Context and Motivation: Attack-Defense Trees (ADTs) are a graphical notation used to model and assess security requirements. ADTs are widely popular, as they can facilitate communication between different stakeholders involved in system security evaluation, and they are formal enough to be verified, e.g., with model checkers. Question/Problem: While the quality of this notation has been primarily assessed quantitatively, its understandability has never been evaluated, despite being mentioned as a key factor for its success. Principal idea/Results: In this paper, we conduct an experiment with human subjects to assess the understandability and user acceptance of the ADT notation. The study focuses on performance-based variables and perception-based variables, with the aim of evaluating the relationship between these measures and how they might impact the practical use of the notation. The results confirm the general understandability of ADTs, as well as the intention to use them. Contribution: This is the first study that empirically confirms the understandability of ADTs, thereby contributing to theory in security requirements engineering.