When selecting a horn for a bus, you must prioritize sound pressure level (SPL), typically ranging from 105 dB to 125 dB, to ensure audibility in heavy traffic. The operating voltage is critical; most buses use a 24V DC system, though some light buses may require 12V. Additionally, check the frequency range—low-tone horns (approx. 300-400Hz) provide better long-distance penetration, while high-tone horns (approx. 500Hz+) offer sharper alerts. Ensure the product has an IP67 waterproof and dustproof rating to withstand harsh exterior road conditions.