HUMAN AUDITORY PERCEPTION OF PULSED RADIOFREQUENCY ENERGY

J.A. Elder and C. K. Chou

Motorola Florida Research Laboratories

8000 W. Sunrise Blvd.

Plantation, FL 33322

Corresponding author: Joe A. Elder, Ph.D.

Motorola Florida Research Laboratories, 8000 W. Sunrise Blvd., Plantation, FL 33322

Telephone: (954) 723-4895

Fax: (954) 723-5611

Email: joe.elder@motorola.com

Running title: Auditory Perception of RF Pulses

Elder, page 1

ABSTRACT

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 RF-

induced 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 10-6 oC

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.

Key Words: RF hearing, microwave, thermoelastic

Elder, page 2

INTRODUCTION

In their review article on the radiofrequency (RF) hearing phenomenon, Chou et al. (1982)

wrote:

“The earliest report we have found on the auditory perception of pulsed microwaves

appeared in 1956 as an advertisement of the Airborne Instruments Laboratory in

Vol. 44 of the Proceedings of the IRE. The advertisement described observations

made in 1947 on the hearing of sounds that occurred at the repetition rate of a radar

while the listener stood close to a horn antenna. When the observers first told their

coworkers in the Laboratory of their hearing experiences, they encountered skepticism

and rather pointed questions about their mental health.”

The skepticism surrounding early reports of RF hearing, such as the one quoted above,

was based on our understanding of human hearing. The ear was known to be exquisitely

sensitive to pressure waves and, at that time, to have no sensitivity to electromagnetic waves at

microwave frequencies (300 MHz – 300 GHz). The skepticism helps to explain why the first

systematic study of this phenomenon by Frey (1961) did not appear until many years after the

development of radar in the early 1940’s. Frey described the perception of transient buzzing

sounds by human subjects exposed to RF radiation from a rotating radar antenna. The apparent

location of the sound, which was described as a short distance behind the head, was the same

regardless of the body’s orientation to the radar (Frey, 1961). In later reports (Frey, 1962, 1963),

RF hearing was described as a “buzz, clicking, hiss or knocking” sound. Table 1 contains

descriptions of these and other sounds reported by human beings exposed to pulsed RF fields.

When a metal shield of aluminum flyscreen was placed between the subject and the radar, no RF
Elder, Page 3

sounds were heard (Frey and Messenger, 1973). The sensitive area for detecting RF sounds was

described as a region over the temporal lobe of the brain, because the placement of a small piece

of metal screen (5 x 5 cm) over this area completely stopped the sound (Frey, 1962). The

subjects in Frey (1961) reported an increase in the RF sound level when earplugs were used to

reduce the ambient noise level, an observation confirmed by others (Guy et al., 1975).

The “sound was something like that of a bee buzzing on a window, but with, perhaps,

more high frequencies” according to Ingalls (1967) who used two radars like those described in

Frey (1961). The sound seemed to come from about a meter or two above the head. In another

report (Constant, 1967), the RF sound was described as being in the area of the ear on the side

opposite to the one that was irradiated. All subjects experienced a buzzing sensation at a pulse

repetition rate (PRR) greater than 100/s, whereas individual pulses were heard at a PRR below

100/s. Cain and Rissmann (1978) reported that human subjects heard distinct clicks either inside

the head or behind the head when exposed to pulsed fields. Individual pulses were heard as

distinct and separate clicks, and short pulse trains as chirps with the tone pitch corresponding to

the PRR by two of the study investigators in Guy et al. (1975). The RF-induced sound appeared

to originate from within or near the back of the head. This report also included the note that

transmitted digital codes could be accurately interpreted by the subject when the pulse generator

was keyed manually. Two reports from Russian scientists described the perception of pulsed RF

signals as polytonal sounds and tinnitus (Tyazhelov et al., 1979; Khizhnyak et al., 1980).

These studies show that human perception of pulsed RF radiation, resulting in sounds that

vary with modulation of the signal, is a well-established phenomenon. The following sections

describe the effective radiation parameters including thresholds for RF hearing, the dependence
Elder, Page 4

of RF hearing on acoustic hearing, the mechanism responsible for human perception of pulsed

RF fields, and a discussion of the significance of the effect. Additional information is available

in reviews by Chou et al. (1982); Elder (1984); Lin (1978, 1989, 1990, 2001); Postow and

Swicord (1996) and Stewart (2000).

EFFECTIVE RF RADIATION PARAMETERS

A summary of RF radiation parameters used in human studies is shown in Table 1. The

parameters include frequency, PRR, pulse width, peak power density, average power density,

and energy density/pulse. Threshold values for RF hearing have been reported in several studies

and these are shown in the table also.

