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Ultrasonic vibrations can be perceived as audible sounds when a piezoelectric transducer is pressed against certain areas of the human body. In the range of frequencies investigated (20 to 108 kcy/sec), the threshold of perception seemed to lie near the threshold of feeling (about 10(-4) watt/cm(2)), and the perceived audible sound appeared to be between 8 and 9 kcy/sec, as judged by six test subjects. The threshold of perception and the perceived frequency appear to be dependent upon the hearing characteristics of the individual.

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The observed anomalous radio-frequency radiations from the sun are associated with sun spot activity and are believed to be generated within intermingling streams of charged particles issuing from active areas of the sun. Such streams have the property of greatly amplifying initial spacecharge fluctuations over a range of frequencies determined by the density and velocity distribution of particles in the stream. The theory of generation of radio energy resulting from space-charge interaction between streams of charged particles is reviewed and applied to the solution of the solar radio noise problem. From estimates of average density and velocity distribution of solar particles the frequency of the most intense radiation (30 to 60 megacycles) and the absolute value of radiation intensity at the surface of the earth (7 to 2)XlO _22 (watt/cm 2 /cycle/sec.) are computed and found to agree well with measurements. The most probable spectral distribution of the anomalous solar radiation is derived in the formwhere E m is the maximum intensity corresponding to the wave-length X m .