Change Voice using AVSpeechSynthesis in IOS?

Hi, Being a learner. I am here to participate in this forum to enhance my knowledge on ios programming  & I would like to know that “how to change voice using AVSpeechSynthesis in IOS & i want to change the default voice in system voice.”, which has become difficult to debug. i am also searching for some php questions and answers for interviews. I hope any tech developer will help me in resolving this issue.