Picking up bad vibrations: Hydro-power and Wind power common seismic bed partners in the newly explained phenomenon known as the Hum, by Dr Chris Barnes Bangor Scientific and Educational Consultants. E-mail firstname.lastname@example.org First released into public Domain without full reference list in the interests of advancement of science and human peace of mind : 25th November 2016
The history, characteristics and theories of the Hum are briefly discussed and reviewed. Hydro-power and Wind -power (although at first sight unexpected) are shown to be likely bed partners in producing seismic signals which can lead to the Hum but only in the presence of PME earthing systems or imbalanced ground AC power flow. Seismic propagation depends heavily on underlying rock strata and this together with any airborne infra-sound and acoustic components strongly controls the geo-sporadic nature of the Hum. Seismic propagation also depends on the whole lithosphere from tides to crustal stress. In that sense the Hum has inherently both natural as well as anthropogenic components. Lithosphere, atmosphere and space physics are also inherently coupled. Electrical power systems here on earth also modulate upper atmospheric space physics processes and may even influence seismic processes, see for example ‘Comparison of the diurnal periodicity features of seismic noise, earthquakes, and electric power consumption’ Sidorin (1990). The author has also recently showed a strong dependence on the Bangor Hum on power flows across UK/Irish and UK /Continental power inter-connectors.
The Hum was until recently a totally unexplained geospatial- sporadic acoustic (or in some cases possibly magneto-acoustic or gravito -acoustic) phenomenon first reported extensively in Britain during the 1970’s, more extensively in the USA since the 1990’s and heard by an estimated 2-11% of the populations of those countries. Hearers or Hummers as they have sometimes being called appear to hear or perceive a pulsating very low pitched and subliminal noise especially at night and often indoors. Some have described this like a large fly trapped in bottle. Others describe a low pitched randomly idling engine noise and others describe a very low pitched Morse code like signal. Some even describe whole or partial body vibration or tingling and elevated body temperature when perceiving the phenomenon. Causing occasional sleep disturbance and even in some cases being difficult to screen with ear plugs this has amongst the afflicted caused expected negative emotions and even physical problems. Naturally, something very different from the usual and an anecdotally described unexplained phenomenon has provoked all kinds of explanation from the paranormal to the sensible. Rationally the symptoms described by Hummers could be attributable to LFN and/or infra sound. Some cases of the Hum have been solved in that noise sources have been located. In DEFRA'S definition of the Hum, a solved case is effectively not the Hum. The Hum is only the phenomenon if the source cannot readily be traced.
The first scientifically published attempt to explain the Hum was that of Deming ( ref) who concluded that at least back in the 1990's the Hum seemed to correlate best in time and space with possible flights of certain western military aircraft known as TCAMO ( take charge and move out). However, since the Hum is now heard virtually the World Over, especially more so in countries which have both TN ( TN-C-S) or PME mains earthing or grounding and renewable energy such as pumped storage hydro-power and/or wind-power Deming's explanation would appear less and less likely. Moreover in areas which have historically switched from TT –PME, the temporal evolution of the Hum has followed EXACTLY the temporal evolution of the change in earthing system type. The present author has previously established the statistical link of the Hum in its more contemporary form as being strongly associated with renewable energy systems is based on user visits to GIS based Hum mapping and information websites.
At first sight the action of two apparently very different renewable energy systems ( hydro and wind) seems very different. The present author has previously explained possible links of the Hum and mains electricity grounding systems (refs). Thus the purpose of this paper is to ask and answer the question how can two apparently unrelated renewable energy systems be possibly related in the conundrum which is the Hum and are there circumstances in which the Hum could be a preternatural or at least part preternatural phenomenon?