RF hearing has been reported at frequencies ranging from 216 to 10,000 MHz (see Table

1). Although Ingalls (1967) mentioned 10,000 MHz as an effective frequency, other

investigators found that lower frequencies (8900 and 9500 MHz) at very high exposure levels did

not induce RF sounds. For example, the frequency of 8900 MHz was not effective at an average

power density of 25 mW/cm2 and peak power density of 25,000 mW/cm2 (Frey, 1962). At 216

MHz, the lowest effective frequency reported in the literature, the average power density

threshold was 4 mW/cm2 and the peak power density was 670 mW/cm2 (Frey, 1963). The lowest

threshold value expressed in units of average incident power density is 0.001 mW/cm2 (Cain and

Rissmann, 1978). This low value was due to the slow PRR of only 0.5/s (Table 1) because, for a

given peak power, average power density depends on the pulse repetition rate. The hearing

phenomenon, however, has been shown to depend on the energy in a single pulse and not on

average power density. Guy et al. (1975) found that the threshold for RF hearing of pulsed
Elder, page 5
2450-MHz radiation was related to an energy density of 40 µJ/cm2 per pulse, or energy

absorption per pulse of 16 µJ/g, regardless of the peak power of the pulse or the pulse width (less

than 32 µs); calculations showed that each pulse at this energy density would increase tissue

temperature by about 10-6 oC.

A review of the table reveals that many of the threshold values were determined in a very

quiet environment or subjects used earplugs or earmuffs to decrease the ambient noise level. As

mentioned in the Introduction, earplugs were used by the subjects in Frey’s first report in 1961.

Thus, investigators were generally aware that a quiet environment was required because, in many

cases, the normal noise levels in laboratory and outdoor environments masked the perception of

RF sounds. In Guy et al. (1975), for example, the threshold value cited above was obtained in a

very quiet environment having a background noise level of only 45 dB. When earplugs were

used, the threshold level for one subject decreased from 40 to 28 µJ/cm2. The threshold for a

subject with a hearing deficit was much higher, approximately 135 µJ/cm2.

DEPENDENCE OF RF HEARING ON ACOUSTIC HEARING

The advertisement from Airborne Instruments Laboratory (1956) stated that two persons

with hearing loss above 5 kHz did not perceive RF sounds as well as did observers with normal

hearing up to 15 kHz. Later studies provided more information on the relationship between

acoustic and RF hearing. Frey (1961) reported that a necessary condition for perceiving the RF

sound was the ability to hear audiofrequencies above approximately 5 kHz, although not

necessarily by air conduction. This conclusion was based on results with subjects with normal or

defective hearing. One subject with normal air-conduction hearing below 5 kHz failed to hear

the microwave pulses; the person was subsequently found to have a substantial loss in bone-
Elder, page 6
conduction hearing. Another subject with good bone-conduction hearing but with poor air-

conduction hearing perceived the RF sound at approximately the same power density that

induced threshold perception in subjects with normal hearing. In a later study, humans were

shown to match sounds caused by repetitive exposure to a pair of RF pulses in the MHz range to

acoustic frequencies near 4.8 kHz (Frey and Eichert, 1985).

In addition to determining standard audiograms that measure hearing thresholds for air

conduction at acoustic frequencies of 250 to 8000 Hz and for bone conduction to 4000 Hz, Cain

and Rissmann (1978) measured the hearing ability of eight subjects over the frequency range of 1

to 20 kHz. They found that although there was no apparent correlation between the ability to

perceive pulsed RF fields at 3000 MHz and hearing ability as measured by standard audiograms,

there was a strong correlation between the RF-hearing threshold and thresholds to air-conducted

acoustic signals above 8 kHz. For example, three of the subjects who had normal hearing below

4 kHz, but a hearing deficit at frequencies above 8 kHz, could not hear RF sounds under

conditions in which the other subjects could perceive RF sounds. The studies by Frey (1961),

Frey and Eichert (1985) and Cain and Rissmann (1978) show RF hearing to depend on high-

frequency hearing in the range of about 5 to 8 kHz and bone-conduction hearing at lower

acoustic frequencies. Calculated values of fundamental frequencies of RF sound in the human

head based on animal data or models are somewhat similar, e.g., 7-10 kHz (Chou et al., 1977),

13 kHz (Lin 1977) and 7-9 kHz (Watanabe et al., 2000); the results of these three studies are

described in more detail below.

Elder, page 7

SIMILARITY OF AUDITORY RESPONSE TO MICROWAVE AND CONVENTIONAL

ACOUSTIC STIMULI

The auditory pathway by which acoustic waves detected by the ear become interpreted as

sound in the brain is known in some detail and several studies have been done to determine if the

electrophysiological response of the auditory pathway to RF pulses is similar to the response to

acoustic stimuli. The first stage of sound transduction is mechanical distortion of cochlear hair

cells that result in cochlear microphonics, electrical potentials that mimic the sonic waveforms of

acoustic stimuli. Subsequent to the detection of sound by the cochlea, electric potentials

associated with the detection of sound may be recorded by electrodes placed in neurons at

various locations along the auditory pathway.

