Voice stress analysis (VSA) is a controversial technology that attempts to examine and measure a physiological stress component, by extracting micro tremors from the voice of a person and measuring their amplitude. Voice stress analysis is often applied as a type of lie detector. The technique's effectiveness remains debated. Multiple studies have concluded that VSA's accuracy at determining a subject's truthfulness is not significantly better than chance. However, many police and intelligence agencies have alleged that VSA is indeed useful in determining a person's truthfulness.
The purpose in measuring voice stress is to attempt to determine the truthfulness of responses by an examinee regarding the subject of investigation. These measurements are made by analyzing the "stress level," or agitation, of the subject and using it as a guide to predict a physiological relation to responses given, based upon the reasoning that high levels of stress suggest the examinee is lying. The amplitude of vocal tremors can be measured in hertz by an analysis device; a low stress is implied at approximately ten hertz, and a high level of stress would correspond to a lower level signal. Since every person is presumed to produce different levels of voice stress according to their own physiology, some significant variation in measurement and interpretation is necessary. Also, unlike a polygraph, with which simple "yes" or "no" answers are given to questions, longer answers are needed for sufficient material to make analyses from.
Devices used to analyze voice stress are sometimes used in the presence of the individual under investigation; however, they can also be used without his or her knowledge. Since all that is needed is a voice, a wireless microphone, a telephone connection, or even a tape recording can provide the necessary input signal.
During the analysis process, it is possible that irrelevant or collateral subjects might also be discovered, leading to false readings. Because analyses are subjective, the actual use of voice stress as a determinant of truth is unreliable.
A great deal of voice stress testing has been conducted. The Israeli intelligence community is often cited as a major user of this technology. In the United States, most private use of these devices has been made illegal. However, the CIA and FBI, both instrumental in the creation of these devices about 30 years ago, continue to employ them at times in their own investigations. The Virginia state legislature has disallowed the use of voice stress analysis.
The American Polygraph Association's website lists conclusions from multiple "university-grade studies" into the accuracy of voice stress analysis as a means of detecting the subject's truthfulness. All cast doubt on the validity of the results of such tests; many describe the results as no better than chance.