Is an Air Horn Legal in the US?

Conditional

An air horn is legal on a road car in the US within limits: the horn must be audible but not a siren, and not unreasonably loud. A moderate air horn is generally fine, while a train-style horn that is dangerously loud or sounds like an emergency signal can break state and local rules.

Why?

Federal and state rules require every vehicle to have a working horn that can be heard from a set distance, and they also cap the other end: the horn must not be unreasonably loud or a harsh, alarming sound, and it must not imitate a siren, whistle or emergency signal.

An air horn that is simply louder and deeper than a stock horn usually sits within that, but the very loud "train horn" setups are where trouble starts. A horn loud enough to be dangerous, or one used to startle rather than warn, can run afoul of noise ordinances and the no-siren rule, and some cities are stricter than the state.

So an air horn is road-legal as a horn; the limits are that it stays a warning device, not an ear-splitting or siren-like one. Keep it reasonable and it is fine; a train horn on a street car invites a ticket.

What decides if it's legal

Every vehicle must have a working horn audible from a required distance.
The horn must not be unreasonably loud or a harsh, alarming sound.
It must not mimic a siren or an emergency-vehicle signal.
Some states and cities limit excessively loud horns.

Does it depend on your state?

The must-have-a-horn and no-siren basics are broadly consistent. How loud is "too loud" is where state and city noise rules differ, so an extreme train horn is more exposed in some places than others.

Related legality guides

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Sources

Vehicle modifications and equipment (NHTSA)

This page is general guidance, not legal advice, on the rules in the US for air horn. The detail varies by exact vehicle and changes over time - confirm with your insurer and the EPA, NHTSA and your state DMV before modifying.