Abstract: An eavesdropping device having a microphone and a transmitter is detected by an apparatus including an antenna, a broad-band detector, a limiter-amplifier, and a loudspeaker. The apparatus establishes a regenerative feedback loop with the eavesdropping device and produces a revealing tone. The limiting amplifier shifts the level of the demodulated signal and amplifies all but a peak portion, and combines the signal thus limited with an unmodified signal in a differential amplifier, thus amplifying the peak portion, thus preventing amplifier saturation in an environment having interfering signals which are stronger than the signals transmitted by the eavesdropping device. Cascaded limiting amplifier stages can be used.

Abstract: A method of detecting psychological stress by evaluating manifestations of physiological change in the human voice wherein the utterances of a subject under examination are transduced to electrical signals and processed to emphasize selected characteristics which have been found to change with psycho-physiological state changes. The processed signals are then displayed, as on a strip chart recorder, for observation, comparison and analysis. An especially useful characteristic is an infrasonic modulation in the voice. Apparatus for performing detection of this type includes a transducer, a magnetic recorder, a series diode, a plurality of integrating capacitors, an amplifier and a chart recorder. A second apparatus includes filter means, an FM discriminator and a detector, a waveform integrator, an amplifier and a recorder for producing a visible record.