Human Auditory Perception of Pulsed Radiofrequency Energy

Human auditory perception of pulses of radiofrequency (RF) energy is a well-established phenomenon that is dependent upon the energy in a single pulse and not on average power density. RF-induced sounds can be characterized as the perception of subtle sounds because, in general, a quiet environment is required for the sounds to be heard. The sound is similar to other common sounds such as a click, buzz, hiss, knock or chirp. Effective radiofrequencies range from 216 to 10,000 MHz, but an individual’s ability to hear RF-induced sounds is dependent upon high-frequency acoustic hearing in the kHz range. The fundamental frequency of RFinduced sounds is independent of the radiofrequency but dependent upon head dimensions. The detection of RF-induced sounds is similar to acoustic sound detection once the cochlea is stimulated; however, the site of conversion of RF energy to acoustic energy is peripheral to the cochlea. The thermoelastic expansion theory explains the RF hearing phenomenon. RF-induced sounds involve the perception, via bone conduction, of thermally generated sound transients, that is, audible sounds are produced by rapid thermal expansion resulting from only a 5 x 10 C temperature rise in tissue at the threshold level due to absorption of the energy in the RF pulse. The experimental weigh-of-evidence excludes direct stimulation of the central nervous system by RF pulses. The perception of RF-induced sounds near the threshold exposure level is considered to be a biological effect without an accompanying health effect. This conclusion is supported by a comparison of pressures induced in the body by RF pulses and by clinical ultrasound procedures.