Despite the first recorded cases of the Hum as a British phenomenon in the 1970’s, perhaps the first most reliable descriptions of the phenomenon seem to have come from those first afflicted in the USA in the 1990’s. There, musically inclined Hum hearers would describe the phenomenon as being like an undulating tone, with its frequency matched somewhere between 30-80 Hz and pulsating or modulated at between .5 and 5 Hz. The earliest US Hum in Taos could not be audio recorded. Hearers described the phenomenon to be often louder indoors and at night. At least in Toas NM, AC Magnetic fields at power line frequencies, their harmonics and sub-harmonics in afflicted houses were generally somewhat higher ( up to three times) than ambient. High AC Fields are often features of circulating or imbalanced ground currents. One can at least see how this might have provided fundamental frequencies in the appropriate range for either magnetic and/or subliminal acoustic/vibrational coupling with the human body but not necessarily the modulation effect.
In another famous US Hum case, namely that at Kokomo two industrial sources were traced namely a cooling fan emitting 36 Hz sound and a compressor emitting 10Hz infrasound/seismic vibration. Presumably real or virtual harmonic beats could have occurred ( the latter due to cochlear non-linearity) giving rise to the pulsating modulation which is a strong feature in anecdotal descriptions of the Hum. Kokomo appears to be one of the cases where infrasound and acoustic frequencies have acted together to produce the hum, is the Kokomo Hum, where cooling fans emitting 36 Hz air borne tones and an industrial compressor emitting 10 Hz were thought to be involved. Two lesser known examples are disturbances at Salbu in Norway due to industrial fans ( again airborne LFN) , and a report in the Journal Noise and Health 2004 which involved a communal central heating plant with ground borne vibrations in the region of 48 Hz and 100 Hz combined with airborne infrasound below 10 Hz from its chimney. The possibility of beats here is again very real. The present author supposes in some cases the distinction between infrasound, LFN and the a Hum to be a rather fine line. The symptoms of infrasound exposure have been described by some as similar to those of the Hum (ref). An infrasound or an LFN can be thought of a Hum if its source cannot be readily located or if it cannot in its entirety be audio recorded.
Let us consider Kokomo in more detail. Several publications suggest that cochlear non-linearity occurs in humans at frequencies (2f 1‐f 2) and (f 2‐f 1) (refs). Barnes has suggested for the Hum that 3f1-2f2 may be relevant and this, generally, has been retrospectively confirmed by reference to the work of Julicher et al (2000). It is difficult to see how the frequencies 36 and 10 Hz alone could generate a difference frequency in the range .5-5 Hz. However if the US mains frequency of 60 Hz or its sub-multiples were somehow involved then a difference frequency of 6 Hz may have been generated. Frequencies of 2x48 Hz and 100 Hz however in the Norwegian case above would yield a 4 Hz difference far closer to that traditionally described.
The author’s home residence and locality sometimes experiences the Hum and Hum like phenomena.
Comments non –linearity
slow beats 3f1-2f2
fast beats 2f1 –f2
Hum-like effect 3f1-f2
Hum-like effect 2f1-f2
Hum-like effect 3f1-f2
Based on the ultrasound, magnetic and audio frequencies recorded using a spectral analysis program with and without the Hum present, it has been possible using appropriate frequencies of acoustic and infrasound ( ref) the author has been able to cause hum like effects to be perceived by a few subjects most probably as a result of aural cochlear non-linearity and this explains in part why the hum can rarely if ever be recorded because unless driven into saturation ordinary electronic acoustic mixers would not be expected to act non –linearly. As Deming once posed the question 'is hum internal or external?' So yes, in some cases, particularly with laboratory synthesis, the hum is in people’s heads but only because of the external frequencies applied.