In 1962, Frey proposed that RF hearing might be a result of direct cortical or neural

stimulation but the results of later studies described in this review showed that Frey’s theory was

incorrect. His proposal was based, in part, on his failure to demonstrate that RF pulses stimulate

the cochlea, that is, cochlear microphonics were not recorded at power densities much higher

than those required to elicit auditory nerve responses (Frey, 1967). Guy et al. (1975) also failed

to measure cochlear microphonics but determined that the failure was due to insufficient

absorption of RF energy. In 1975, Chou et al. reported their success in overcoming the technical

problems that had prevented investigators from recording cochlear microphonics from RF-

exposed animals. The results showed that pulses of RF energy activated the cochlea because

cochlear microphonics were recorded that were similar to those evoked by acoustic stimuli. The

demonstration that RF sounds are perceived by the normal auditory system via the cochlea

provided evidence against the proposal that RF pulses directly simulated the central nervous
Elder, page 8
system.

Taylor and Ashleman (1974) and Guy et al. (1975) showed the importance of the cochlea

by finding that destruction of the cochlea abolished RF-evoked potentials recorded at higher

levels in the auditory pathway. These results indicated that the locus of the initial interaction of

pulse-modulated microwave energy with the auditory system is peripheral to the cochlea.

In cats with an undamaged cochlea, Taylor and Ashleman (1974) measured the

electrophysiological response in three successive levels of the cat auditory nervous system

(eighth cranial nerve, medial geniculate nucleus, and primary auditory cortex) to both acoustic

and pulsed-microwave (2450-MHz) stimuli. They found similar responses to microwave stimuli

and conventional acoustic stimuli. Lebovitz and Seaman (1977a,b) reached the same conclusion

based on the similar response of single auditory neurons in the cat to pulsed 915-MHz fields and

acoustic clicks. The detection of these electric potentials in auditory neurons was expected based

on the results of studies that demonstrated subjective auditory perception (Frey, 1962), auditory

evoked potentials (Taylor and Ashleman, 1974), and cochlear microphonics (Chou et al., 1975).

It is known that acoustic stimuli can cause evoked potentials, called “cross-modal”

responses, in central nervous system sites outside the auditory pathway. Similar “cross-modal”

responses due to the auditory response to RF pulses were recorded by Guy et al. (1975). This

finding indicated that electric potentials recorded from any CNS location could be misinterpreted

as a direct interaction of RF energy with the particular neural system in which the recording was

made, as reported by Frey (1967).

In an experiment in which the thresholds of evoked electrical responses from the medial-

geniculate body in the auditory pathway in cats were determined as a function of background

noise, Guy et al. (1975) found that as the noise level (50- to 15,000-Hz bandwidth) increased
Elder, page 9
from 60 to 80 dB, there was only a negligible increase in the threshold for microwave stimuli, a

moderate increase in the threshold for a piezoelectric bone-conduction source, and a large

increase in the threshold for loudspeaker-produced stimuli. The finding that the evoked response

to microwave stimuli did not increase in relation to background noise, which included acoustic

frequencies to 15,000 Hz, indicated that pulsed RF energy interacted with the high-frequency

portion of the auditory system.

Additional support for the dependence of RF hearing on high-frequency hearing was

provided by theoretical analysis of acoustic vibrations induced in the heads of animals and

humans based on thermal expansion in spheres exposed to pulses of RF energy (Lin, 1977). The

frequency of the induced sound was found to be a function of head size and of acoustic

properties of brain tissue; hence, the acoustic pitch perceived by a given subject is the same

regardless of the frequency of RF radiation. The calculations of Lin show that the fundamental

frequency predicted by the model varies inversely with the radius of the head, i.e., the larger the

radius, the lower the frequency of the perceived RF sound. The estimated fundamental

frequency of vibration in guinea pigs, cats, and adult humans were 45, 38, and 13 kHz,

respectively; the frequency for an infant human head was estimated to be about 18 kHz. These

calculations provide further evidence that a necessary condition for auditory perception by adult

humans is the ability to hear sound waves at frequencies above about 5 kHz (Frey, 1961;

Rissmann and Cain, 1975).

The results of Lin (1977) appear to be in good agreement with the measurements of Chou

et al. (1975), who found cochlear microphonics of 50 kHz in guinea pigs exposed to RF pulses.

