Onomatopoeia:
* BOOM! (classic and simple)
* KABOOM! (more exaggerated)
* BANG! (for a smaller explosion)
* CRACK! (for a quick, sharp explosion)
Descriptive words:
* A deafening roar
* A thunderous blast
* A shattering explosion
* A violent eruption
Sound effects:
You can find sound effects of explosions online or in sound libraries. These can help you visualize and hear the sound.
Ultimately, the way you "spell" the sound of an explosion depends on the context and the effect you want to create.