A Collection of Scientific
Works and Comments on the HUM
- Letter to the
The article ‘The Hum: An anomalous sound heard
around the world’ Journal of Scientific Exploration, Volume 18, No4,
pp571-595-2004 gives an excellent history and background to the Hum yet sadly,
seems to be fundamentally flawed in its main conclusion.
It seems
illogical to suppose that very low frequency radio transmissions from TCAMO
aircraft can be the sole cause of the Hum, particularly in the light of the
author quoting on the article’s last page, page 590, stating that there are no reports of the Hum near to even higher powered ground based vlf broadcast installations. There are two possible
ways of constructively reconciling this. One would be if the HUM required an
additional radio or acoustic frequency co-factor or component found at all
specific HUM sites but not close to the
I would also like to clarify the papers’ comments on the
aurora. There are several references which pre-date the paper which explain
sound generation by the aurora and one which purports to have audio –recorded
the extremely weak sound of the aurora. Thus the aurora being a four fold
emitter of sound, broad spectrum radio waves, magnetic pulsations and often
light as well is nature’s electrophonic concert for the taking. Presumably the
body reacts to the aurora by enhanced perception involving dueling senses. If
we can begin to understand human sensitivity to the aurora we may understand the
Hum as well.
Elf and vlf radio transmissions
are capable of traveling for thousands of kilometers on this pretext of
causality alone; it seems strange how
one could escape the Hum by traveling only a few tens of miles, see page 575.
In a similar vain, it is interesting to note how there seem to be these
anecdotal reports refereed to in the paper of
an acclimation period of uptown
48 hours when ‘hummers’ move home. The paper refers to HUM hearers having
frequency matched tones in the range 40-80 Hz. A moving car generates very high
levels of infrasound and low frequency noise in the range 15-200 Hz i.e encompassing the Hum range. Being a ‘hummer’ myself I have personally
encountered this effect. I often have to drive to meeting about 100 miles
distant and return home the same day. My wife is also a ‘hummer’ and even on
occasion when the Hum is booming in according to her I cannot hear it for at
least the first night I return. I believe therefore traveling noise
(particularly in car) induces a sort of
low frequency threshold shift (TTS) in the ear of hummers.
I have lots of other material to report but want to keep
this brief. I will endeavor to publish more extensively in the future. I trust
your readership will find my comments useful.
Yours sincerely,
Dr
2. Search
for the Cause of the HUM, the case for infrasound.
CHRIS
BARNES
Abstract
The history, cases and characteristics of the HUM are discussed in terms
of three pre –existing theories of the HUM; electromagnetic, infrasonic and
gravitational. On the strength of the available evidence a new hypothesis is
advanced, that the HUM is both external and internal to the subject and that
for perception of the HUM at least two components are required with one
component being infrasonic. This hypothesis is tested and validated for three
related subjects who have considerable right-ear monaural sensitivity to
infrasound. Further experiments suggest
that exposure to certain radio frequencies above 30 MHz might increase human
sensitivity to infrasound and hence the HUM.
Proliferation infrastructure capable of radiating infrasound and
electromagnetic technology together then might account for the ever increasing
numbers of people reporting the HUM.
Introduction
The HUM is an anomalous
sound, mainly but not exclusively described by sufferers as that of an distant
idling diesel engine. It is heard by estimates of between 2-11% of the
population throughout the World most essentially in Westernised nations. Being
a highly subjective phenomenon there is little wonder that there are few
published scientific works on the HUM to be found.
It is particularly difficult
to do Science with such anomalous phenomena because one is mainly dependent on
anecdotal reports. However, if these contain enough congruent detail certainly
some significance ought to be afforded.
With the advent of the better communications, in particular the internet
it is possible to search for such similarities in HUM reports on different
forums, websites and bulletin boards.
Reported
Cases and Causes of the HUM
Sparse reports of the HUM
began in
The most famous cases in the
a known source of seismic
infrasound (refs). There was also the inception of the high pressure gas grid
both also significant sources of acoustic sound ( refs) and infrasound ( refs),
particularly hydroelectric power and pumped storage schemes( ref). Most recent sources of sound and infrasound
in developed nations are wind turbines and undersea coastal oil and gas
exploration (refs).