In a later report, Chou et al. (1977) found the frequency of cochlear microphonics in guinea pigs

and cats to correlate well with the longest dimension of the brain cavity and, based on these data,
Elder, page 10

estimated the frequency of the microwave-induced cochlear microphonics in human beings to be

between 7 and 10 kHz.

Gandhi and Riazi (1986) calculated RF hearing thresholds at 30-300 GHz, but there is

little if any physiological significance of these calculations to RF hearing because a) the

fundamental frequencies in the head are on the order of several hundred kilohertz, well above the

maximum acoustic frequency of 20 kHz for human hearing, and b) there are no reports of human

perception of RF pulses at frequencies higher than 10 GHz (see Table 1).

The results of the above studies of evoked electrical potentials in the auditory system,

including the demonstration of pulsed-RF-evoked cochlear microphonics, strongly indicate that

the detection of RF-induced auditory sensations is similar to that of acoustic sound detection, the

site of conversion from RF to acoustic energy is peripheral to the cochlea, the fundamental

frequency of RF sound is independent of the radiofrequency but dependent upon the dimensions

of the head, and the pulsed RF energy interacts with the high-frequency portion of the auditory

system. To hear RF sounds, one must be exposed to pulses of RF energy in the MHz range and

be capable of hearing acoustic waves in the kHz range.

MECHANISM OF RF HEARING: THERMOELASTIC EXPANSION

One of the first challenges to Frey’s proposal of direct neural stimulation (Frey, 1961,

1962) came from Sommer and von Gierke (1964) who suggested that stimulation of the cochlea

through electromechanical field forces by air or bone conduction appeared to be a more likely

explanation of the RF hearing phenomenon. Other scientists who helped lay the foundation for

identifying the mechanism are White (1963) and Gournay (1966). White (1963) showed that
Elder, page 11

pressure waves could be detected in water exposed to pulses of RF energy and his analysis of

waves in this system predicted that, as a result of thermal expansion, the resulting temperature

gradient would generate stress waves that propagate away from the site of energy absorption.

Gournay (1966) extended White’s analysis to show that for single long pulses, the induced stress

wave is a function of peak power density and, for shorter pulses, the stress wave is a function of

the peak power density and pulse width (or energy density per pulse).

Foster and Finch (1974) extended Gournay's analysis to a physiological solution exposed

to RF pulses similar to those that produce sounds in humans. They showed both theoretically

and experimentally that pressure changes would result from the absorption of RF pulses which

could produce significant acoustic energy in the solution. They concluded that audible sounds

were produced by rapid thermal expansion, resulting from only a 5 x 10-6 oC temperature rise in

the physiological solution, due to absorption of the energy in the RF pulse. This conclusion led

to their proposal that thermoelastic expansion is the mechanism for RF hearing. This mechanism

is consistent with the following results.

1) RF pulses that would elicit sounds perceived by a human produced acoustic transients

recorded with a hydrophone immersed in a solution (0.15 N KCl) having an electrical

conductivity similar to that of tissue. In addition, acoustic transients were detected in

blood, muscle, and brain exposed in vitro to pulses of RF energy.

2) The RF-induced pressure wave generated in distilled water inverted in phase when the

water was cooled below 4 °C, and the response vanished at 4 °C, in agreement with the

temperature dependence of the thermoelastic properties of water.

3) The thermoelastic theory predicts that the maximal pressure in the medium is
 Elder, page 12

4) proportional to the total energy of the pulse for short pulses and is proportional to the

peak power for long pulses. The relationship between pulse width and the RF-generated

acoustic transient in the KCl solution was consistent with the theory.

Based on these findings, Foster and Finch concluded that RF-induced sounds involve

perception, via bone conduction, of the thermally generated sound transients caused by the

absorption of energy in RF pulses. The pulse can be sufficiently brief ( 50 µs) such that the

maximum increase in tissue temperature after each pulse is very small (<10-5 oC). The peak

power intensity of the pulse, however, must be moderately intense (typically 500 to 5000

mW/cm2 at the surface of the head). These values are within the range of effective peak power

intensities of 90-50,000 mW/cm2 in the human studies shown in Table 1.

A year before the thermoelastic theory was proposed by Foster and Finch (1974), Frey and

Messenger (1973) published the results of a human study that are in agreement with the theory.

That is, the loudness of the RF hearing sensation in the human subjects depended upon the

incident-peak-power density for pulse widths <30 µs; for shorter pulses, their data show that

loudness is a function of the total energy per pulse. The threshold dependence on pulse width

reported by Chou and Guy (1979) is in agreement with the predictions of the thermoelastic

mechanism. They showed that the threshold for RF hearing in guinea pigs, as measured by

auditory brainstem-evoked electrical responses, is related to the incident energy per pulse for

pulse widths <30 µs and is related to the peak power for longer pulses.