Clearly, the early US cases of the Hum were very sporadic and far from generic. Similarly were the early reports of the Hum in the UK in the 1970’s at Largs and Bristol. However, the description of the sounds perceived in the phenomena were closely matching. Clearly, also there are distinct groups of frequencies which seem to be able to produce the Hum, or at least illicit a Hum like perception in some individuals. If we consider the well-known UK Hums; the cause of the original Bristol Hum has remained, perhaps, one of England’s most highly guarded secrets and eventually the Hum in that location appeared to lapse from notoriety or popularity. Some suggested a common connection between Largs and Bristol was railway infrastructure. In the author’s search for an explanation of the Hum, new infrastructure has featured heavily. In the early 1970’s Britain got most of its pumped storage hydropower systems, PME mains earthing began to be used and new high pressure natural gas mains were installed. One needed to systematically enquire if these systems could provide any or all of the frequencies perceived in the Hum. One also has to enquire into the possible propagation pathways for these frequencies into domestic presses and hearers’ bodies. Interestingly, a Hum in Bristol has recently returned and some are linking it to nearby wind generation infrastructure. There also remains the possibility that at least one component of the Hum may be preternatural. For example, very strong narrow band seismic tremor signals at frequencies of 1.5-4 Hz have been observed at some 15 or more sites world-wide over natural reservoirs of oil, gas. Water or mix phase hydrocarbon deposits ( refs). It seems logical to suppose if these frequencies could mix in some underneath or in a building structure or in the ear with power-line frequencies then one could have a phenomena tantamount to the Hum. The seismic aspect of the Hum must not be overlooked. In interviews and surveys with Hummers the author has found that a considerable percentage only began hearing the Hum after they had been involved in an Earthquake. In other words do humans develop in inane sensitivity to seismic vibration as a tool of self-preservation?
Reports of even a preternatural Hum, worldwide, caused in this manner would still continue to grow as the use of PME grew. This would then be at least in part a preternatural Hum. Such a Hum may account for very sparse reports in Britain as far back as the 1950’s? However, it would not and does not account for associations of Hums with renewable energy systems which is predominantly what is presently seen the world over. It may give an additional guide however on the types of (seismic) frequencies we need to look for.
Frequencies supplied by hydro-power
We have some examples of the types of frequency combinations which can elicit Hums as outlined above. The question is then to see if hydropower too could supply any of these frequencies either directly or by ( parametric) or other mixing either internal or external to the Hum hearer.
All large rotating industrial machinery is capable of providing acoustic noise and vibration. Traditional hydroelectric power was established in the USA and Canada long before the advent of the Hum. However such machinery is capable of producing underground seismic vibration which can travel very large distances. For example, Kværna 1990 has studied short-term fluctuations in the seismic noise field at the NORESS array and have traced seismic signals with dominant frequencies between 2.5 and 2.9 Hz to sources at a nearby hydroelectric power plant. Similarly Hjortenberg and Risbo (1974) studied monochromatic components of the seismic noise in the NORSAR area. The most prominent peaks were 2·08 Hz and 2·78 Hz. The geographical distribution and the variation with time was also studied, as was the particle motion and wave propagation. A source for the 2·78 Hz peak has was found at a hydroelectric power plant, but the sources for the other spectral peaks were at the time still unknown. The 2.78 Hz frequency varied very slightly in close association with that of the power network. This suggests a phase locking or coherence between the two. Bolkmann and Baisch (1999) showed that seismic signals from synchronous machines on the Czech border could be detected as far away as Bavaria. Dovgan (2012) discusses the complete seismic monitoring of a hydroelectric power station. 2.7 Hz is a rotational frequency of the machine and 44.6 Hz is the lowest resonance of the dam. Zhu et al (2013) also discuss seismic monitoring as an important and previously overlooked technique for condition monitoring of a hydropower station. Kasahara et al discusses large water pumping stations as seismic sources in the frequency range 10-50 Hz. Penstock vibrations of pumped storage Francis turbines fall around the 8-10 Hz range, see Dorfler et al ( 2012) and Vortex swirl at frequencies of between .3 and 5.5 Hz can occur, see for example but not exclusively Whal which can also affect power output amplitude and frequency.
Others have monitored velocities in the sighting area of the Norwegian Seismic Array (Norsar) have been monitored over a time period of 1 week by using a hydroelectric power plant as a continuous wave generator. Propagation phase angle differences have been measured over travel distances ranging from 4.7 to 13.7 km, and group velocities of the order of 3.5 km/s are derived. This is close to the expected (phase) velocity for S waves, and the particle motions derived at a distance of 4.7 km also correspond well with those for S waves. The obtained precisions are 10−3 for a time period of about 2 hours and 10−4 when 1 week of data is used. The phase difference data exhibit a clear semi-diurnal variation which probably corresponds to variations in the seismic velocities with peak to trough amplitudes around 10−3. There is some possibility that the observed variations could be caused by tidal effects. Observed spatial variations show that the influence of the local geology is significant.
