Introduction to RF Radiation Safety / 7 / 7.3 / 7.3.3

7.3.3 Microwave Auditory Effect

The microwave auditory effect, or Frey effect, is an uncommon but established sensory response in which a person exposed to particular high-peak-power microwave pulses may perceive clicks, buzzing, knocking, or chirping without an external acoustic source.

The accepted mechanism is rapid thermoelastic expansion. A brief pulse deposits a small amount of energy in head tissues, producing a minute and rapid temperature change and a pressure wave that reaches the cochlea. The effect is not direct electromagnetic stimulation of the auditory nerve and does not require a harmful bulk temperature rise.

The perception normally ceases when the exposure ends and is not itself evidence of tissue injury. Nor does it by itself prove that a health-based exposure restriction has been exceeded. An unexpected occurrence in an occupational setting should nevertheless prompt review of the pulse characteristics and exposure circumstances.

The effect demonstrates that temporal characteristics can influence biological response. ICNIRP's brief-exposure restrictions are intended more generally to prevent excessive transient temperature rise from rapid energy deposition; they should not be described as limits derived specifically from the microwave auditory effect.

7.3.4 Indirect Biological Effects

As described in Section 7.2, RF fields can affect safety indirectly by inducing voltages or currents in conductive objects. Contact current or a spark discharge may produce tingling, shock, involuntary movement, or a localized burn. Grounding, bonding, shielding, isolation, and safe separation are therefore important controls near high-power RF sources and large conductive structures.

Implanted and body-worn medical devices require separate consideration. RF fields may interfere with device electronics or couple to conductive leads, potentially affecting operation or producing localized heating. The result depends on the device, frequency, source, geometry, and exposure conditions.

General human-exposure restrictions are not electromagnetic compatibility limits and do not guarantee the safe operation of a medical device. Manufacturer instructions, advice from a qualified health professional, and applicable device-specific standards must be followed.