The results on threshold and loudness may be summarized as follows. The energy in the first

30 µs or so of the pulse determines the threshold and loudness levels regardless of pulse widths

greater than about 30 µs. For wider pulses (>90 µs), loudness is related to peak power rather than
Elder, page 13

energy because the energy associated with the first 30 µs of the pulse increases directly with peak

power. Thus, if sufficient energy is deposited within a 30-µs period, an RF-induced sound will

result without regard to pulse width. And, for pulses >30 µs, loudness increases with an increase

in peak power. Thus, the auditory response undergoes a gradual transition from an energy-

related effect at pulse widths <30 µs to an effect dependent on peak power at pulse widths >90 µs

(Frey and Messenger, 1973; Chou and Guy, 1979).

A psychophysical experiment with 18 subjects examined the adequacy of the

thermoelastic hypothesis and the perceptual qualities of RF-induced sounds (Tyazhelov et al.,

1979). Audiofrequency signals were presented alternately to or concurrently with microwave

pulses (see Table 1) under conditions in which the subject could adjust the amplitude, frequency,

and phase of the audio signal. Long pulses (~100 µs) resulted in a lower pitch of the RF sound

and two subjects who had a high-frequency auditory limit of 10 kHz could not hear short RF

pulses but could hear long pulses. These observations on human perception of long pulses are

consistent with the results of electrophysiological responses in cats, that is, long pulses of 250 to

300 µs led to a decrease in sensitivity of high-frequency auditory responses (Lebovitz and

Seaman 1977). Tyazhelov et al. (1979) concluded that the thermoelastic hypothesis adequately

explained some of their findings for RF pulses of high peak power and short width (<50 µs), but

they questioned the applicability of the hypothesis to some observations involving near-threshold

pulses of low-power, long-duration, and high-repetition rate [see Chou et al. (1982) for a critique

of Tyazhelov et al. (1979)]. In a subsequent paper, Tyazhelov and colleagues suggested that the

thermoelastic theory accounted for the low frequency, but not the high frequency, RF sounds

(Khizhnyak et al., 1980); however, no other reports have been found that support their proposed
Elder, page 14

model for high frequency RF sounds.

Other animal studies, in addition to those already discussed, support and extend our

understanding of RF hearing and the thermoelastic mechanism. Several investigators have

determined the threshold for the RF-induced auditory sensation in laboratory animals (Table 2).

In cats exposed to pulses of 918- and 2450-MHz radiation, the threshold was related to the

incident energy density per pulse. The cat’s threshold energy density per pulse was about one-

half of the human threshold (Guy et al., 1975). The thresholds in Cain and Rissmann (1978) are

in general agreement with the results in Guy et al. (1975), but a lower threshold was reported by

Seaman and Lebovitz (1989). At higher frequencies between 8670 and 9160 MHz, Guy et al.

(1975b) found that the threshold values of power density and of energy density per pulse were an

order of magnitude higher than those at 918 and 2450 MHz (Table 2), but it is noted that no

auditory response was obtained at the two higher frequencies unless the brain was exposed by

removing part of the skull.

In guinea pigs, the threshold dependence on pulse width was found to be in agreement

with the predictions of the thermoelastic expansion mechanism; that is, the threshold was related

to the incident energy per pulse for short pulse widths (<30 µs) and was related to the peak power

for longer pulses. At the shortest pulse width (10 µs), the threshold was about 6 µJ/g (Chou and

Guy, 1979).

Chou et al. (1985) documented the dose response relationship of the auditory brainstem-

evoked response (BER) in rats exposed to pulses of 2450 MHz fields in circularly polarized

waveguides. The results were consistent with the thermal expansion theory because the same

BER response was evoked when the incident energy density or absorbed energy density per
Elder, page 15

pulse was the same, regardless of pulse width.

By measuring acoustic pressure waves with a miniature hydrophone transducer implanted

in the brains of rats, cats and guinea pigs exposed to pulses of RF energy, Olsen and Lin (1983)

confirmed earlier theoretical predictions of pressure waves in the head. In later work, Lin et al.

(1988) observed that the speed of RF-induced pressure waves in the cat brain was similar to that

of conventional acoustic wave propagation. These results support the thermoelastic expansion

theory.

The hypothesis of Foster and Finch (1974) predicts that the RF hearing effect is related to

thermoelastically induced mechanical vibrations in the head. Vibrations of this type can be

produced by other means, such as by a laser pulse or by a pulsed piezoelectric crystal in contact

with the skull (Chou et al., 1976). Frey and Coren (1979) used a holographic technique to test

whether the skull and the tissues of the head of an animal have the predicted vibrations when

exposed to a pulsed RF field. No displacements were recorded, but a subsequent paper by Chou

et al. (1980) demonstrated that the holographic technique used by Frey and Coren (1979) did not

have the sensitivity to detect displacements related to vibrations from microwave-induced

thermoelastic expansion in biological tissues.

