When selecting a round horn, you must prioritize Sound Pressure Level (SPL), typically ranging from 105dB to 118dB, to ensure audibility in high-noise environments. The frequency range (High tone vs. Low tone) is critical; for automotive applications, a dual-tone setup (e.g., 410Hz and 510Hz) is standard. Additionally, verify the voltage compatibility (12V for passenger cars, 24V for trucks/machinery) and ensure the diaphragm material is made of high-quality manganese steel for longevity and consistent vibration.