It is clear then that hydropower in its various guises could provide a range of seismic vibration frequencies which may under some conditions be associated with the Hum. The ear is of course not directly sensitive to the lowest of such seismic frequencies but the body could potentially feel vertical vibrations as a result of say a Rayleigh wave impinging on property. Imbalanced AC ground currents have become in both the US and UK since the advent of PME. I hypothesize that these may give seismo- electric vibrations in certain properties which in themselves could be modulated of mixed with the aforesaid hydroelectric seismic signals due to non- linearity in the rock/porous media and/or in the human cochlea and/or saccule, resulting in the perception of Hum like effects. This would account for a genesis of the Hum as phenomenon in response to the type of electrical grounding systems in use.
Frequencies supplied by wind power
Although disputed by many wind-power and wind-farms do produce large quantities of infra-sound. So here is a potential way in which they could cause the Hum at a distance. At far closer quarters, a phenomenon called OAM ( other acoustic modulation) is now accepted as being that which can make wind-turbine noise so disturbing and annoying to humans.
However, what is far lesser known is that wind-turbines also supply seismic noise. Further and even more fascinating and borne out from a conversation the author had with a gentleman in the UK recently afflicted by the Hum from a single nearby wind-turbine is the possibly that due to flexural or other modes in the tower, a wind turbine could actually supply seismic noise even when the wind is not blowing strong enough to turn its blades.
The questions to be asked here are:
I) How far could such noise be expected to travel?
II) Are the frequencies supplied sufficiently similar to those supplied by hydro-power or those experienced in other cases of disturbing LFN to establish a link with the Hum?
III) Are turbine tower seismic signals in the absence of wind strong enough to turn turbine blades sufficient to cause or contribute to a/the Hum?
The following is a direct extract taken from the public domain document 'Bad vibrations: windfarms and seismic monitors' see Blachman-Biatch (2011)
Start of Extract:
The Guardian recently reported that the UK Ministry of Defence (MoD) blocked a planning application from REG Windpower to build a wind farm near Eskdalemuir. The MoD prevented construction because the vibrations from the wind farms would disturb seismic monitoring activities at Eskdalemuir. Although this frustrated REG Windpower, the Eskdalemuir monitor is part of the CTBT’s International Monitoring System (IMS). Preventing interference with this system is important, and the MoD has a strong reason to deny the application. The MoD would object to any new turbines within 50 km of the station to ensure monitoring is not disturbed. Wind power developers are free to build outside of the 50 km zone. The problem is that Eskdalemuir is an ideal space for wind farms because it is open and sparsely populated. Unfortunately, those are the same qualities that make it an excellent seismic monitoring site.
Wind farms’ seismic impact
In 1998, the UK has ratified the Comprehensive Nuclear Test-Ban Treaty (CTBT). This treaty bans all nuclear explosions in all environments. Treaty compliance is verified through an elaborate network comprising 337 monitoring stations worldwide. The stations employ four types of technology: seismic, hydroacoustic, infrasound and radionuclide. The seismic network consists of 50 primary and 120 auxiliary stations. Eskdalemuir is an auxiliary station in this network. The Preparatory Commission for the Comprehensive Nuclear Test Ban Treaty Organization is charged with constructing and operating facilities. State parties to the treaty, on their part, have pledged not to interfere in their operation.
The problem is that wind turbines built too close to a monitoring station interfere with the station’s detection capabilities. Although wind turbines are designed to be balanced, imperfections in construction and uneven wind forces disrupt this balance. As the turbine blades rotate these imbalances cause the turbine itself to vibrate in tandem with the blades. These vibrations are translated into the ground, creating an interfering seismic signal. Large numbers of wind farms therefore raise the average signal noise within a particular area. If the average noise level is too high, the scientists cannot easily distinguish a distant nuclear explosion from background noise. REG Windpower’s proposed building wind farms within 50 km of Eskdalemuir. According to a 2004 study by the Applied & Environmental Geophysics Research Group (AEGRG), this is too close.