Wilson et al. (1980) described an autoradiographic technique in which [14C]2-deoxy-D-

glucose was used to map auditory activity in the brain of rats exposed to acoustic stimuli and to

pulsed- and continuous-wave radiation. With this technique, in vivo determination of metabolic

activity (i.e., glucose utilization and associated functional activity in the brain) can be visualized.

Prior to exposure to the acoustic stimuli or to microwaves, one middle ear was ablated to block

detection of sound waves in one side of the head. The expected bilateral asymmetry of
Elder, page 16

radioactive tracer uptake in the auditory system of rats exposed to acoustic clicks or weak

background noise was demonstrated. In contrast, a symmetrical uptake of tracer was found in

the brain of animals exposed to pulsed radiation. These autoradiographic results confirmed the

finding that RF hearing does not involve the middle ear (Frey, 1961; Chou and Galambos, 1979).

Unexpectedly, Wilson et al. (1980) found similar patterns of radioactive tracer uptake in the

auditory system of rats exposed to continuous wave radiation and to pulsed radiation. These

results with a continuous wave field, however, have not been independently replicated and there

are no known reports of continuous wave signals causing RF-induced sound in humans or

experimental animals.

In summary, evidence from both human and laboratory animal studies indicates that

thermoelastic expansion is the mechanism that explains the RF hearing phenomenon. The

evidence includes measurements of acoustic transients in water, physiological (KCl) solution,

and tissues (Foster and Finch, 1974) as well as in muscle-simulating materials (Olsen and

Hammer, 1980); the relationship of the threshold value to pulse duration (Foster and Finch,1974;

Frey and Messenger, 1973; Chou and Guy, 1979); the characteristics of the RF-induced cochear

microphonics in laboratory animals (Chou et al., 1975, 1977) and calculations of the

fundamental frequencies in the human head (Lin 1978; Chou et al., 1977) that correlate well with

the perception of high frequency sounds in the kHz range.

SIGNIFICANCE OF RF HEARING

The potential for human exposure to pulsed fields that could induce RF hearing raises two

questions in regard to the significance of the effect. One, what is the psychological impact of RF
Elder, page 17

sounds? Two, aside from the perception of sounds, what is the physiological significance of

exposure to pulsed RF radiation at intensities at and above the threshold for hearing?

The perception of RF sounds at threshold exposure levels is considered to be a biological

effect without a health effect and, therefore, is not an adverse effect.1 This conclusion is based on

the following points. The sounds associated with RF hearing are not unusual but are similar to

other common sounds such as a click, buzz, hiss, knock or chirp (see Table 1). Furthermore, RF

hearing can be characterized as the perception of subtle sounds because, in general, a quiet

environment is required for the sounds to be heard. It was noted in this review that most of the

human subjects in the studies listed in Table 1 used earplugs to create conditions sufficiently

quiet to hear RF sounds. The apparent location of the sounds, however, may vary from within,

behind or above the head. Under some exposure situations that may lead to prolonged periods of

RF sounds, the sounds might become an annoyance but our knowledge of the effective exposure

conditions is sufficient to develop measures to eliminate RF sounds determined to be annoying.

One solution is to move farther away from the source. A review of the human studies in Table 1

reveals that most of the studies were done in laboratory settings in which the subjects were a few

feet from the RF antenna. In three of the four field studies, the distance of the subjects from the

radar ranged from about six feet up to several hundred feet. Such close proximity was needed to

achieve the effective, moderately high, peak power intensities ranging from 90-50,000 mW/cm2

(see Table 1). This information on distance and effective exposure levels indicates that anyone

1
An adverse effect is a biological effect characterized by a harmful change in health. For example, such changes can
include organic disease, impaired mental function, behavioral dysfunction, reduced longevity, and defective or
deficient reproduction. Adverse effects do not include: 1. Biological effects without a health effect. 2. Changes in
subjective feelings of well-being that are a result of anxiety about RF effects or impacts of RF infrastructure that are
not related to RF emissions. 3. Indirect effects caused by electromagnetic interference with electronic devices. These
indirect effects are covered by other standards. (This definition was developed by the IEEE CES SCC28/SC4
Revision Working Group.)
Elder, page 18

reporting RF hearing would be relatively close to a pulsed source operating in the 216 –10,000

MHz range (Table 1). If it is not possible to increase the distance from the source, remediation

measures could include metal shielding and changes in the operating procedure of the RF device.

Aside from the perception of sound, it is important to address the physiological significance

of exposure to RF pulses at the threshold for hearing. One approach is to compare the magnitude

of the pressure of the RF-induced acoustic wave in the head to pressures from other sources.

