Sound absorption by openings has been rarely considered in room acoustics. In fact, information about small openings (such as ventilation grids) may sometimes be found, but nothing is said about larger openings, possibly as a consequence of the less likely occurrence in a design. In order to fill this gap, measurements were carried out in scale models, measuring the equivalent absorption due to different openings and comparing it with theoretical results. A “practical” model, showing a simple dependence on the opening dimension, was finally obtained and subsequently validated by measurements in a real room.