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Related to this question (sort of): Thunderwave in constricted area

Say you found a 1 ft. × 1ft. × 8 ft. hole in a cave that led to the outside, and you loaded a projectile of sorts – let's say a large rock, smaller than the diameter of the hole. If you then cast thunderwave inside of the hole, would the sound and pressure move the projectile out of the hole in a fast manner? If so, what would be the distance someone could use for the cannon?

A 5e Trebuchet has a 600/900 ft. range, so 1/3rd of that for the cannon as it's improvised – and I would say half the damage, so like... 3d8 damage? Obviously, it would take a skilled magic user to use mold earth multiple times to aim and move this thing which would put it at a huge disadvantage... But could it be done?

Keep in mind that I'm looking to see if this could be feasible, not practical or balanced. I don't see anything that says it can't be done, but not anything that says it could be either. The Rule 0 answer always works; however, this is not what I'm looking for.

Laurel
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Thatguy
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2 Answers2

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The spell only does what its description says it does.

The rules for casting a spell state:

Each spell description in Chapter 11 begins with a block of information, including the spell’s name, level, school of magic, casting time, range, components, and duration. The rest of a spell entry describes the spell’s effect.

A spell’s effect is exactly what is printed in its spell description. Since the description of thunderwave mentions nothing about launching objects like a cannon, it doesn’t do that. Instead, it only says:

unsecured objects that are completely within the area of effect are automatically pushed 10 feet away from you by the spell's effect

If these objects did damage, the spell would explain that, as it does in the description of the spell catapult:

Choose one object weighing 1 to 5 pounds within range that isn’t being worn or carried. The object flies in a straight line up to 90 feet in a direction you choose before falling to the ground, stopping early if it impacts against a solid surface. If the object would strike a creature, that creature must make a Dexterity saving throw. On a failed save, the object strikes the target and stops moving. When the object strikes something, the object and what it strikes each take 3d8 bludgeoning damage.

We know the object from catapult deals damage. Why? The spell description explains that. “It doesn’t say I can’t” is never a reason for a spell to do something, because spell descriptions tell you what a spell’s effect is; there are no secret or hidden rules for spell effects.

Kuerten
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Thomas Markov
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Even if were feasible, the range is incalculable.

Thomas's answer already addresses the core issue — Thunderwave doesn't act like a cannon because it doesn't say it does — but even if you were looking for an outside-of-RAW answer to this, you're not going to get something quantifiable from the spell (on top of 5e not being a physics simulator).

If so, what would be the distance someone could use for the cannon?

The potentially-useful part of the spell is this (and presumably you're not looking for a cannon with 10' of range):

In addition, unsecured objects that are completely within the area of effect are automatically pushed 10 feet away from you by the spell's effect, and the spell emits a thunderous boom audible out to 300 feet.

"A thunderous boom audible out to 300 feet" doesn't make all that much sense. Generously assuming "audible" is 60dB (about the volume of traffic or nearby conversation), that's 96dB at 5' from the origin point, or about the volume of a hand drill. If the origin sound is assumed to be on the level of a shotgun (165dB) from five feet away, that would still be 115dB at 300' and readily-audible from a mile away. The volume of the sound isn't well-expressed, so we can't derive any single level of force from it: either it's not written well or the sound itself is magically-attenuated and thus outside standard physics.

More importantly, all objects are pushed 10' away. It doesn't matter if it's a small feather or a tungsten block, it's moving 10' in an unstated amount of time. Again, there's no way to derive any single level of force from the evocation, as there's no consistent amount of force being directed against any object. All you can say is that an unattended object (like a rock in a hole) gets moved 10'.

Overall, on top of Thunderwave not behaving like a cannon, even if you were to rule that it did so, nothing in the spell is going to give you any singular range.

Shivers
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