TABLE 31: RFR-AUDITORY EFFECT IN HUMANS
| Authors | Effects Sought or Examined | Exposure Modality | Effects Reported | Notes & Comments | |||||||||||||||||||||||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Frey (1961) | RFR-auditory effect in human volunteers. | 6-µs pulses of 1.3-GHz RFR at 244 pps; 1-µs pulses of 3.0-GHz RFR at 400 pps. Ambient noise levels were about 70 and 80 dB, but with earplugs were lowered by about 25-30 dB. | The mean threshold of average power density for RFR perception was about 0.4 mW/cm² at 1.3 GHz for eight subjects and 2 mW/cm² at 3.0 GHz for seven subjects. The corresponding peak power densities were 270 and 5,000 mW/cm². Some subjects with various types of hearing losses had higher perception thresholds. | The author did not provide any variances or other statistical data.
The subjects were unable to match the RFR sounds to audio sine waves. With band-pass controlled white noise, best match was obtained by removing all frequencies below 5 kHz. Frey (1962)
| The auditory effect in volunteers, some who had an audiogram notch (a significant hearing loss) around 5 kHz.
| Pulses of 125, 250, 500, 1,000, and 2,000 µs of 425-MHz RFR at 27 pps; 2.5-µs pulses of 8.9-GHz RFR at 400 pps. The noise levels were 70-90 dB; with Flent ear stopples, the noise levels were lowered by about 20 dB from 100 Hz to 2 kHz and about 35 dB at 10 kHz.
| The average-power-density thresholds for perception of 125-, 250-, 500-, and 1,000-µs 425-MHz pulses were 1.0, 1.9, 3.2, and 7.1 mW/cm², with peak power densities of 300, 280, 240, and 260 mW/cm², all comparable to the 1.3-GHz threshold in Frey (1961). However, the 3-GHz peak-power threshold in Frey (1961) was much higher, about 5 W/cm², and the 8.9-GHz RFR was not perceived for peak-power densities as high as 25 W/cm², differences ascribed by the author to smaller penetration depths.
| The author speculated about possible sites and mechanisms of detection of RFR pulses, including interaction of the RFR with neuron fields in the brain, but found the data to be inconclusive. Again, no statistics were presented.
| Subjects who had the 5-kHz notch (and adequate hearing above and below 5 kHz) did not perceive RFR pulses as sound. White (1963)
| Elastic-wave theory of sound generation from thermal expansion due to transient surface heating.
| Bombardment of the surfaces of metals, plastics, water, and piezoelectric crystal by RFR pulses or an electron beam.
| Elastic waves were detected in each of the surfaces studied. Mixing (production of beat frequencies) was found when two pulses of different RFR frequencies were absorbed simultaneously. Use of a barium titanate crystal to detect elastic waves from heating with a single 2-µs pulse of electrons or RFR yielded easily detected signals at pulse power densities down to 2 W/cm², corresponded to a computed peak surface-temperature rise of 0.001 °C.
| The author did a theoretical analysis of the process, based on assuming an input heat flux that varies harmonically with time, to relate the amplitude of the elastic waves to the characteristics of the input flux and thermal and elastic properties of the body.
| Frey and Messenger (1973)
| Further human studies on the RFR-auditory effect.
| 1.245-GHz pulses at 50 pps; pulse width varied from 10 to 70 µs: Average power density held at 0.32 mW/cm² for peak power densities of 640 to 91 mW/cm²; peak power density held at 370 mW/cm² for average power densities of 0.19 to 1.3 mW/cm².
| Each subject was requested to judge the loudness of pulsed-RFR signals relative to an initial reference signal. The median values of the loudness versus peak power density and versus average power density were graphed for each test, without deviations. The authors calculated that the peak-power-density threshold for perception of RFR pulses is 80 mW/cm², a value much lower than reported subsequently by Guy et al. (1975b) and by Cain and Rissman (1978).
| No data for each subject were given. The accuracy of these results could not be evaluated because of the absence of data on the scatter of responses by each subject and because the subjective judgments of the relative loudness may be imprecise.
| Foster and Finch (1974)
| Further confirmation of the elastic theory of sound generation by RFR pulses.
| They used 2.45-GHz RFR pulses in several combinations of pulse power density and pulse width, and a hydrophone immersed in salt water for detection.
| The experimental results confirmed: the findings in water of White (1963); calculations showing that for short pulses, the peak sound pressure is proportional to the energy per pulse, but for long pulses it is proportional to the incident power density; and that the transition between the two peak-sound-pressure regimes occurs for pulse durations between 20 and 25 µs.
| The authors also found that such acoustic transients were not obtained in water at 4 °C (where its thermal expansion coefficient is 0) and that acoustic signals between 0 and 4 °C were reverse in polarity from those for temperatures above 4 °C, results supporting the thermoelastic expansion hypothesis.
| Sharp et al. (1974)
| Transduction of RFR pulses into sound at the surfaces of RFR-absorbers.
| 14-µs pulses of 1.5-GHz RFR triggered randomly at about 3 pps while regions of the subject's head were shielded with RFR absorber. The power per pulse was 4.5 kW and the pulse power density ranged from 750 to 1,500 mW/cm².
