What is the role of AI in Natural language processing ?
For example Siri. Is Ai used to make siri get better at understanding the users voice ?
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I think the source of your confusion is the fuzziness of the definition of AI.
AI can be the subfield of computer science that deals with problems where computers need to become more intelligent in some way. These subfields include NLP, Machine Learning, Knowledge Representation, Automated Reasoning, speech recognition, speech synthesis etc. By this definition Siri certainly uses AI, since it recognizes your speech, translates the spoken phrase from human language to a query on a knowledge database, and then translates the answer back to human language, and then to human speech. The first and last two of these steps are considered NLP (speech recognition, natural language parsing, natural language generation, speech synthesis).
Another definition of AI (strong AI) is the philosophical problem of whether computers can ever mimic/portray all the intelligence of human beings, and whether that raises them to the same level of humans. For all Siri's achievements, I don't think anyone would call Siri an example of strong AI.
Finally the example you give of Siri getting better at recognizing your voice is called Machine Learning. This is the field of study of programs that adapt to users or their surroundings. I don't think Siri uses machine learning as you use it, but the whole system may well contain machine learning parts that were trained before the system was rolled out.
NLP is the branch of computer science focused on developing automated systems that allow computers to communicate with people using natural language like English, German,...language. It is part of AI. Many techniques from machine learning, statistical processing,... from AI are used in NLP.
In AI, NLP is also called AI-complete or AI-hard. It is equivalent to solving the central artificial intelligence problem—have computers as intelligent as human.
You can find more information in Wikipedia page