Some have suggested the HUM
has exotic electromagnetic causes, such as very low frequency transmissions
from military aircraft known as TACAMO an abbreviation standing for ‘take
charge and move out’. (Deming 2004). The fact that the HUM is experienced
inside a Faraday screen and a stationary vehicle tends to rule out at least the
electric field component of electromagnetic sources as an option, but it should
not be forgotten that magnetic fields will permeate these situations unadulterated. The fact that the HUM can still be heard
inside anechoic chambers tends to rule
out that the HUM is an acoustic signal but it is notoriously difficult to
prevent airborne very low audio frequencies and infrasound entering such a
chamber or earth bound vibrations. Alternatively, the behaviour in anechoic
chambers in particular is suggestive
that the HUM is either internal or has a cause which is neither electromagnetic
nor acoustic. Dawes (2006) has suggested
that the HUM may be due to the influence of the power grid on the earth’s
gravitational field. Further it has been
suggested that the HUM may not have an external cause at all and that it may be
all in the mind, a function of the stress of modern living or of a
physiological state known as tensor tympani syndrome (ref). Barnes (2007) has
suggested, in reply to Deming (2004), that an electromagnetic HUM may require
an acoustic co-factor and that the infrasound and low frequency noise generated
by car interiors might explain why ‘Hummers’ or people afflicted by the HUM
gain up to forty-eight hours relief after a significant vehicular journey. The hypothesis developed here is
that the HUM is both internal and external in that it depends on more than one
external signal and processing of those signals by human audition. This is not
inconsistent with Barnes (2007) previous findings or suggestions. It will be
shown that two or more external signals of appropriate frequency with at least
one in the infrasonic range can produce a ‘HUM like’ experience under
laboratory conditions in certain experimental subjects and that certain radio
frequencies appear to enhance low frequency acoustic perception in those
subjects.
Characteristics
of the HUM
Possible causes of the HUM
might be ascribable after considering common threads in its anecdotally
described characteristics, such as; frequency or rather frequency of tones
matched by sufferers, pulse repetition rate, times the phenomena is
experienced, locations where the phenomenon is and is not experienced, effects
of the weather and environment on the intensity of the HUM, effects of other
senses on the HUM and finally remedies which sufferers use for escaping or
attenuating the HUM.
HUM
frequency and amplitude
HUM sufferers often tone-match
its frequency in the range 30-80 Hz where it is described as a quasi
–periodic pulsation withy pulse
repetition frequencies varying between 0.5 and 5 Hz. To this end many suffers
describe the HUM as sounding like either a fly or wasp trapped in a bottle or
even a distant throbbing diesel engine or rumbling machinery. Sufferers also
describe the amplitude of the HUM as varying from the barely discernable yet
frustratingly annoying to the downright unbearable wherein they feel their
heads and even whole bodies are vibrating and their ear drums are popping. To
this initial end when the phenomenon is quiet possibly accounts for it mainly
being experienced at night. Despite the
assertions of sufferers and a paper suggesting the cause of the HUM is due to
specific aircraft borne electromagnetic transmissions (Deming 2004) no
per-existing study has yet found any signal either acoustic or electromagnetic
in the requisite frequency range which has modulation which behaves as
reported. This has led some to conclude that the HUM may be all in people’s
minds or at least due to some kind of physiological ailment such as Tensor
Tympani Syndrome (ref). However one
should perhaps err with a note of caution because low frequency acoustic
measuring equipment is notoriously insensitive and inaccurate ( ref) and it is
known that there is a certain sub-set of the population with extremely
sensitive low frequency hearing, some of whom can even hear infrasound (
ref).
HUM time of
day
Many HUM sufferers report the
phenomenon only at night. There are four possible reasons for this. Firstly in
most locations with the exception of City areas it is inherently quieter at
night and there are less masking noises.
Secondly with low or medium frequency radio waves or infrasound as a
potential cause of the HUM, these would both propagate better at night.
Thirdly, pumped storage hydroelectric power schemes tend to pump water more at
night. Finally, there is less vehicular movement. It is possible that vehicular
movement would randomise the arrival of any coherent signals if such a
phenomenon were required in HUM perception.
HUM
locations
The HUM as reported elsewhere
(refs) and anecdotally by sufferers on numerous internet forums and chat rooms is
often, although not exclusively, experienced in coastal locations or near
mountainous regions, both places where natural infrasound generation is
possible. The HUM is rarely experienced
outdoors but is mainly experienced in houses or stationary vehicles. Somehow houses and vehicles must amplify the
HUM. One possibility is that they simply block out masking noises such as wind.
Another is that they facilitate transmission of ground borne vibrations to the
body. Houses with chimneys might particularly be expected to amplify the HUM if
it had an infrasonic component due to the Helmholtz
effect (ref) . Similarly cars have structural components which resonate at
infrasonic frequencies (ref). The HUM
is experienced inside Faraday screens or cages and also inside anechoic
chambers.
Weather and
environment
There are all kinds of
reports of the HUM varying with the weather but they are mainly inconsistent.
The most credible seems to be the report of the HUM ceasing after very heavy
snow fall which halted vehicular movement.
