Originally Posted by ItsJustMe

Probably not going to find it. Car horns draw several amps at 12 volts. That's a lot of power. Sound is logarithmic; getting a little more sound takes a lot more energy.

The bottom line is that to provide that kind of sound levels, a standard single car horn draws 6 amps at 12 volts, which is 72 watts. You aren't going to be able to get the same level of sound with less power input than that. Lower voltage, more amps. 6 amps is already very challenging for most batteries that are fairly light.

Add to that, cars actually have TWO horns, so double that.

What you really need is a source of a big whack of power for a short period of time. It would be neat to figure out a way to have a brake that made noise on demand, since brakes are dissipating a heck of a lot of energy quickly.