The need for a ‘noise budget’
It would unfortunate indeed if CTBT implementation would create an obstacle to the establishment of wind farms. Therefore, over the years, the MoD has made several decisions to enable greater wind power development.
Before 2004, the MoD objected to any wind farms within 80 km of Eskdalemuir. This objection removed 40 per cent of the UK’s potential wind energy capacity, or up to up to 1.6 gigawatts (GW) of wind energy.
As noted above, in 2004, the AEGRG released a study on the seismic impact of wind farms. This study establishes a set of guidelines that would allow the UK to develop more wind farms in Eskdalemuir. These guidelines also ensure that the wind farms would not damage the Eskdalemuir station’s monitoring capacity. As a part of the study, the AEGRG suggested the MoD revise its blanket ban. Instead, the MoD could enforce a ‘noise budget’ for the area 50 km from the station in all directions. And no one would be allowed to build wind farms in the area 10km from Eskdalemuir. Within the 50 km zone, the budget would be equal to the seismic noise of a windy day at Eskdalemuir, or 0.336 nanometres of uncertainty in vibration measurement.
This solution allowed businesses to construct wind farms ten to 50 km from Eskdalemuir, while ensuring that those farms would not interfere with seismic monitoring. These restrictions were relaxed after another AEGRG study to allow seismically-quiet ‘micro’ wind turbines. These turbines, which contribute an insignificant amount to the noise budget, can be built within 50 km of Eskdalemuir.
Building better wind farms
If wind power developers want to build wind farms near Eskdalemuir, they must integrate technology that diminishes the seismic impact of wind turbines. According to the AEGRG, technologies reduce the vibration of mechanical systems are available, though they are relatively new. For example, in April 2011, Reactrec Ltd, a UK company specialized in solving vibration problems, launched a ‘Seismically Quiet Tower’ (SQT) system that can significantly decrease turbine vibrations. Another possible solution may hang weights inside the turbines to deaden vibrations.
But proponents of wind power may not want to limit their scope to technology that decreases seismic activity. They could also potentially help their cause by funding or developing more accurate seismic monitors. Such monitors might allow a greater noise budget which could allow more wind farms near seismic monitors.
Wind farms in the Eskdalemuir area are an important part of
the UK’s wind power program. However, seismic monitoring stations are also a
key component of the CTBT’s monitoring regime. But it may not be necessary to
choose between the two. Developers are now able to either build outside of the
50 km zone or use technology to decrease the turbines’ vibrations. And with the
introduction of micro wind turbines, seismologists and developers may no longer
have to fight over the same windy and barren patch of land.
Last changed: Sep 15 2011 at 7:06 PM
End of Extract
Saccorotti et al also studied seismic noise from Wind Farms at the Virgo Gravitational Wave Observatory, Italy. Inter alia they found among the different spectral peaks thus discriminated, the one at frequency 1.7 Hz is associated with the greatest power, and under particular conditions it can be observed at distances as large as 11 km from the wind farm. The spatial decay of amplitudes exhibits a complicated pattern, which we interpret in terms of the combination of direct surface waves and body waves refracted at a deep (≈800 m) interface between the Plio-Pleistocenic marine, fluvial, and lacustrine sediments and the Miocene carbonate basement. We develop a model for wave attenuation that allows determining the amplitude of the radiation from individual turbines, which is estimated on the order of for wind speeds over the 8–14 m/s range. On the basis of this model, we then develop a predictive relationship for assessing the possible impact of future wind farm projects. 1.7 Seis
Effectively, what they have found is that at least with the underlying soil and rock conditions in their locale is that a seismic signal of 1.7 Hz ( the usual blade crossing frequency) can travel some 11km.
This frequency is of the same order as that which can be supplied by hydro-power as a result of either mechanical motor generator signals or vortex swirl oscillations.