The implication is that either the HUM is itself some way due to
vehicular movement or that the snow on the ground is effecting the propagation
of whatever is carrying the HUM. In
respect of the former roads with continuous traffic flows such as motorways are
known sources of infrasound. The present
author has evidence that the HUM depends on wind speed and direction and also
on high level winds or jet streams and hopes to present this elsewhere.
Other senses
There are reports of people
stating that the HUM intensity increases when they look at artificial light or
moonlight. Sound and infrasound are more
likely to propagate better under a moonlit sky as the atmospheric boundary
layer is likely to be more stable.
Alternatively it is possible that atmospheric scintillation is occurring
at a rate linked with one or more components of the HUM. The body may be able
to detect this through synasthesia or duelling of the
senses. Humans are known to make use of this in the cognition of learning
(ref).
Relief from
the HUIM
The HUM is reported to come
and go at various locations almost spontaneously and at others to present
almost incessantly. Some state that they
can only get relief from the HUM by travelling by car for several hundred kilometres
but yet even then when staying at an alternative location the HUM catches up
with them after a couple of days. The
present author has noticed this effect to be very pronounced even when
travelling away from and back home in the same day. In the hypothesis advanced here that at least
one component of the HUM is due to infrasound, car travel ought to cause a
temporary shift in the threshold of the ear’s sensitivity to such sound. It is
not known in which way car travel might affect the HUM if it were due
gravitational field effects. Some, but not all, claim some relief from the HUM
by using earplugs, suggesting the perhaps at least some component, if not all
of the HUM may be acoustic, if not in the usually sense of the word. A wholly
infrasonic HUM would be almost impossible to attenuate using earplugs because
they are less effective at low frequencies and body resonances and bone
conduction to the hearing apparatus would come into play
Some claim that they get
relief from the HUM by descending into deep underground limestone caves. The
HUM is not reported inside moving vehicles as far as the author is aware,
presumably because the broadband infrasonic and acoustic sound levels grossly
outweigh the HUM.
Deep underground all forms of
surface wave energy; acoustic, infrasonic surface vibration and electromagnetic
(except extremely low frequency) will be heavily damped and attenuated. It is impossible to say then with any
certainty which cause of the HUM the ‘limestone cave’ effect supports. It is possible however to deduce from this
that it is unlikely the HUM is not due to any bulk or very deep vibration mode
of the earth. Surface induced seismic
vibrations from specially designed vibrators can in some circumstances be detected
as much as 350 Km distant from the source and 50 Km deep (ref).
Experimental
case for infrasound
A series of experiments have
been performed so as to test the hypothesis that at least one component of the
HUM is infrasonic. People who perceive the HUM are known collectively as HUMMERS.
The author’s wife has been a HUMMER since October 2003, the author and his son
started hearing the HUM about 15 months later. The author’s sister-in law is
also a HUMMER. The first three subjects perceive the HUM intermittently in
their
Experiment 1 Infrasonic Hearing
An experiment was conducted
to ascertain if the subjects could hear infrasound. Freeware Audio synthesis
software courtesy V.Burel was employed on a Fujutsi –Seimens Amilo Notebook Computer driving HD-3030 stereo
headphones. All three subjects could
clearly perceive low frequency down 20 Hz and lower to infrasound down to 5Hz
monaurally in their right ears. The author
and his son have normal hearing in their left ears and could perceive
sounds from 16 KHz down to 27 Hz in their left ears. The author’s wife has age related high
frequency hearing loss and could barely perceive 10KHz in either ear but could
perceive infrasound monaurally in both ears with the left ear being some 10dB
less sensitive than the right. The
author’s wife and son lost tonal sensation at about 30 Hz simply describing the
infrasound as a ‘buzzing’. On occasions the author felt that tonal
discrimination continued whilst lowering the frequency from say 40 to 15 Hz but
could discern discrete clicks below the lower frequency. On other occasions
lowering the actual frequency in the range around 25 Hz seemed to produce an actual
increase in perceived frequency, in line with previous reported findings that
tonal discrimination of infrasound is not possible (refs).
Experiment 2 Measuring infrasound at HUM sites
After reading the work of
Deming (2004) the author decided initially to test an electromagnetic
hypothesis of the Hum. The subjects
visited various mobile phone cell towers, a 400 MHz TETRA mast, UHF TV station
which also carries VHF FM and DAB and finally a medium wave transmitter
station. When they heard the HUM in their parked car near the TETRA mast and TV
station and underneath 400 kV power lines, this at first seemed to corroborate
the electromagnetic hypothesis.
However, their experience was not consistent and seemed to depend on the
weather conditions. At these locations however, unlike their home, when
present, the HUM was present and could sometimes be perceived by day as well as
by night.
