## 2 Background
### Speech in AG Design  
Speech occupies just a portion of our everyday acoustic environment, yet it constitutes the primary medium for information distribution [16]. Research in the field of psychoacoustics has demonstrated the increased sensitivity of human hearing in the mid-frequency range (from lower-mid to higher-mid) [17], hinting at the species' evolutionary focus on the voice and the messages it expresses. In audio-visual media, the voice has been said to have a captivating power compared to other elements, a phenomenon which has been called \"vococentrism\" [18]. When included in an audio-only environment, such as an audio film, words and phrases become so vital for understanding the action that they can distract from other non-verbal sounds or even overshadow them [19]. In AG design, speech is regarded as one of the three basic auditory elements along with music and sound, namely the one responsible for the transmission of knowledge: from the announcement of news or advice to the description of scenes, actions, and plot [20; 21]. Researchers consider the dependency on verbal information to be one of the genre's decisive factors, with some even discerning between two types of AG: those using spoken descriptions and those using only non-verbal audio cues [10; 22]. Speech has also served as one of the main modalities to facilitate accessibility for the visually impaired. Since the early history of AG around the end of 1970s, any text-based game can become accessible to blind players via a screen reader [8]. The existing uses of speech in AG design include: (i) instruction, (ii) description, (iii) narration, and (iv) mechanics. These categories are not to be seen as disconnected from one another, but rather as intertwined manifestations of one common, inherent characteristic: they all rely on the semantic function of speech, on the meaning it carries. The following examples, drawn not only from AGs but also from the broader field of audio-interactive applications, demonstrate the broad range of the semantic use of speech. On the most basic level, verbal audio delivers the necessary instructions for setting up and starting the game. Listening to the available options is particularly useful to visually impaired players who cannot rely on any graphical interface [23]. The instructions on how to enter the game are often complemented by a brief description of the goal and relevant tasks [24]. In other cases, an audio tutorial guides new players throughout their first contact with the game environment [25]. Spoken instructions are also included in the main game to describe the scene and remind players of current tasks [26]. This support can be in the form of an audio guide that introduces players to the important elements of each stage [27]. [PERSON] et al. have reviewed various educational AGs on subjects including mathematics, programming, and biology, in which speech delivers instructions, announces the tasks at hand, and explains the educational content [28]. In terms of description, verbal audio is regarded as the most direct way to describe visual information. Albeit tedious and not without disadvantages compared to other sonification techniques, speech is more flexible in attributing subtle differentiations and/or sub-categories of an information structure [29]. Personal audio guides typically utilise spoken descriptions of cultural content to offer augmented-reality-enhanced tourist experiences [30]. Verbal description of points of interest can not only refer to the real world but also represent in detail a fictional world, such as the medieval setting with images and monsters in [31]. The more an audio guide is combined with the game element of exploration, the more mere description of information is enriched with narrative elements. Players are told the game's story and provided with cues as to the necessary route or other hints about required actions, to follow that story [32]. In that context, speech lies at the intersection of describing the game world and being part of the game's mechanics.