This establishes one possible link between two unlikely 'bed-partners' in the conundrum of the Hum
Another study from Massey University, New Zealand is equally revealing, see Bakker (2009).
I have reproduced the Public Domain Abstract of that Study in its Entirety.
START 'Residents on a river plain at the foot of the Tararua Ranges, New Zealand, experience ongoing noise problems, including sleep deprivation, thought to emanate from a nearby wind farm in the ranges to the east (closest V90 turbine is 3 km away). The problem is worst when wind is from the eastern quadrant. Installation of 'Hush Glass' only partly alleviated the problem indoors.
Continuous time series recording of seismic noise using a buried L4 geophone and acoustic surface microphone attached to a wall inside the house, was conducted during March 2009. Use of night hours records minimised extraneous noise, and seismic noise from vegetation was also guarded against by analysis of site wind records.
Early analysis of 196s seismic samples identifies noise bursts lasting 10 seconds or more, every minute or so, associated with easterly wind conditions; with broad spectral power peaks centred on approximately 10 and 28 Hz. Audio playback of the seismic records was identified by the residents as similar to the noise they experienced.
We conclude that seismic energy from the turbines, most likely as Rayleigh waves, is coupled through its concrete foundations into the house, where various vibrational modes are stimulated, thus producing the effects experienced. We note that residents experience these strongest when lying down, i.e. when best aurally coupled to the foundations.
These results provide an initial indication that seismic effects should be assessed in consideration of offset distances from turbines to residences.
Ongoing work will consider such factors as directionality of seismic noise, proximity to the range front fault as possibly accentuating seismic response, either through standing wave or dispersion, and constructive/destructive interference between turbines associated with wind variability as a cause of the intermittent nature of the phenomenon. End
Frequencies of 10 and 28 Hz are very similar to those encountered in the original Kokomo Hum and I would expect on the basis of cochlear non-linearity for them to have the potential to cause the Hum or Hum -like effects. Frequencies of 10 Hz are similar to those of Penstock vibrations in a Francis turbine establishing yet another possible link in our search for an explanation of unlikely Hum bedfellows.
Westwood et al (2015) have confirmed that vibrations from wind turbines in frequency range .93 -5 Hz do indeed enter the ground but their study only deals with the near field to distances of about 300m.
Dean ( 2008) has authored a discussion paper on the potential environmental threat from Wind Turbine mechanical vibrations. Dean concludes that
'Their power and frequencies have been measured, and in Ireland magnified similar vibrations caused significant harm. Given the nature of this problem the author is calling on relevant authorities to exercise increased caution when considering turbine installations until urgent threat assessment is completed. '
Styles et al (2005) describe an extensive monitoring programme to characterise the low frequency vibration spectra produced by wind turbines of various types, both fixed and variable speed. They demonstrated that small but significant harmonic vibrations controlled by the modal vibrations of the towers and excited by blade passing, tower braking and wind loading while parked, can propagate tens of kilometres and be detected on broadband seismometers.
Stammler and Ceranna (2016) discuss continuous seismic signals of wind turbines (WTs) d at 13 sites of the Gräfenberg (GRF) array in Germany. The stations of the GRF array have operated continuously for 40 years and comprise the longest available digital broadband array data set. By comparing time spans before and after installation of WTs in the vicinity of the stations, their influence on background noise can be quantified. Here, a strong dependence is shown between local wind speed and the observed effects on noise spectra. Station sites with WTs within distances up to 5 km are exposed to significant disturbance in the background noise; even at distances of 15 km such signals are still visible. The geological setting at GRF with sedimentary layer below all stations seems to favor propagation of these signals. Moreover, we observe different decay patterns for signals below and above 2 Hz, which could be related to the geometry of this layer. Overall, our observations clearly document deteriorating effects of WTs to highly sensitive seismological stations.