Realising that infrasound can
be generated by Aeolian modes of power lines (Irvine 2006), it was decided to
try and detect infrasound under the power lines and when both subjects were
reporting the Hum. A ‘big ear’ dynamic
microphone was constructed based on a guitar amplifier loudspeaker feeding
through a 1: 25 step up matching transformer giving a sensitivity better than (-60 dbSPL from 8 to 80 Hz)
into
the sound card of the Fujitsu –Siemens computer running Spectrum Lab software
set up as a 0-50 Hz spectrum analyser with colour palette adjusted to indicate
an appreciable acoustic dynamic range of approaching 120 dB.
A number of other locations
were chosen for infrasound analysis on the basis of them being either close to
a known or expected source of infrasound such as at Dinorwig
pumped storage Hydroelectric plant which has six 500 rpm synchronous pump
turbines and is expected to produce seismic infrasound (Grainger and McCann
1977) at 8.33 Hz (Pritchard 1and 2 (1988)).Other locations where both subjects
could either perceive the Hum very readily or not at all were also tested. In
all cases both subjects listed acutely for the Hum prior to booting the laptop
and opening the spectrum analysis program so there would be no tendency for
suggestion from the observed traces.
Infrasound spectra were also recorded at the home address of the
subjects both when the Hum was present and absent. The effect of passing motor
vehicles was also recorded.
Results
The spectrograms of the
results between 0-53 Hz were logged in the laptop memory and the corresponding
JPEG files used to produce the figures below. The x-axes represent frequency
0-53 Hz and the y-axes time, 3 minutes per minor white division.
Figure 1
Both subjects could hear the
Hum overwhelmingly loud at Dinorwig, grid reference
…. The infrasound spectrum recorded at this location is shown in Figure 1. A distinct band of infrasound between 2-11 Hz
can clearly be seen, with monochromatic bursts at approximately 3.5, 6.5 and
8.5 Hz. The signal at 50 Hz is thought not to be acoustic, rather an artefact
due to magnetic field induction directly into the speaker voice coil from a
nearby transformer.
Figure 2
A wider frequency spectrum
obtained at another location, grid reference …. across the lower lake at Dinorwig shows some narrower band infrasound at
approximately 3.5, 10 and 32 Hz and a large amount of acoustic noise in the
region of 60-70 Hz, see Figure 2.
Figure 3
Figure 3 shows the very noisy
infrasound spectrum recorded underneath super grid 400 kV power conductors at
grid reference ………………….. and within that spectrum shows three bands of almost
monochromatic infrasound at
approximately 11, 13.5 and 32
Hz. The huge signal at 50 Hz is most
likely due to electromagnetic induction as no audible 50 Hz was perceived by
either subject yet both perceived the Hum at this point.
Figure 4
Figure 4 is the infrasonic
spectrum recorded at a site in the countryside where the HUM was absent as
reported by all the subjects, grid reference ……
The part of the spectrum between 4 and 53 Hz shows an almost complete
absence of infrasound and acoustic noise.
There are some very weak spontaneous infrasound bursts at 1.5 and 3.5 Hz
but obviously in view of the subjects reports these were neither strong enough,
continuous enough or on a suitable frequency to initiate Hum perception in
either of the subjects.
Figure 5
The result shown in Figure 5
above is for that of a site at grid reference ……………..where both subjects
perceived the Hum in their parked vehicle but where no obvious source of the
Hum was known. As can be seen there is a
strong monochromatic signal at approximately 11 Hz. There is also evidence of monochromatic
acoustic signals in the region of 19, 29 and 32 Hz and broad-band acoustic noise to above 50
Hz. The very broad -band signal bursts
lasting some 6-9 seconds and some 60 decibels greater in amplitude are due to
passing vehicles.
Figure 6
Figure 6 shows the spectrum
in the main bedroom at the author’s residence on the morning of
Figure 7
Figure 7 shows the spectrum a
few minutes earlier when the Hum is not present at the authors’ address.
There is broad band acoustic
noise 22 Hz upwards and some bursts of 50Hz interference but only random weak incoherent
bursts in the range 2-16 Hz. There were some fifteen passing vehicles during
the three minute recording. Cancellation of the lowest frequency infrasound
signals may have resulted in loss of the HUM. Such signals can be disrupted by
vehicles on a local basis (Daigle 1984) and this probably accounts for lack of
Hum reports in large cities, see later.
Figure 8
The
trace in figure 8 was obtained late morning when the Hum was not audible at the
author’s house and a few days after the data of figure 7. Although there are some random acoustic
bursts around 20 Hz and some more narrowband signals at 30 Hz, the infrasonic
part of the spectrum below 20 Hz is incredibly quiet. It seems therefore for HUM perception in the
subjects of this study at least some infrasound well below 20 Hz is required.
