Car Horn

Spec out the horn we'll be using here.

8.2.1.C - Supplemental, replaceable batteries carried in the solar car may be used to power: main power switch, radios, commercially available electronic panel meters with internal batteries, cell phones, driver ventilation fans (if solely used for driver ventilation), and the horn. All other systems present on the car must be powered off the main battery.
9.4.I - Solar cars must be equipped with a horn that can be heard at a sound power level between 75 and 102 dBA at a distance of 15 m in front of the solar car. The horn must be permanently mounted, operated from the steering wheel. Horn must be able to operate for up to 5 minutes continuously at the required volume.

An audible warning device complying with the intent of UNECE Regulation 28 must be fitted to the solar car.
- 2.25.2 Sound pressure level must be more than LA = 105 dB measured 2 m from the horn.
  The horn should be mounted so that solar car occupants are not subjected to excessive sound pressure levels.
- 2.25.3 The device must emit a continuous and uniform sound.

Wikipedia Article on car horns - some interesting history and examples: https://en.wikipedia.org/wiki/Vehicle_horn

Car horns usually have 2 frequencies in their horns, using a significant amount of power for each sounder, usually around 50-60W each.

Apparently Tesla's horn sounds are customizable - this would be a cool feature to add but not necessary.