Study type: Medical/biological study (experimental study)

Microwave-induced acoustic effects in mammalian auditory systems and physical materials. med./bio.

Published in: Ann N Y Acad Sci 1975; 247: 194-218

Aim of study (acc. to author)

One of the most widely observed biologic effects of low average power electromagnetic energy is the auditory sensation evoked in human irradiated by pulsed microwaves. The effect appears as an audible clicking or buzzing sensation that originates from within and near the back of the head.
The study reported here was designed to apply an approach for establishing the threshold of the effect in human and animals as a function of pulse power or energy, pulse shape, and carrier frequency; the locus of action of the effect, that is, whether it is initiated at a central or at a peripheral site; and whether the stimulation is due to direct action of the electromagnetic fields on the nervous system or to transduced acoustic energy acting on the auditory system.

Endpoint

Exposure

Exposure Parameters
Exposure 1: 918 MHz
Modulation type: pulsed
  • power density: 17.4 µW/cm² mean (19.4 µW/cm², 22.6 µW/cm², 20.6 µW/cm², 24.3 µW/cm² and 28.3 µW/cm²)
  • power density: 5.8 W/cm² peak value (3.88 W/cm², 2.26 W/cm², 1.37 W/cm², 1.17 W/cm², 0.97 W/cm² and 0.80 W/cm²)
Exposure 2: 2.45 GHz
Modulation type: pulsed
  • power density: 17.8 µW/cm² mean (20.3 µW/cm², 21.6 µW/cm², 28.0 µW/cm², 33.0 µW/cm², 15.2 µW/cm² and 47.0 µW/cm²)
  • power density: 35.6 W/cm² peak value (17.8 W/cm², 10.0 W/cm², 5.0 W/cm², 4.0 W/cm², 2.2 W/cm², 1.9 W/cm², 1.7 W/cm², 0.6 W/cm² and 1.5 W/c)
Exposure 3: 8.5–10 GHz
Modulation type: pulsed

Exposure 1

Main characteristics
Frequency 918 MHz
Modulation
Modulation type pulsed
Pulse width 32 µs
Packets per second 1
Additional info

different pulse widths of 3, 5, 10, 15, 20, 25 µs were also used.

Exposure setup
Exposure source
Distance between exposed object and exposure source 0.08 m
Parameters
Measurand Value Type Method Mass Remarks
power density 17.4 µW/cm² mean measured - 19.4 µW/cm², 22.6 µW/cm², 20.6 µW/cm², 24.3 µW/cm² and 28.3 µW/cm²
power density 5.8 W/cm² peak value measured - 3.88 W/cm², 2.26 W/cm², 1.37 W/cm², 1.17 W/cm², 0.97 W/cm² and 0.80 W/cm²

Exposure 2

Main characteristics
Frequency 2.45 GHz
Modulation
Modulation type pulsed
Pulse width 32 µs
Packets per second 1
Additional info

different pulse widths of 0.5, 1, 2, 4, 5, 10, 15, 20, 25 µs were also used.

Exposure setup
Exposure source
Distance between exposed object and exposure source 0.08 m
Parameters
Measurand Value Type Method Mass Remarks
power density 17.8 µW/cm² mean measured - 20.3 µW/cm², 21.6 µW/cm², 28.0 µW/cm², 33.0 µW/cm², 15.2 µW/cm² and 47.0 µW/cm²
power density 35.6 W/cm² peak value measured - 17.8 W/cm², 10.0 W/cm², 5.0 W/cm², 4.0 W/cm², 2.2 W/cm², 1.9 W/cm², 1.7 W/cm², 0.6 W/cm² and 1.5 W/c

Exposure 3

Main characteristics
Frequency 8.5–10 GHz
Modulation
Modulation type pulsed
Pulse width 32 µs
Packets per second 1
Additional info

different pulse widths of 0.5, 1, 2, 4, 5, 10, 15, 20, 25 µs were also used.

Exposure setup
Exposure source
Distance between exposed object and exposure source 0.08 m
Parameters
Measurand Value Type Method Mass Remarks
power density 4.72 W/m² mean measured - 1240 µW/cm²
power density 14.8 W/cm² peak value measured - to 38.8 W/cm²

Exposed system:

Methods Endpoint/measurement parameters/methodology

Investigated system:
Investigated organ system:
Time of investigation:
  • during exposure

Main outcome of study (acc. to author)

The threshold for microwave pulse-evoked auditory responses in both humans and cats is related to the incident energy per pulse, with values of approximately 20 µJ/cm² for cats to 40 µJ/cm² for humans for pulses less than 30 µsec wide. These values correspond to an estimated peak absorbed power density of 10-16 mJ/kg as measured in the cat head and 16 mJ/kg as estimated for a human head.This energy density is capable of increasing the tissue temperature by only 5 x 10-6°C. As background noise was increased, the threshold for evoked responses in the medial geniculate nucleus of the cat remained stable for pulsed microwave stimuli but increased for acoustic stimuli. The most likely mechanism of electromagnetic field interaction appears to be a conversion of electromagnetic energy to acoustic energy due to thermal expansion.

Study character:

Study funded by