Thus I conclude that wind-farms are capable of transmitting seismic vibrations for up-to 10km and beyond, interestingly even when parked. Further I conclude that rather unexpectedly they are potential bedfellows for hydro-power in the phenomenon which is the Hum. Fortunately, Reactec Ltd have developed a Seismically Quiet Tower (SQT) system which can be retro-fitted or installed during construction. This can significantly attenuate the vibrations produced and delivered to the ground in the frequency band deleterious to the discrimination capability of the Eskdalemuir station, I.e those at 2-6 Hz. The SQT can, and is planned to, be fitted to existing close-in wind turbines to reduce their contribution to the vibration budget and potentially release budget for new development elsewhere in the 50 km zone of concern around Eskdalemuir. Keele University have carried out a programme of modelling and seismic monitoring of this system and have confirmed that it does significantly reduce the vibration spectrum in the region of 2 to 6 Hz which has the added benefit of reducing many fatigue loads on the turbine tower itself. It remains to be seen how much if at all the system will bring relief to Hum sufferers.
Further questions raised
I) How does PME earthing fit in with the above and is it relevant at all?
II) How does magnetic Hum perception fir in with the above or is it artefactual?
Quite simply there are parts of the world which utilise wind power but do not have PME earthing where on the basis of lack of of apparent reports, complainants or visits to the World Hum Database website, the Hum may not be a problem.
The hypothesis I proposes is that via the electro-seismic effect, PME accentuates transmission of Hum frequencies into houses. Low frequency seismic signals can couple into building structures that is without question. So with PME earthing one would expect buildings to vibrate at power line frequencies and their harmonics and various sub-harmonics that is also without question. Hornbostel and Thompson (2007) confirm that electro-seismic processes may be both linear and non -limear.
Therefore given the possibility of a non -linear electro-seismic process I would expect power line frequencies and their attend higher and fractional order harmonics to mix additively and subtractively with any seismic frequencies already present in the ground, the result being the Hum.
Essentially, because of induced structural vibration overlying houses will contain a range of acoustic and (electro)-magnetic signals which will potentially be coherent. I have discussed this effect previously ( refs).
Magnetic Hum Perception
Following the principle of Occam's razor one might be tempted to forget magnetic Hum perception as a fanciful, theoretical notion. It may potentially be needed to explain absolutely all the anecdotally reported properties of the Hum especially the perception of the Hum in caves containing electrically and magnetically permeable rock and the fact that there is reported no perception in limestone caves. Although this could be a facet of just electromagnetic/magneto-seismic or electro-seismic generation.
It may only be relevant to the perception of the Hum in deaf people who possibly perceive magnetically and by sensing vibration.
But human beings are incredibly complex entities which have evolved with nervous systems and brain waves which appear to be coherently synchronised with the fields of Gaia and the Universe. Any anthropogenic changes to these fields then, perhaps has the potential to be sensed.
A strong case has been advanced to suggest that Seismic signals from renewable energy sources and PME earthing systems act together to bring us the Hum. Hydro-power and Wind -power, at first sight two very unlikely bedfellows in the story of the Hum have been shown to be equally capable facilitators in terms of the narrow-band seismic frequencies they produce.
Seismic propagation depends heavily on underlying rock strata and this together with any airborne infra-sound and acoustic components strongly controls the geo-sporadic nature of the Hum.
Seismic propagation also depends on the whole lithosphere from tides to crustal stress. In that sense the Hum has inherently both natural as well as anthropogenic components.
Lithosphere, atmosphere and space physics are also inherently coupled. Electrical power systems here on earth also modulate upper atmospheric space physics processes and may even influence seismic processes, see for example 'Comparison of the diurnal periodicity features of seismic noise, earthquakes, and electric power consumption' Sidorin ( 1990).
Indeed, the author recently showed a strong dependence on the Bangor Hum on power flows across UK/Irish and UK /Continental power inter-connectors (ref).
The author has dedicated some 12 years' research to the Hum. It is sincerely believed that this work may represent the final piece of the jigsaw puzzle which has been the Hum.
It is further hoped that those charged with developing our future technology will take on my findings to the benefit of human kind.
The author wishes to thank his wife Gwyneth and his son Dwain for both being experimental subjects in some of the Hum work and for valuable discussions in the topic.
Full Reference List to follow