It should perhaps be noted
that electromagnetic signals such as those of TCAMO and others as a potential
source of some cases of the HUM is not entirely ruled out by this present
hypothesis and study. The mechanism of the electrophonic interaction perceived
here would be simply because such signals can generate sounds by means of
passive inter-modulation or passive demodulation either directly at their
antenna or due to non-linear effects at corroded metallic surfaces or vibration
due to magnetic induction. Depending on the precise frequency and modulation
frequency or data rate of such signals, generation of secondary infrasound is a
possibility. Following this hypothesis,
the data from figure 10 are fascinating. These data were gathered about 100
metres from a TETRA (Trunked Emergency Terrestrial
Radio) radio mast transmitting at some 26 dBW in a
country area where there was little wind or vehicular movement and where the HUM
could be heard by all the subjects. Besides some broad band infrasound between 3
and 8 Hz and a narrower signal in the region of 12 Hz, there is also a clear
but quite weak narrowband signal at 17.6 Hz, one of TETRA’S pulse repetition
frequencies. It is presently not known
if a signal at this frequency alone would be sufficient to cause the Hum.
Figure 9
Other
subjective effects observed in this study
All three subjects find that
subjective Hum level reduces with increasing wind speed, however when more
detailed evaluations are made, there appear to be imposed sinusoidal variations
in this behaviour. Wind is known to destroy the coherence of seismic infrasound
(Withers et al 1996) or in the case of the HUM might simply be providing a more
familiar and tolerable broad band masking noise. Withers et al (1996) have shown that winds
with speeds as low 3m/s can, in certain circumstances, destroy the coherence of
seismic infrasound at 15 Hz and below whereas winds of greater than 8m/s were
required to reduce the coherence of sound in the 23-55Hz frequency band.
Assuming linearity between wind speed and coherent frequency destroyed and applying this method to data
recorded at the authors’ home for the north –west wind direction which most easily quells the
Hum, extrapolation from the graph suggests the arrival of coherent infrasound
at frequencies of approximately 3.4, 8.8 and 29 Hz, in remarkable agreement
with those measured by the ‘big-ear’.
All three subjects perceive
the following effect when using earplugs to try and defend against the
HUM. Without ear plugs, the HUM has its
usual pulsating engine or wasp in bottle tone. The pulse repletion frequency is
estimated to vary quasi-periodically from roughly 1-3 Hz. On nights when the
HUM is particularly intense, Inserting ear plugs removes the tonal component
and leaves a sound which can only be described as a fast hammering in the
region of 10 Hz or so. Earplugs would be
expected to remove higher frequency components and not low frequency
components. The inference is that possibly two or more of the frequency
components as recorded by the big ear or as detected by the wind-speed
experiments must somehow beat together in the ears or heads of the subjects to
produce the effect which is the HUM.
This is in strong support of the hypothesis that the HUM is both
external and internal and that a signal which is the HUM can therefore never be
simply measured in the environment, unless that is additional signal processing
is applied.
Experiment 3 Hum simulation
Following the above
subjective effects and initial hypothesis that the HUM needs more than one
external component for perception and that internal perception is a function of
human audition and that at all the above HUM sites infrasound appears to be present
at two or more frequencies below 20 Hz in addition often to low frequency sound
in the region of 30 Hz it was decided to
see if conditions could be set up experimentally which would synthesise the
HUM. The V.Burel
software was set up on two other computers and all three computer sound cards
were fed into separate loudspeaker systems.
The subjects sat about 1 metre away from the loudspeaker systems and
listened binaurally. It was found that HUM like effects could be perceived for
any infrasound in the region of 9-17 Hz when outputted at similar amplitude to
any audio tone in the region 29-75 Hz. A third tone was not always necessary
but equal amplitude tones of 4, 10 and 29 Hz gave a particularly disturbing
effect and the subjects felt quite giddy for several hours afterwards. HUM like
effects with typical quasi-periodicity were also maximised for two tones one
audio in the range 29-75 Hz one infrasonic wherein the infrasonic tone fell
such that its third overtone was within 2Hz or so of the audio tone. The results of these experiments give very
strong support to the notion that at least one component of the HUM is
infrasonic and that it might never be possible to measure a single HUM
characteristic or single per se in that the quasi-periodicity which subjects
experience may well be an internal effect related to the latency periods of
linear and non-linear products in hearing. Non linear acoustic products of the
human cochlea are know to be of the form 2f1 – f2 at frequencies of the order of a few kilohertz
(ref) but odd harmonic generation is
known at lower frequencies ( ref) in line with the above result.
Alternatively, the more complex and pulsating
behaviour for the HUM as actually observed seems to be consistent with a signal
arriving via more than one medium or pathway.