Based on calculated pressures resulting from the absorbed energy of 915-MHz pulses in human

head models, Watanabe et al. (2000) found the RF-induced pressure at the hearing threshold to

be only 0.18 Pa. This threshold value is more than 42,000X lower than ultrasound-induced

pressure (7700 Pa, spatial peak temporal average) during medical diagnosis, which includes

exposure of the fetus; the factor would be much greater if the comparison was to the higher

spatial peak temporal power of the ultrasound pulses. Another comparison shows that the

pressure at the RF hearing threshold is 1,000,000X lower than the pressures at the surface of the

brain that produce changes in the EEG and moderate brain damage (1.5 X 105 Pa and 3 X 105 Pa,

respectively) based on studies of traumatic head injury (see Raslear et al., 1993, p. 476). When

compared to pressures exerted by medical ultrasound exposure and traumatic injury, it is highly

unlikely that the RF hearing effect at the threshold level is hazardous with regard to the strength

of the pressure waves, the dominant force in comparison to electrostrictive force and radiation

pressure (see Guy et al., 1975; Gandhi and Riazi, 1986). The comparison with ultrasound

pressures suggests that RF-induced pressures would have to be several orders of magnitude

greater than the pressure at the hearing threshold to cause adverse effects.

Very high intensity RF pulses will induce adverse effects such as convulsions and a state of
Elder, page 19

unconsciousness (stun effect) as demonstrated by Guy and Chou (1982). These authors

determined the threshold for these effects in rats exposed to a single, high intensity, 915-MHz

pulse that caused an elevation in brain temperature of 8 oC resulting in petit or grand mal seizures

lasting for one minute after exposure, followed by a four-to-five-minute unconscious state. The

brain temperature returned to normal within five minutes after exposure and the animals began

moving when the brain temperature returned to within 1 oC of normal. Limited histopathological

examination of four exposed rats revealed significant changes including neuronal demyelination

at one day after exposure and brain swelling at one month after exposure. The threshold for the

stun effect was 680 J, regardless of peak power and pulse width, or about 28 kJ/kg, expressed in

terms of peak specific absorption. The stun threshold, a clearly adverse effect, is about

100,000X higher than the thresholds for auditory responses in rats (5-180 mJ/kg) and humans

(16 mJ/kg) (Guy et al., 1975).

Small but significant changes in the otoacoustic emissions from the cochlea may serve as

an indicator of outer hair cell subclinical or clinical pathology. A recently published paper found

no functional changes in otoacoutic emissions of RF-exposed rats at average SARs in the head of

0.2 (950 MHz) and 1 W/kg (936 and 950 MHz) (Marino et al., 2000). Although the field was

not pulsed and RF sounds would not occur, this report is important because it addresses

potentially functional effects in the auditory system of exposed animals.

 Elder, page 20

CONCLUSIONS

Human perception of pulses of RF radiation is a well-established phenomenon that is not

an adverse effect. RF-induced sounds are similar to other common sounds such as a click, buzz,

hiss, knock or chirp. Furthermore, the phenomenon can be characterized as the perception of

subtle sounds because, in general, a quiet environment is required for the sounds to be heard.

The detection of RF-induced auditory sensations is similar to acoustic sound detection

once the cochlea is stimulated; however, the site of conversion from RF to acoustic energy is

peripheral to the cochlea. To hear the sounds, individuals must be capable of hearing high-

frequency acoustic waves in the kHz range and the exposure to pulsed RF fields must be in the

MHz range. The effective radiofrequencies reported in the literature range from 216 to 10,000

MHz.

The hearing phenomenon depends on the energy in a single pulse and not on average

power density. Guy et al. (1975) found that the threshold for RF-induced hearing of pulsed

2450-MHz radiation was related to an energy density of 40 µJ/cm2 per pulse, or energy

absorption per pulse of 16 µJ/g.

The thermoelastic expansion theory explains the phenomenon, that is, audible sounds are

produced by rapid thermal expansion, resulting from only a 5 x 10-6 oC temperature rise in tissue

due to absorption of the energy in the RF pulse. The experimental weight-of-evidence does not

support direct stimulation of the central nervous system by RF pulses. No published reports

support the suggestion by Tyazhelov et al. (1979) that the theory does not explain all

characteristics of RF hearing.
 Elder, page 21

A comparison with routine ultrasound pressures during medical diagnosis, including

exposure of the fetus, suggests that RF-induced pressures more than about five orders of

magnitude greater than the pressure at the hearing threshold would be unlikely to cause

significant biological effects.

