## 4 Discussion
### Intelligibility  
Because of the lack of redundant auditory and visual cues found in synthesised speech, listeners must allocate more attentional resources to process synthesised speech, as compared to natural speech [76]. These factors, such as a lack of visual cues, are likely to influence the comprehensibility of any communication mode (natural or synthesised), when the speaker cannot be seen, such as what one might experience with telephone speech [77]. Similarly, natural speech is often comprehended faster than synthesised speech, especially when there is a high level of background noise or the listener's attention is divided [76]. However, as the quality of synthetic speech improves, the margin between synthetic speech and natural speech decreases, as does the cognitive load required [9]. In terms of what individuals prefer, it appears that people prefer listening to voices that match the vocal identity of the user [5, 9, 78]. Despite this, there is evidence to suggest that individualswould rather choose high intelligibility of the speech output instead [5,77,78]. A high level of intelligibility is therefore crucial in the development of synthetic speech.  
It has been shown that human listeners adjust in subtle but systematic ways to understand synthetic speech. Understanding synthetic speech often requires a greater cognitive load than understanding natural speech, but this cognitive load decreases when the listener becomes accustomed with the voice [9,76]. For children and adults, comprehensibility of the synthesised speech signal often improves after greater exposure [45]. This is also true for individuals with intellectual disabilities, as studies showed that their perception of the synthetic speech improved after systematic exposure to it [11,65].  
Results have suggested that context and the length of the utterance play a role in the intelligibility of synthesised speech [66,76]. In other words, longer utterances are more intelligible than single words, unless the listeners are given a closed response set (i.e., having a set of predefined answers and pointing to a picture in response to a stimulus) [11,65]. Longer utterances typically contain more linguistic context than single words, which listeners can use to increase signal-independent information. This is clinically important as some children with CCN may have comorbid difficulties, such as decreased working memory and impaired language skills, resulting in decreased use and comprehension of longer utterances [1]. Thus, single-word AAC devices are often common starting points for children with CCN. However, if their speech output is supported with a symbol or picture on their AAC device, it may assist their communication partners in understanding them. It is well-recognised that successful AAC communication depends on both the AAC user and their communication partners [2,8,79,80].  
Intelligibility of the speech output on a speech-generating AAC device is essential for several reasons. If children are unable to communicate with their natural voice, the speech output will assist them in learning how to use the AAC system; it provides feedback to the child and their communication partner and it allows for successful interaction opportunities with new communication partners [66]. Moreover, the speech output may provide additional verbal modelling, which could result in an increase in the child's spontaneous speech production [81]. Researchers have discussed how sound produced by an iPad or a SGD may act as a reinforcer for a child user, which subsequently motivates them to use it to communicate [81, 82, 83]. In one study, children with intellectual disabilities were also able to generalise their knowledge of the acoustic-phonetic properties of synthetic speech to novel stimuli [65]. This clearly has some vitally important clinical implications for children with intellectual disabilities and would likely result in an increase in their participation in academic environments.