| With a sound-level meter to measure the delay times for acoustic propagation for distances of 0.3 to 0.6 m between the absorber and microphone, the authors confirmed that the absorber transduced the RFR pulses into acoustic signals. Varying the carrier frequency from 1.2 to 1.6 GHz or using 2.45 GHz made little difference in the level or quality of the sound. The threshold pulse power for audibility was 275 W, yielding estimated pulse power densities in the range 46-92 mW/cm².
| The authors, while shifting the RFR absorber over various areas of the subject's head, noticed that the apparent locus of the sound moved from the subject's head to the absorber.
| Guy et al. (1975b)
| RFR-auditory power-density thresholds and modulation characteristics.
| Exposures of the back of the subject's head to 2.45-GHz pulses of duration varied from 1 to 32 µs. Exposures were to trains of 3 pps each, with 100 ms between pulses. The ambient noise level was 45 dB.
| For Subject 1 with a normal audiogram hearing threshold, the threshold for RFR auditory perception was a constant peak energy density of 40 µJ/cm² per pulse irrespective of pulse duration. With ear plugs, the threshold was only 28 µJ/cm² per pulse. Similar results were obtained for Subject 2, who had a deep notch at 3.5 kHz in both ears, but the threshold was 135 µJ/cm² per pulse, or about three times higher than for Subject 1.
| The two subjects studied were requested to signal when they perceived sound. They heard each pulse as a click, and heard pulse trains as chirps that corresponded to the pulse repetition rate. The subjects accurately interpreted Morse code transmitted by manual keying of the pulse generator.
| Lin (1977c)
| Analysis of equations of spherical models of human and animal heads of brain-equivalent material for acoustic resonant frequencies.
| Theoretical paper on resonant frequencies generated by exposure to RFR pulses.
| Analyzed were heads of guinea pigs, cats, and human adults and infants. The results showed that the fundamental and higher-harmonic frequencies produced by RFR pulses are independent of carrier frequency, but dependent on head size, with the fundamental frequency inversely proportional to the radius of the head.
| The theoretically predicted fundamental frequencies for humans were 13 kHz for an adult and 18 kHz for an infant.
| Cain and Rissman (1978)
| Experimental study of RFR-auditory effect in 2 cats, 2 chinchillas, 1 beagle, 8 humans.
| 5-, 10-, 15-, or 20-µs pulses of 3.0-GHz RFR at 1 pulse every 2 seconds. The humans wore ear muffs during exposure, to reduce the ambient noise level (to 45 dB).
| Subjects 1-5 could hear 15-µs pulses, with peak-power-density thresholds of 300, 300, 300, 600, and 1,000 mW/cm². They also could hear 10-µs pulses, with thresholds of 1,800, 225, 600, 2,000, and 2,000 mW/cm² . Only Subject 1 could hear 5-µs pulses, with a threshold of 2,500 mW/cm². By contrast, Subjects 6-8 could not hear 5-, 10-, or 15-µs pulses at the highest peak power density but could hear 20-µs pulses. In summary, 300 mW/cm² can be taken as the nominal human pulse-power-density threshold for pulse durations of 10 µs or longer.
| Only results for the humans are summarized here. Seven subjects were given standard audiograms, and the thresholds for binaural hearing were determined for frequencies in the range 1-20 kHz.
| The authors had exposed humans to pulses of 3.0-GHz RFR at peak power densities as high as 2,500 mW/cm² with no apparent ill effects. Tyazhelov et al. (1979)
| The qualities of the sounds perceived by humans from exposure to RFR pulses. Small hollow tubes from a speaker to the ears were used to present audiofrequency sounds without or concurrent with the RFR.
| The parietal area of each subject's head was exposed to 800-MHz RFR pulses of widths 5 to 150 µs, presented either at a pulse repetition rate (PRR) of 50 to 2,000 pps or pulse trains lasting 0.1 to 0.5 seconds at a rate of 0.2 to 2.0 trains per second.
| Three subjects who had HFALs below 10 kHz could not perceive 10-30 µs pulses, results that were consonant with those of Cain and Rissman (1978). Only 1 Of 15 subjects with HFALs above 10 kHz could not hear the pulses. Subjects with HFALs below 15 kHz were unable to distinguish between the sounds from a 5,000-pps and a 10,000-pps signal, and subjects with more extended HFALs heard a higher pitch for a 5,000-pps signal than a 10,000-pps signal. Subjects reported hearing beat-frequency notes when presented acoustic tones above 8 kHz concurrently with 10-µs to 30-µs pulses at PRRs just above or below 8 kHz; the subjects could cancel the perception of the RFR and audio when matching them in frequency, amplitude, and phase.
| The high-frequency auditory limit (HFAL) of each subject was tested for tones from 1 kHz upward. The subjects had means for varying amplitude, frequency, and phase of the audio signals to try to match their timbre and loudness to the perceived RFR.
| The authors suggested that many of their results are consistent with the thermoelastic hypothesis, but that others, such as the suppression of the perception of a 5,000-pps train of RFR pulses by a 10-kHz acoustic tone, were at variance with that model. |
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