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 Table 1. Summary of Human Studies Describing Auditory Effects of Pulsed RF Radiation
Exposure Conditions
Number Frequency Pulse Pulse Peak Power Av. Power Energy Density
Effect Comment of (MHz) Repetition Width Density Density Per Pulse Noise Level Reference
Subjects Rate (s-1) (us) (mW/cm2) (mW/cm2) (µJ/cm2) (dB)
Not 1,300 600 2 (peak power Airborne Instruments Lab (1956)
RF hearing: heard
given ~0.5 MW)
repetition rate of radar
as “high frequency
components”
Threshold 8 3,000 0.5 5 2500 0.006 12.5 45 (+plastic Cain and Rissmann (1978);
RF hearing:
Values 10 225-2,000 0.001-0.01 2.3-20.0 foam earmuffs) Rissmann and Cain (1975)
“distinct”
15 300-1,000 0.002-0.007 4.5-15.0
clicks
RF hearing: 3 3,000 <100-1,000 1-2 2,500-50,000 5 40 Constant (1967)
buzz heard 6,500 <100-1,000 1-2 2,500-50,000 5
at PRR>100;
individual
pulses heard
at PRR<100

No auditory response 3,000 <100-1,000 0.5 10,000-100,000 5

6,500 <100-1,000 0.5 10,000-100,000 5
No auditory
Response 9,500 <100-1,000 0.5-2 2,500-100,000 5
RF hearing: “buzz, Threshold Not 216 - - 670 4.0 70-90 (+ear Frey (1962,1963)
clicking, hiss, or values given 425 27 125 263 1.0 stopples)
knocking” 425 27 250 271 1.9
425 27 500 229 3.2
425 27 1,000 254 7.1
No auditory 8,900 400 2.5 25,000 25 70-90 (+ear Frey (1962)
Response stopples)
RF hearing: Matched Subjects 3 1,200 12.5- <0.5 Frey and Eichert (1985)
RF sound to 4.8 kHz were 50
acoustic sounds trained
musicians
RF hearing: 4 1,245 50 10 370 0.19 Frey and Messenger (1973)
“buzzing sound” 1,245 50 70 90 0.32
RF hearing: “clicks, Threshold 2 2,450 3 1-32 1,250-40,000 0.1 40* 45 Guy et al. (1975)
chirps” values
RF hearing: Threshold Not 1,310 244 6 (12 v/cm) 0.3 Ingalls (1967)
Buzz values given 2,982 400 1 (18 v/cm) 0.18
(not at 10 10,000 - - -
GHz)
RF hearing: “tinitus” Not - 100-20,000 10-160 - - Khizhnyak et al. (1980)
given
RF hearing: polytonal 18 800 1,000-1,200 10-30 >500 - - 40 (+ear Tyazhelov et al. (1979)
sound stopples)
*Calculated peak-absorbed-energy density per pulse is 16 mJ/kg.
Table 2. Summary of Studies Concerning Threshold Values for Auditory-Evoked Potentials in Animals

Exposure Conditions
Energy Peak Absorbed
Repetition Pulse Peak Power Av. Power Density Energy y
Effect Species Frequency Rate Width Density Density Per Pulse Density Per Reference
(n) (MHz) (s-1) (µs) (mW/cm2) (mW/cm2) (µJ/cm2) (µJ/g)
Response Cat (2) 3000 0.5 5 2,200, 2,800 11, 14 Cain and
obtained with [also dog 10 1,300 13 Rissmann
scalp and 15 580 8.7 (1978);
electrodes chinchill Rissmann and
a] Cain (1975)
Response Guinea 918 100 1-10 * * 20 Chou et al.
obtained pig (5) 1975)
from round
window with
carbon lead
Response Guinea 918 30 10-500 62-156 0.02-1.4 1.56-46.8 6-180 Chou and Guy
obtained with pig (1979)
carbon- (n not
loaded given)
Teflon
electrodes
Electrode Cat (11) 1200-1525 12-130 10 60 0.03 Frey (1967)
implanted in
brain stem
Response Cat (2) 918 1 3-32 800-5,800 0.017-0.028 17.4-28.3 12-3-20.0 Guy et al.
obtained 2450 1 0.5-32 600-35,6000 0.015-0.047 15.2-47.0 8.7-26.7 (1975)
from medial 8,670- 1 32 14,800-38,800 0.472-1.24 472-1,240
geniculate 9,160
with glass
electrode
Response Cat 915 <10 25-250 - 1.0 - 4-40 Lebovitz and
obtained (n not Seaman (1977)
from given)
individual
auditory
neurons with
glass
electrode
Neuronal Cat 915 20-700 0.6 Seaman and
action Lebovitz (1989)
potentials in
cochlea
*Direct comparison of power density in the circular waveguide exposure system to free-field power density is improper because the efficiency of energy coupling is
10 times higher than for free-field exposure (See Chou et al. 1975, p. 362).