The spelling of "ipsen" follows the rules of English phonetics, which indicate that the sound /ɪp/ is represented by the letters "ip" and the sound /sɛn/ by "sen". The International Phonetic Alphabet (IPA) can help us understand the exact sounds of these letters. In IPA, "ipsen" is transcribed as /'ɪpsən/, with the first syllable pronounced as a short "i" sound followed by a "p" sound, while the second syllable has an "s" sound followed by an "en" sound.