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

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.

• human
• animal
• cat
• partial body: head

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.