The FM quality of the voice is susceptible to the amount of stress that one may be under when speaking. The PSE senses the difference and records the change in the inaudible FM qualities of the voice on a chart. When the chart is interpreted by an experienced examiner, it reveals the key stress areas of the person being questioned. The audible portion is what we hear. The inaudible modulation comes from involuntary areas, those not totally controlled by the brain or thought processes.
Internal stress is reflected in the inaudible variations of the voice.
The key to successful use of the PSE is in the preparation of selected questions keyed to the individual and structured to reveal normal or truthful answers and answers that are false. Once the personal pattern has been established, any evasive or false answers reveal stress. If a person is not telling the truth, analysis of their voice patterns will reveal it. The PSE has great advantages over similar instrumentation in that it does not have to be used at the time of the interview or interrogation. A tape recorder is used to make a permanent record of the interview and this tape can later be fed into the PSE where the voice reactions are recorded on a chart.