Here's why:
* Sounds are not spelled: We use letters to represent sounds in written language, but sounds themselves are not made up of letters.
* Whispering is a manner of speaking: It's not a specific sound. You can whisper different words or phrases, and the sound will change depending on what you're saying.
Instead of spelling a whispering sound, you could:
* Describe it: "The wind whispered through the trees."
* Use onomatopoeia: "Shhh... the baby is sleeping."
* Use italics or boldface: *Whispering* or whispering to indicate a quiet voice in writing.