ABSTRACT

Electrophonic hearing, stimulated by an audio-frequency current passed through different types of electrode Systems attached to various areas of the head and body, has been previously investigated. More recently, human auditory system response to modulated electromagnetic energy has been reported. The experiments to be discussed in this paper were designed to study the hearing phenomena in electrostatic fields when the whole head or parts of its surface are exposed to an alternating electrostatic field of audio frequency with and without a superimposed DC field. The threshold data obtained suggest there is no other auditory stimulation excepting mechanical tissue excitation by the electrostatic forces connected with such fields. Calculated threshold data for stimulation by amplitude modulated RF fields, assuming the same electromechanical excitation of normal bone and air conduction bearing, are presented and compared to the hearing phenomena in such fields reported by others.

ELECTRICAL current of audio frequency passed through the human head by employing various types of electrode systems gives rise to hearing sensations. These hearing sensations in principle were first described as far back as 1800 by Volta and later investigated in detail by Stevens and others. Stevens named this phenomena the electrophonic effect. The intensity and purity of the hearing sensations and the electrical stimulus requirements appeared to depend in a complicated fashion on the location and type of electrode system used. Most results indicated clearly that the hearing sensations had their origin in electromechanical phenomena generating vibrations in bone or tissue structures outside the cochlea and being perceived through the cochlea in the normal way. Some of the original speculations and hopes that the electrophonic effect might be the result of a direct stimulation of auditory nerve activity were soon disproved. Although evidence has been presented to show that it is possible in selected cases to stimulate the eighth nerve directly with sinusoidal current applied with electrodes in the middle ear, such indiscriminate nerve stimulation is perceived only as broad band noise, and therefore, not attractive for communication transmission. Recently, amplitude modulated carrier frequencies ranging from 50 kHz to the radio frequency range have been used with various electrode modifications to produce hearing phenomena in normal human subjects 2, 12 and to attempt speech information transmission to deaf subjects.12 13 All evidence available for this special form of electrophonic hearing points toward the fact that in these cases electromechanical excitation of tissue sets up vibrations, which are carried by bone conduction to the inner ear, stimulating the cochlea in the normal manner. It appears as if the limited, optimistic results reported with hard of hearing and "deaf" subjects can be explained by assuming such bone conduction stimulation. Subjects with a severe hearing loss, characterised as clinically deaf, apparently received additional clues from vibration receptors in the skin when stimulated at high stimulus intensities. Excitation of nerves in the skin by electrical 1 and mechanical 8 stimulation has been studied repeatedly. Although the potential usefulness of this sensory mode for speech communication has not been fully explored, the capabilities and limitation of single point excitation with unpreprocessed audio signals are fairly well known. ‘The learning capability’ for a limited vocabulary has been demonstrated many times together with the shortcomings in discrimination. The latter appears to be restricted to the recognition of the temporal sequence of intensities characteristic to words and sentences and provides in no way for analysis of the spectral information contained in speech, necessary for its recognition.

In 1961 Frey. reported that the human auditory system can directly detect radio frequency energy transmitted through air by electromagnetic waves. Several radio frequencies, transmitted by a pulse modulated microwave transmitter, were used. Subjects, exposed to peak power densities of 200 to 300 milliwatts per cm2 and electric field strengths in the order of 15 volts/cm, reported hearing these modulation pulses. By shielding other areas of the head, Frey found that the temporal area was most sensitive to RF stimulation. Although the mechanism was left in doubt and experimental data were not complete enough for clear differentiation, the explanation has been offered and widely discussed that this phenomenon may be the result of direct cortical or nerve fibre stimulation.

In addition to the technical papers listed, there have been rumours and much hearsay about the hearing phenomena generated in audio frequency and radio frequency electromagnetic fields. In most cases, where the resulting hearing phenomena were accessible to controlled experimentation, the effects could be explained as artifacts. A recent article in a national non-technical journal described an invention allowing transmission of sound directly to the brain by radio frequency waves Although experimental results and details on this invention were never released, it appears most likely that the original claims of this invention arc not true and that the system makes use of an electrophonic effect based on electromechanical tissue excitation.

 

The absence of any attempt to evaluate quantitatively the transduction of electrical energy into mechanical vibrations for the cases of electrophonic hearing and to relate these vibrations to the sensitivity of the ear for hearing by air and bone conduction stimulated the present investigation. It is hoped that studies of the type reported will assist in estimating at least the order of magnitude for electromechanical stimulation to be expected for different types of electrophomc experiments, and will attempt to explain quantitatively

many of the phenomena oobserved by various investigators. The obvious and easily accessible electromechanical component could be first utilised or eliminated in

attempting to explain electrophonic observations. Only if this approach is unsuccessful, one should resort to one of the more unconventional hypotheses of transmitting auditory information through other sensory channels or directly to the brain.

The reported investigation depicts the most simple type and configuration of electromechanical stimulation, calculates the hearing sensations to be expected, and compares the theoretical results with simple well-controlled laboratory experiments in electrostatic fields. The discussion applies these results to an explanation of some of the various types of electrophonic effects previously described
Study of Three cases of Electrostatic Excitation:-
The action of electrostatic fields on the human head was investigated for the three conditions illustrated in Figure 1. In the situation 1.a., the head was exposed to a uniform electric field produced between the electrodes of a large plate condenser. Providing the wavelength used is large compared to the distance between the plates (<10E8 Hz) the field can be considered as quasi-stationary.

The relatively simple electrostatic force formulas can then be applied to calculate the force exerted by the field on the head. The head can be approximated by a conducting dielectric) sphere and the stress distribution on its surface determined. The stresses in this situation, when alternating fields are applied, are in phase over the whole surface, tending to compress and dilate the head mechanically’ in its fundamental mode. This mechanical action of the electric field on the head is similar to the mechanical action of a low frequency sound wave on the human head provided the wave length of the sound wave is large compared to the head. (In addition, no asymmetrical resultant net force on the head occurs in the electric case). Assuming equal pressure distribution over the head for the electrostatic field and the acoustic field case, the resulting hearing phenomena should be the same. For the acoustic excitation this situation has been extensively studied. The auditory threshold for stimulating the ear by "body" or ‘tone conduction in a free sound field" is known. (For this mode of excitation no sound is allowed to reach the ear directly by air conduction, i.e. by travel through the auditory meatus to the tympanic membrane). Auditory sensations must, therefore, occur in such an electrostatic field when the electrostatic pressure on the head surface is of the same magnitude as the sound pressure required for stimulating bone conduction in a free sound field. Good quantitative agreement can only be expected at the low frequencies since the pressure distribution over the head changes considerably at higher frequencies in a progressive sound wave, whereas the stress distribution from electrostatic forces remains unchanged by the audio frequency modulation of the field.

In situation 1.b. the electrostatic field was restricted to a small area of the head and existed only between an electrode held at close distance from the body surface.

In this situation the electrostatic field should be audible through electromechanical force stimulation alone, when the electrostatic force equals or exceeds the threshold force for bone conduction excitation of this area. The threshold of bone conduction, when small areas of the head are driven by’ mechanical drivers, are well known.3 It is a characterisation for this type of bone conduction threshold that below 2000 Hz values differ depending upon the ear canal being open or closed by an effective earplug. With earplug the threshold is lower, i.e. less force on the head is required to stimulate the inner ear

In situation Ic, an electrode was inserted in the external auditory meatus and placed close to the tympanic membrane, the electrostatic forces, therefore, were concentrated on the tympanic membrane. Comparison of this effect with normal airborne acoustic stimulation of the tympanic membrane should be possible.