In order to obtain an effective speech communication in rooms it is advisable, besides reaching the full intelligibility of words, to minimize the effort paid by the listener in the recognition of the speech material. This twofold requirement is not easily described by the current room acoustic indicators, which are mainly concerned either with a subjective rating by means of word recognition scores or with using listeners' impressions of reported listening difficulties. In this work, the problem is tackled by introducing the concept of "listening efficiency," which is defined as a combination of the accuracy of intelligibility and of the effort spent on achieving this goal. This indicator is here developed, and an application of the former and of the "listening efficiency" is presented in the field of classroom acoustics. Listening tests with pupils and adults were performed and the subsequent statistical analyses indicated several interesting findings. In particular, listening efficiency is able to clearly discriminate between equal intelligibility scores obtained under different acoustical conditions, permitting room acoustics to be tailored for specific groups, such as children.