The spelling of the word "HIRSI" can be explained through the use of IPA phonetic transcription. The word is pronounced as /hɪrsi/, with the first syllable "hir" pronounced with the short "i" vowel sound /ɪ/ and the second syllable "si" pronounced with the long "i" vowel sound /iː/. The letter "s" in the word is pronounced as a voiceless alveolar fricative /s/. The spelling of "HIRSI" follows the basic rules of English phonetics, with the letter "s" used to represent the /s/ sound and the vowels representing their respective sounds.