The fits Smart HJC 21B is the 2nd Generation Standard made with Bluetooth technology Communication System model that supports 21B compatible HJC helmets, with structural interfaces that users can easily mount on HJC helmets. Based on the BT 5.1 low power system, the 21B offers made with Bluetooth technology 4-way intercom functionality, with over 22 hours of talk time. The enhanced sound provides users with an HD speaker, to enrich mid-to low-pitched sounds. made with Bluetooth technology 5.1. Advanced noise control. Intercom 1600mm. Voice prompt. Universal intercom. HD speakers. Audio multitasking. 4 way intercom. This helmet is DOT approved and FMVSS 218 certified for legal on-road motorcycle use in the United States.