In order to study speech sounds,we must have a way to write down the sounds.Linguists have invented a phonetic alphabet,a system for writing speech sounds,so that there is a specific and concrete way to refer to each individual sound. Writing down speech sounds by using a phonetic alphabet is called phonetic transcription.
A spelling system cannot be used as a tool for phonetic transcription. This is because orthography,or alphabetical spelling system. It is not a good phonetic alphabet, and hence not an adequate tool for the phonetic transcription,because of the following discrepancies between letters and sounds.