Built-in level-dependent function allows the wearer to hear ambient sounds, such as conversations, machine sounds and warning signals. 3.5 mm listen only stereo input (limited to 82 dB) for connection to external devices (e.g. mobile phone, radio, two-way radio and iPod). Voice guided menu system to prevent you from taking the headset off. Auto power off turns headset off after 4 hours of non-use, to save energy and a low-battery warning informs you when to change batteries.