In this respect the Hum is known to have quasi-periodic fluctuations in
amplitude anecdotally reported to be between 0.5 and 2Hz. Imagine a 10 Hz infrasound wave propagated
by three media, namely; air, water and rock, at speeds of 330 m/s, 1500 m/s and
approximately 5km/s. For in-phase
coincidence on arrival times from a fixed source this yields a 1.66 second
difference for the rock-air case or 0.6 Hz, a 0.5 second difference or 2 Hz for
the water air case and a 3 Hz difference for the water rock case. Given that
the attenuation coefficients of sound in all three media are different and may
vary independently this would be sufficient to produce the amplitude pulsation
effect experienced by Hummers. For
example the seismic propagation of anthropogenic sound from turbines is known
to change preceding and post earthquake due to changes in stress and strain in
the planetary structure and rocks (Yakovlev and Aleshin 1994). This
may account for anecdotal reports of people hearing the HUM louder before major
earthquakes and its amplitude subsiding afterwards. The present subjects have
certainly noticed this effect. One possible anthropogenic source involved
in
Moving vehicles also seem to
momentarily attenuate the Hum at the authors’ residence whereas random noises
inside the house do not. This effect is
indeed ironic when some have said that the cause of the Hum itself may be more
remote fast moving vehicles on motorways, see Rybak
(2000) and Fox (1992). Vehicles produce broad band infrasonic noise in addition
to narrower band engine noise. The Hum
seems to have a dead time of up to 5 seconds after the passage of a
vehicle. For example at 10 Hz this
represents some 500 cycles of Hum. In
Fourier domain technology this is about the number of cycles required to
specify a coherent sine wave. Therefore
it is tempting to suggest then that the body is coherently detecting the Hum in
some way. Since these frequencies are
close to those of natural alpha brain rhythms of
the coherent oscillations of the thalamic pacemaker cells in the human brain (Brazier,
M. A. B. (1970), The Electrical Activity of the Nervous
System, London: Pitman) it is possible the
brain may entrain at Hum frequencies.
Feasibly, this might account
for sensitisation or cancellation by other noise sources as reported by Deming
(2004) or even experience of the same or similar sensations as a result of
appropriately pulsed electromagnetic sources.
Movement of vehicles will
also disrupt the propagation of both seismic and airborne infrasound on a local
basis (Daigle 984). Also, turbulence in
their wakes may produce an effect similar to the wind above. At the author’s residence such disruption
seems to occur over about a 300 metre radius, consistent with a significant
fraction of a wavelength of a bulk seismic wave at say 10 Hz and several wavelengths
of the same frequency in air. From above
it can be seen by simple calculation that when there are more than about 700
vehicles per hour the Hum will simply not be heard. At night there are far fewer vehicles. In a busy city there are often several
thousand vehicles per hour even on side streets thereby minimising the chances
of ever hearing the Hum.
It is feasible that the Hum
has a seismic component of the Hum may propagate quite close to the earth’s
surface. There are anecdotal reports on Hum forum websites of the Hum ceasing when
the ground was loaded with several feet of snow which would have a pressure
damping effect. Similarly there are
reports of the Hum starting up again when the snow melted and vehicular
movement re-commenced. Fox (1992) has
actually suggested the vehicular movement might be the cause of the Hum. That
vehicles on motorways emit surprising narrowband infrasound in two distinctive
frequency bands has recently been shown by Rybak
(2000).
Experiment 4 radio frequency enhancement
Since some have said the HUM
is or may be electromagnetic in origin an experiment was performed to see if
low frequency hearing is affected in the presence of radio frequencies. Many studies conclude that normal hearing is
for instance totally unaffected by GSM cell phone signals. Continuous carrier wave excitations at
frequencies of 7, 30, 50 and 144 MHz were employed with the subjects standing
near indoor transmitting antennas with field strengths of …….Volts per metre. No acoustic effects were recoded with radio
frequency only. For infrasound in the
range 17- 30 Hz played monaurally all subjects reported a slight increase in
the perceived amplitude in the right ear at transmitter frequencies of 50 and
144 MHz. The author alone noticed a
slight increase in low frequency acoustic perception around 30 Hz in his
‘infrasound- deaf’ left ear. More and more people the world –over seem to
be reporting sensitivity to the HUM and although at least one component of the
HUM is infrasonic we have the strong possibility that electromagnetic emissions
may be enhancing people’s low frequency hearing! It is interesting to note that
the right ear has been described elsewhere as being most sensitive to radio
frequency radiation (See at al). The
present author hopes to report on relationships between radio transmitter sites
and HUM sites at some stage in the future and also on other mechanisms by which
infrasonic and acoustic sound and radio waves could interact.
