how to set the threshold for classifying speech signal and silence

Hello,all As a part of my project, I need to use Matlab for voice/unvoice separation of the recorded speech file.For now I can record a file in Matlab, and calculate Zerro-Crossing Rate(ZCR) and frame energy. high energy is voiced sound and less energy is unvoiced sound. But I cant set the threshold for classifying voiced and unvoiced speech. Please help me to set the threshold. Thank you in advance. -----------Rahiba