First of all, there is no such thing as a lie detector. 

All too often both polygraph and voice stress analysis (VSA) are referred to as such. Neither of these two devices detect lies. Both are only stress detectors that provide feedback for emotional responses to questions asked. Therefore, either can only be considered a tool to help an examiner arrive at a subjective opinion regarding the veracity of someone's responses to questions asked. When used properly, both can help to focus intelligence collection and investigative efforts.