Discussion of results and
wider context of the Hum
The conclusion is that narrow
band or monochromatic infrasound below 20 Hz is only found in locations where
the subjects perceive the Hum would appear to corroborate the initial hypothesis. Further it would appear that at least for
the subjects of this investigation the precise frequency of the infrasound is relatively
unimportant provided it is relatively monochromatic and lies somewhere between
5-17 Hz. This also encompasses the range of human alpha brain rhythms. The presence of two monochromatic frequencies
close together as at the author’s residence does not seem to significantly
change the nature of the HUM as perceived.
Closer inspections of all the
above spectra which show infrasound of less than 20 Hz (figures 1-7 and 9) have
some amplitude variation as a function of time scales of the order of a second
or so. Although these fluctuations are not dramatic, they may be sufficient to
account for the pulsating nature of the Hum particularly given comments made by
Moller and Pederson (2004) on how small changes in
infrasound amplitude can dramatically alter perception.
Alternatively the
quasi-periodic fluctuations people report as implicit in the HUM may be related
to different internal latency periods in the audition process (ref).
In all but one of the spectra
wherein the subjects reported the Hum, figure 1, there is also acoustic noise
and a narrow band acoustic signal at or close to 32 Hz, coincidentally an
acoustic frequency which some have associated with the Hum. It is possible this represents the third or
fourth overtone or harmonic of a lower infrasonic component. Such components are known to be generated by
non-linear seismic interactions in certain soil types (Pavlenko
2001). Indeed it would seem that a
frequency around 32/33 Hz is very prevalent in the environment. There is
reference to this being due to seismic harmonics from turbines (Yakovlev and Aleshin 1994) or to
signals from fans, pumps and compressors (Cowan 2003) or from turboprop
aircraft at 2000 rpm (Farokhi 1990).
Close to locations which are
known to radiate infrasound on a regular basis the present subjects perceive
the Hum in their car at any time of day or night whereas at home the HUM is
mainly perceived at night after about 9 pm, throughout the night thereafter and
in the morning until as late as 10 am.
This is suggestive of either a local infrasound source which switches on
and off at these times or equally suggestive of a more distant infrasound
source being able to propagate into the author’s residence exclusively at night.
It is known the sound propagated much better in the nocturnal boundary layer at
night (Waxler 2003) and it is also known that two
higher channels for atmospheric ducting of infrasound exist, one in the
stratosphere and one in the thermosphere (Gibson and Drob). Due to the fact that infrasound from more
distant sources also travels further at night, some light is now also shed on
anecdotal reports of the HUM being a mainly nocturnal phenomenon.
It is hoped to report on
sources of infrasound for the Hum and effects of the Jet Stream on the Hum in
the near future.
Conclusions
- The hypothesis that the HUM is both internal and
external in that it depends on more than one external signal and
processing of those signals by human audition has been strongly supported.
- Using at least one in the infrasonic range can
produce a ‘HUM like’ experience under laboratory conditions in certain
experimental subjects.
- Certain radio frequencies above 30 MHz appear to
enhance low frequency acoustic perception in those subjects.
- At least
some subjects perceive the HUM as a result of their general enhanced
sensitivity to infrasound.
- The hypothesis and results fit in well with many
anecdotal reports of the HUM and its properties.
- The HUM as perceived by the present experimental subjects
appears to be associated with at least narrow band infrasound in the
frequency range up to 17 Hz, these frequencies are corroborated by both
electronic measurement and wind speed measurement methods.
- At most of
the sites where the subjects heard the HUM, acoustic signals were also
present, mainly broad-band, with the exception of a signal in the region
of 32 Hz which seems prolific.
- A possible explanation for the pulsating
modulation of the HUM is advanced in terms of multi-path or multi-media HUM
signals
- An alternative explanation in terms of internal
audition latency exists.
- The HUM may be experienced very locally to a
continuous infrasound source or at a distance when air -borne infrasound
propagation is better at night; this explains why for some the HUM is
mainly a nocturnal phenomenon.
- Generation of infrasound in the air at the
antenna or at passive inter -modulation interfaces by electromagnetic
signals is not ruled out by this study but is not thought to be the main
or only cause of the HUM.
- The effect of moving vehicles probably accounts
for why the Hum is rarely if ever perceived in large cities.
- Modern electromagnetic technologies may be
enhancing people’s sensitivity to the HUM.
Acknowledgments
The author wishes to
acknowledge his wife Gwyneth for her patience during the preparation of the
manuscript and to further thank his son Dwain and sister-in-law Marian for
their contribution as experimental subjects.
