple experiment is used to show the change of the ultrasonic spectrum during the cure of an epoxy. The results clearly show a direct correlation of amplitude and state of cure where the amplitude reaches a global minimum at the glass point.