| This spell eliminates all sound near the caster. This makes reading scrolls, casting spells, praying or yelling in the caster's vicinity impossible. (Applies to caster too, of course.) This spell will not hide your presence, since its oppressive, unnatural effect will almost certainly alert any living creature that something is very wrong.
Silence starts out with a radius depending on power, which will then shrink, eventually covering only the caster, before the effect times out. Some creatures, such as demons, cast magic without speaking and so will be unaffected, and those with innate magical abilities will still be able to use them.
Silence is a level 5 Hexes/Air Magic spell which makes reading scrolls, casting spells, praying, or yelling in the caster's vicinity impossible (this applies to the caster too, of course). The duration and radius of the spell effect is somewhat randomized, but the higher the power of the spell, the more likely it is to be bigger and last longer.
In spite of the flavor text, very stealthy characters can still go unnoticed when silenced; it merely hits you with a -50 penalty to your stealth score. On the other hand, monsters that are already aware of you will be unable to shout while silenced, and loud actions performed within the aura will produce no noise. Bear in mind that demonic monsters ignore silence effects.
Using a scroll of silence creates a similar effect at a power of 25. On average, this will create a starting radius of 4 or 5.
Silence is an excellent method for preventing powerful enemy casters from accessing their most dangerous abilities. So long as you are capable of dealing damage without use of spells, you can reduce many powerful uniques and devastating enemies to pitiful melee opponents. Just be sure to defeat them before the spell wears down too much, as a fight can rapidly turn ugly during that period at the end of the spell in which only you are silenced.
If you happen to have any potions of cancellation, consider quaffing one any time you need to end your own silence aura early. It'll eradicate any other status effects you may be enjoying, but it's better than being stuck unable to read a scroll of blinking or teleportation when you most need one. Just remember that it'll have no effect on the silence auras of your enemies.
The radius starts shrinking with 43 turns to go, and during the last 6 turns only the center tile (you) is silenced. Pre-squared radius is reduced linearly from 37 = 6*6+1, which means that radius is reduced like sqrt(dur) (slowly at large radius, and faster near the end).
For reference, cast_silence has:
you.increase_duration(DUR_SILENCE, 10 + random2avg(pow,2), 100);
That's at least 10 turns, at most 100. At 50 power (mid-game caster) you'll get an average of 35, which means you'll usually start below maximal radius.