Voice Recorder, 8GB Digital Audio Sound Recorder Dictaphone, Digital Voice Activated Recorder with MP3 Player,High-Sensitivity Dual Microphone, Multifunctional Rechargeable Recorder for Lectures It can work when charging frequency: 400MHZ distance for sending and receiving signals: 500-1500m working time for sending signals: 68 hours Low-voltage prompts, built-in rechargeable battery.