Spell noise is the noise generated when casting spells.
How noise is determined
There is a casting noise which is the spell level with factor applied (rounded up). The factor is 1 for conjuration spells, 1/2 for poison and air spells and 3/4 for the others.
A modifier (noise_mod) can be set in spl-data.h to change this base casting noise. It is added to the spell level before applying the school-dependant factor. The noise will be made at the caster's position.
A second source of noise is the spell itself. The noise will be made at the target's position. Some number of spells don't make any noise at all, like Bolt of Draining, and they end up with a noise of 0. For bolt spells, the noise is made at each square where it hits a monster, and the last square the spell reached even if it doesn't hit anything.
This is from spl-cast.cc for reference:
- Silent: 0
- Almost silent: 1
- Quiet: 2
- A bit loud: 4
- Loud: 8
- Very loud: 15
- Extremely loud: 20
- Deafening: 30