Voice Stress Analyzer – How It Works
Since ancient times, man has tried to determine if his fellow man is telling the truth. A medieval “truth by trial” technique called for sticking a suspected liar’s hand into a fire; if it was not burned, a person was judged innocent. The ancient Chinese test required a suspected wrongdoer to chew rice powder while being questioned. If the powder stayed dry, the person was condemned on the theory that tension caused by lying blocked the salivary glands. During the time of the Pilgrims, suspected liars were often thrown fully-clothed into the middle of a lake; if they drowned they were innocent; if they floated, guilty.
Technology has brought us a long way from these ancient methods of detecting lies. Today many law enforcement agencies rely on the polygraph to determine lies from truth. A less intrusive truth verification device, the Voice Stress Analyzer (VSA), is now also in use.
The VSA was first used during the Vietnam War by U.S. soldiers in an attempt to determine if Vietnamese prisoners were Viet Cong guerrillas or civilians. During that time it was known as the PSE, or Psychological Stress Evaluator. After the war, private developers took over the technology, but it wasn’t until the late 1980s that the VSA became a tool widely used in police investigations.
The body has two nervous systems, the Central Nervous System and the Autonomic Nervous System. The Central system can be considered as concerning itself with those physical and sensory functions which occur at, or above the threshold of awareness. In other words, we have control over this system.
The Autonomic system is concerned primarily with those functions which occur below the threshold of awareness and operates on an automatic, self-regulating basis. The beating of the heart, the rise and fall of the lungs, the digestion of food, all complex processes of the body which must continue without any conscious effort are functions of the autonomic nervous system.
The heartbeat, for example, normally occurs at a rate kept within certain bounds. However, under the influence of physical stress such as exertion or effort, or emotional stress, such as fear, anger, or excitement, the heart rate will change. This is called the “fight or flight syndrome.”
Since we are concerning ourselves with stress caused by psychological stimuli and not teaching physiology, we will accept the premise that there are many other functions going on in the body which are inherently related and vital to our being, but our main concern in this is the voice and how it is affected by psychological stress.
Use of the VSA revolves around two basic components of speech: Amplitude Modulation (AM), and Frequency Modulation (FM). AM sound is audible, FM is not.
Under relaxed conditions, the human voice box is under control of the central nervous system. With the onset of stress, as when people lie, changes occur in the voice that causes vocal cords to react. These changes can be detected by the VSA. Since these changes can be measured, the VSA indicates the presence of stress and thus, depending on its application, deception.
The VSA measures changes in a person’s voice caused by stress. Under stress, the nervous system kicks into high gear, sending the voice response into an abnormal range. A normal-range response (the truth) appears as peaks on the CVSA print out. Lies, or stress, look like squared-off blocks.
The Truth …………. A Lie
The pattern on the left is an example of a person answering a question and displaying no stress. The pattern on the right is displaying tremendous stress when the person answered the second question. Both were answered with a “No” as seen by the minus sign (-) next to the question number.
The difference should be obvious in this case. Of course, it is never always this easy to read a chart, but whenever a pattern is in a diagonal as it is on the left; it is a no-stress pattern and a truthful response. The pattern on the right is blocking, not diagonal, and is an example of an answer given under stress.