Volume control for a car horn

I have a 12V/6A power supply connected to a car horn (12V/6A). I had intended to place a pot in series as a volume control, however even a 0.47Ω resister causes the horn to not sound. Is there anything short of getting a new horn/power supply that can quite this thing down? I will be connecting this to an alarm clock for one hell of an alarm

Thanks!

Run it from a lower voltage, or wrap it in sticky tape or foam.

Slide a block of foam down the horn. If you need more volumn cut some passages in it. Think mute.