The head as an electromagnetic cavity resonator or an acoustical resonator

Introduction

The head can be considered as an electromagnetic cavity resonator or an acoustical resonator.

For background on electromagnetic cavity resonators please refer to Physics section "Vacuum Electronic Devices - Cavity Resonators".

The head modelled as a ~10 cm sphere is an electromagnetic cavity resonator with a resonant frequency ranging from 400 to 700 MHz depending on its size 

It thereby "traps" waves of the resonance frequency via standing-wave generation.

If a signal for reading or influencing brain activity uses a carrier wave of the above frequency range, it will demonstrate maximal interaction with the brain and will therefore exhibit enhanced delivery and EEG enactment. 

The above frequencies instead of just transversing the head, set up standing waves inside the head, are confined or "trapped" within it and deposit electromagnetic energy.

If we wish to transfer electromagnetic waves, for instance from their production source to their utilization location, as in the case of the microwave oven, where we transfer EM waves from the magnetron to the heating chamber, or in the case of RADAR, we may use a metal pipe which is called a waveguide (given that it guides waves towards a location). In general, electromagnetic waves can be found in pipes or closed cavities as in the case of the microwave heating chamber. A closed cavity has certain resonance frequencies depending on its dimensions. This means that if electromagnetic waves of a frequency that is equal to a resonance frequency of the cavity are applied or sent to a cavity, then as the waves move back and forth, the opposing-moving waves will interfere and will form standing waves inside the cavity. As a result, electromagnetic energy will be stored in it. The ability of the cavity to sustain the oscillations is represented by the Q factor.

The lowest resonant frequency of a cavity is termed the fundamental frequency. This corresponds to the frequency for which the half-wavelength (λ/2) is equal to the width of the cavity. It is interesting to mention that a microwave oven uses a wavelength that can fit inside it and accomplishes resonance and generation of standing waves. Consumer ovens use a frequency of 2.45 gigahertz (GHz) corresponding to a wavelength of 12.2 centimetres (4.80 inches). Different experimental settings e.g. using moist paper inside the microwave oven can be used to visualize the standing wave pattern.

Using modelling approaches where the human head is represented by a sphere of a radius of approximately 10 cm, it has been determined that its resonant frequency is a function of its size, ranging from around 700 MHz in the infant to 400 MHz in adults (Gandhi et al., 1977).

Excerpt from the ARRL Handbook for Radio Amateurs on web adress http://www.arrl.org/rf-radiation-and-electromagnetic-field-safety

under expandable section (indicated by the “+” sign) entitled "Thermal Effects of RF Energy": 

"At frequencies near the body's natural resonant frequency, RF energy is absorbed more efficiently, and maximum heating occurs. In adults, this frequency usually is about 35 MHz if the person is grounded, and about 70 MHz if the person's body is insulated from the ground. Also, body parts may be resonant; the adult head, for example is resonant around 400 MHz, while a baby's smaller head resonates near 700 MHz. Body size thus determines the frequency at which most RF energy is absorbed. As the frequency is increased above resonance, less RF heating generally occurs. However, additional longitudinal resonances occur at about 1 GHz near the body surface."

ARRL is the national association for Amateur Radio in the US. It was founded in 1914 as "The American Radio Relay League". Note: Excerpt in bold cited by McLean C. (2019).

If we take into account the detailed shape of the cavity of the head, including the shape of the brain and both its dimensions as well as its structural characteristics, a finer determination can be provided. Similar studies use techniques such as Finite-Difference Time-Domain (FDTD) modelling.

Based on the above, the frequencies of the mentioned range instead of just transversing the head, will form standing waves inside the head and will be confined or "trapped" within it resulting in electromagnetic energy being deposited in the head.

It could be suggested that if a signal for reading or influencing brain activity uses a carrier wave of the above frequency range, it will benefit from maximal interaction with the brain and will therefore demonstrate enhanced delivery and EEG enactment.

It is suggested that remote neural monitoring is based on neural monitoring techniques such as fMRI and MR current density imaging as well as EPR/ESR and Proton Electron Double Resonance imaging (Overhauser Magnetic Resonance Imaging or OMRI). It is interesting to note "the microwave resonator (cavity)" section from the Wikipedia page on Electron Paramagnetic Resonance (ERP):

"The microwave resonator is designed to enhance the microwave magnetic field at the sample in order to induce EPR transitions. It is a metal box with a rectangular or cylindrical shape that resonates with microwaves (like an organ pipe with sound waves). At the resonance frequency of the cavity microwaves remain inside the cavity and are not reflected back. Resonance means the cavity stores microwave energy, its ability to do this is given by the quality fact (Q) defined by the following equation: (...)"

Former senior DARPA scientist reports transmissions of frequencies in the range of head resonance with the purpose of targeting citizens