Casting Silence on a door without affecting a guard beside it

Question

Today I stumbled upon a excellent yet seemingly useless magic item: Squeaky Shoes of Invisibility!

Wearer of this shoes become invisible indefinitely, but release a squeaking sound whenever you take a step.

This is an imagined magic item, but it raised a question for me about placing a Silence spell. For the purpose of this question let's also say the squeaking is about as loud as a rubber duck.

My first thought is to cast Silence near a guard, but carefully placed so as to not include him in the sphere, so I can become invisible and silent simultaneously.

For the duration, no sound can be created within or pass through a 20-foot-radius sphere centered on a point you choose within range. Any creature or object entirely inside the sphere is immune to thunder damage, and creatures are deafened while entirely inside it. Casting a spell that includes a verbal component is impossible there.

However, in this imaginary castle, the guard is guarding beside an open door. Suppose that the guard is alert to his surrounding so he will notice if suddenly he become deafened. How can I cast this Silence to cover the door without affecting him? Since the diameter of affected area is 20', I need to carefully position the center of the spell.

The width of the door is 6 feet.

Possible options I have considered, but don't know if this will work:

• Control the AoE of the spell to become smaller.
• Cast the spell center behind the wall. The spell doesn't specify you have to be able to see the center of the spell.
• Cast the spell center within the wall

I'm looking for the right way to place Silence without affecting the guard.

Answer 1

Your only option is to be careful

The shape of spell areas is fixed and nothing in the book gives you permission to directly change it. There are some features that deal with this problem (like sculpt spells for evocation wizards), but none apply to silence. As to casting in relation to a wall:

To target something, you must have a clear path to it, so it can't be behind total cover. If you place an area of effect at a point that you can't see and an obstruction, such as a wall, is between you and that point, the point of origin comes into being on the near side of that obstruction. (PHB 204)

So you do not necessarily have to be able to see the center (it could be dark or obscured), but if there is a wall, it will not work as you intended. There is one thing that you could make use of, though:

A spell's effect expands in straight lines from the point of origin. If no unblocked straight line extends from the point of origin to a location within the area of effect, that location isn't included in the spell's area. To block one of these imaginary lines, an obstruction must provide total cover (PHB 204)

So if you position the center right, the wall could block the area the guard is in from the effect.

Answer 2

You're out of luck

In the scenario you are describing, it doesn't really matter how you position your spell. Your silence has a radius of 20 feet, so at best you'll be 40 feet away from your guard, inside a zone of silence. If you step out, your shoes start making sounds again.

Now I'd imagine that the guard should be able to hear it from 40 feet away from him. (5e does not give any lists on hearing distances, but if you've ever had a dog with a squeaky toy, you'll hear that darn thing on the other side of the house.)

This leaves you inside a silence effect and needing to cast a new one before taking another step, which you can't do without a vocal component, at which point the question becomes "can a guard hear me casting a spell from 40 feet away if I end my concentration on the silence spell?"

Instead, may I suggest simply walking on your hands and trying not to bump your shoes into anything, or, as suggested by SevenSidedDie, crawling on hands and knees.