# Generating Robotic Speech Prosody for Human Robot Interaction: A Preliminary Study
## 2 Literature Reviews on Speech Prosody and Robot  
To express emotion using prosodic vocal speech, it is important to analyze which features present in direct verbal communication will be generated by the model. Machine learning techniques can provide a better understanding regarding which features of speech signals can be used to classify emotional speech [10]. Pitch, timing (speaking rate in Reference [21]), and intensity (volume in Reference [21]) are the features of speech that are usually related to the uses of emotion through vocal prosody. The literature indicates the big three of vocal prosody, namely pitch, timing, and loudness [22]. Pitch refers to how the human ear perceives the fundamental frequency of the sounds. Low frequencies are perceived as bass sounds and the highest are perceived as treble sounds, although human speech is more complex than a signal composed by one particular periodic sinusoid. Timing is related to speed of the speech and the pauses found between words. The speed of someone speaking is typically measured in words per second, whereas the pauses can be measured in seconds. Intensity is related to how loud or quiet a person's voice sounds, and can be seen as the power level of the voice [10]. Specific combinations of these three factors can represent some group of emotions. For example, rapid speech can indicate that the speaker is happy or angry. A slow rate of speech typically indicates that the speaker is sad [10]. The specific case of dynamic pitch, also known as intonation, has an important role in providing information regarding emotion [23], and pitch can also be used to infer interpersonal relationships between individuals [21]. Changes in dynamic pitch alone can express the degrees of friendliness and authority, for example [23].  
Several robots with different way of speech have been suggested in recent studies. [PERSON] and [PERSON] explored the vocal prosody of a speech synthesizer called MARY which was embedded in a robot used in an interactive experiment [24]. In a series of interactions with two NAO robots, it was observed that children display more expressions when interacting with a robot that displays emotion and adapts its expressions [25]. In this specific experiment, the voice of the robot was influenced by its arousal, where the robot would speak louder under higher arousal. Some studies also mention the affect used in conversation. [PERSON] et al. revealed that the feeling of a robot's speech being natural and comfortable varies depending on the user [16].  
In case of the robots with verbal communication for specific uses (e.g., elderly care), it is challenging. The communication between robots and elderly people can be difficult as a result of being too vague because elderly people are likely to forget the terms used by a robot and may not use precise delivery in their speech [26]. Studies have shown that the elderly tend to have less ability to process dynamic pitch [23], particularly when dealing with weak pitch contours with rising pitch patterns. Moreover, these studies suggest that this problem may be prominent when the dynamic pitch is carried by natural speech and when the pitch contour is not strong [23]. Making changes in speech to highlight the pitch contours could improve the ability of the elderly to perceive speech accurately when interacting with robotic systems.