References
3. World
Sources of Infrasound for the Hum and the
effect the Jet Stream has upon subjective Hum levels in
Introduction
Sources capable of generating
infrasound such as gas mains, factories and traffic have in the past been
blamed for the Hum (Fox 1992). Such infrastructure expanded enormously in the
It has recently been shown experimentally that
at least one infrasonic component below 20 Hz is involved in the HUM and
further that HUM like effects can be synthesised and simulated using low
frequency sound and infrasound. (Barnes 2007). This revives and reinforces the
infrasound hypothesis. Barnes (2007) has further established, at least for
certain subjects that the anomalous auditory phenomenon known as the Hum is due
to monochromatic infrasound in the range 5-17 Hz is manifest when very local
infrasound is present at appropriate frequencies, for example, as in the case
of subjects listening when seated in a parked vehicle outside a pumped storage
station or underneath wind vibrating power lines. However, it seems it can
equally be manifest by propagation from a more remote source, which is more
likely at nighttimes, when the majority of those afflicted seem to experience
the Hum. The infrasound measured appears
to be monochromatic and slowly pulsating in amplitude, thus it may be that when
Hummers experience the Hum at home and at night, more than one source of
infrasound is involved. Some sources
of infrasound can travel many hundreds of kilometres, so ultimately it might
not always be readily possible to trace the source for every single case of the
Hum. However differentiation between
most and least likely sources ought to be possible on the basis of their
temporal, frequency and amplitude properties and upon their historical
evolution compared with the time line in growth of the Hum phenomenon.
Sources
of infrasound for the Hum
Sources of infrasound as
possible causal pre-requisites for the Hum need either to be constant radiators
or at least radiate a significant amount of the time, particularly at night,
and possibly need to be capable of producing multi-path effects or use multiple
propagation media to allow for the amplitude variation of the Hum. Mainly anthropogenic sources of infrasound
fit this requirement, although some natural sources also radiate with
surprising constancy. Existing over
geological epochs and pre-dating even the existence of human kind, let alone
their twentieth and twenty-first century experience of the Hum, natural infrasound
would seem at first sight simply not to fit the bill, unless, that is, something is sensitising more and more of the
population to infrasound in general. If
on the other hand sensitisation were a distinct possibility then natural
infrasound may be of relevance, it is hoped, perhaps to report on possible
evidence which corroborates such a sensitisation hypothesis at a later
date.
Anthropogenic infrasound from infrastructure is usually be expected to
be generated at or close to ground level and
is a possible candidate also
allowing propagation into multiple transmission media, so would seem to
be a more acceptable cause for infrasonic Hum, but does not readily explain anecdotal reports of the Hum being
momentarily altered by the passage of passenger planes. Unless, that is, some of this infrasound is
being propagated very large upward distances and is then re-reflected which has
been shown to be the case (Mutschlecner and Whitaker
1990). This fact has recently been
re-asserted by Krasnov et al (2005) and has been
proven experimentally by Koshovvyy et al (2007). The propagation of infrasound to earth from a
natural source, the aurora, is known to be affected by the jet stream (Johnson
1976). Indeed some are intentionally experimenting by projecting high power
synthetic and monochromatic infrasound into the ionosphere (Rapoport
et al 2003). The present frequency of
such experiments and how they might affect Hummers is not readily known. Whatever the sound source injected,
effectively several channels exist for propagating infrasound from the Hum.
These are seismic (refs) and airborne. Of the airborne channels, one can exist in the night-time boundary
layer (Waxler) and it is also known that a further
two higher channels for atmospheric ducting of infrasound exist, one in the
stratosphere and one in the thermosphere (Gibson and Drob).
It may be that to produce the pulsating effect of the Hum propagation between
the source and the hearer has to be through more than one of these channels,
with associated phase delays between them.
Alternatively or additionally
infrasound generated by planes
themselves (Bedard Jnr. and
Cook 1968) and (Crowley and Blaney 1987) or from their wake vortices ( Hardin et al
2004) may be involved as a cause of, or contributor to the Hum, which with the
ever increasing expansion of air traffic
can be seen is another distinct possibility. The recent vast growth in air passenger
travel coupled with the use of aircraft with higher by-pass ratios which are
known to generate more infrasound (Baklanov and Zayykin Paper 58) is thus a very feasible contributing
factor in some parts of the World.
Land based anthropogenic
sources of infrasound include; the power grid, power generation in particular
hydroelectric and pumped storage (refs), cooling fans (Cowan 2003),
compressors, chimneys, tunnels, suspension bridge structures, motorways (Rybak 2000) traffic (Fox 1992) and the gas grid (www.springerlink.com/index/Q6JH11813263UP67.pdf).
High pressure gas grids capable of generating infrasound pulsations
close to 10Hz(Bell,PGIInternational)www.afms.org/Docs/gas/George_Bell_Pulsation_Paper.pdf now span much of
Britain. Strictly speaking faster pulses from the system control valves about
100 Hz are modulated onto slower pulsations of between 0-50 Hz arising from the
compressors. The example pressure
waveform given by
The