One facet of the Hum is Pumped Storage Hydropower, the Francis Turbine a common denominator connecting the UK and Italy, by Dr Chris Barnes, Bangor Scientific and Educational Consultants, June 28th 2013  e-mail

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The Hum is very briefly reviewed.  A possible connection with the Hum and UK pumped storage hydropower schemes is reiterated which is reinforced by recent contact with Luca Rizzardi an Italian acoustician studying the Hum in that country. Francis turbines appear to be a common denominator.  The predominant frequency recorded does not appear  to be capable of accounting for the pervasiveness of the Hum and is thus probably symptom rather than cause.  Infrasound directly transmitted and arising by other earth and space borne mechanisms from power flow oscillations is re-investigated. A connection with wind power is also realised.  Hums due to these aspects of power systems are hopefully expected to be abated in future as better wide area real time control is realised.            


The Hum is a geo-sporadic auditory phenomenon or possibly group of phenomena which are reckoned to  between (2% and 11% of the World’s population [1].  Contemporary Hums were first documented in newspaper reports in the UK in the 1970’s [2] and in the USA in the 1990’s [3].  Hums as distinct from more mundane forms of LFN are characterised by their apparent lack of traceability and rarely have been recorded.    They are also characterised by the commonality in the description of the noise given, and the noise which is said by those afflicted to sound like a distant irregularly idling engine which cannot always readily be screened out by ear plugs .  Hums are often  tone matched by musically inclined hearers at between 36  Hz and 80 Hz and with quasi-periodic amplitude and frequency fluctuations of between .5 Hz and 5 Hz.  


Occasionally both natural and anthropogenic sources have been found for some Hums.   For example a hum on the big island of Hawaii was traced to volcanic action [4] and see also Fee et al [5].  Since  volcanoes  produce acoustic sound, infrasound and seismic vibrations, this gives us a possible clue to the Hum.  Another US Hum, the Kokomo hum was traced to separate industrial compressors and fans producing frequencies of 10 Hz and        36 Hz, see Cowan (2008) [6].  Similar to volcanic infrasound,  these anthropogenic sources would also provide infrasound, acoustic sound and, if ground mounted equipment was involved, seismic vibration.


The Hum used to be an apparent problem of only the developed Western World which led some such as Deming to propose it may  be associated with the operations of TCAMO aircraft and indeed in the 1990’s Deming suggested that the Hum seemed to best correlate in time and space with such operations, see Deming (2004) [7].  Such aircraft could produce acoustic sound, infrasound and ELF amongst other radio transmissions.


More recently, however, in the twenty-first century cases of the Hum have sprung up throughout the world and in places where TCAMO would not be expected to be in use.  Barnes has recently evaluated mapping on the World Hum database in addition to the locations of visitors to Dawes’ Hum website.  Based on this evaluation it would appear that the twenty-first century Hum is associated with renewable electricity generation sites including wind power, pumped storage hydro-power and solar power [8].      


Hums began in the UK in the 1970’s about the time that pumped storage hydro-power in North Wales and Scotland was being developed.    The author has shown an association of   a Hum in Bangor and the surrounding area with the Dinorwig pumped storage hydro-power scheme [9]. 


It is quite probable   that two other well known UK Hums could be associated with similar schemes.   Largs in Scotland has had a famous Hum since the 1980's which could be associated with the closely located Cruachan  hydropower scheme.  Loch Ness has a Hum which could equally be associated with  the nearby Foyers scheme, see Wilson (2008) [10].


Amateur Hum investigator John Dawes has also had a lifetime belief that the Hum in Britain since the 1970’s is somehow connected with the electricity industry [11].   Dawes even concurs with the present author that in some facets signals from electricity grids may even interact in the ionosphere to produce Hums [12].  The present author has even commented how such interactions may impact the weather and climate[13].


The Italian connection.

One difficulty with doing science on the Hum is that it is a highly subjective phenomenon or group of phenomena where one often has to rely on anecdotal reports.   The present author has had the advantage that he hears the Hum himself and has a live- in subject, namely his wife who also hears the Hum.  Another possible criticism which could be levelled the present author is that most of his Hum work has simply been self-published and not peer reviewed, and not reproduced or endorsed by others nor is he presently directly affiliated with an academic institution.    


Very recently indeed the author has been contacted by an Itlaian acoustic scientist with a University affiliation, namely Luca Rizzardi  who has been working with people who hear the Hum in  Belluno  a province in North East Italy.  Rizzardi has also published evidence of this contact on a well known internet Hum forum [14] .


  Furthermore Rizzardi in his honours degree dissertation, (2012)  [15]  has referred extensively to my work and has reached the conclusion that the Hum in some Italian regions is probably caused by pumped storage hydro plants. This is a conclusion the present author has previously reached regarding the Hum in the Bangor Wales region.  Moreover the Italian plants use Francis turbines which are the same type as those used in the UK.   Rizzardi has successfully recorded the Hum about 1 km from a hydro plant which uses Francis turbines.  The frequencies recorded are in the region of 30 Hz and some infrasound well below 11Hz   the exact frequency of which is difficult to ascertain given the scales employed.    I have previously ascribed on possible cause of the Hum to infrasound in the presence of low frequency sound and a flat higher frequency acoustic spectrum [16].   This also seems to be a feature of Rizzardi’s spectrograph. Rizzardi also comments on the behaviour of the Hum as a function of ground conditions as influenced by weather.   He notes that the Hum is often weaker or dies out all together when the ground is frozen or wet but maximises in long, hot dry spells.  This is suggestive of ground propagation over the distance involved and there is independently published data on the effects of moisture on such propagation, see Hess et al (1990) [17].    The frequencies recorded are quite close to those recorded by the present author close to the Dinorwig Hydro Plant in North Wales and in his house  and neither are they that different from the frequencies associated with the Kokomo Hum.   It   is worth noting that Rizzardi believes the problem frequency to be 30 Hz on the basis of its inclusion in a spectrum measured locally by the present author and on the premise that a fairly close frequency of 32 Hz is included in the famous ’56 Hz’ hum spectrum measured by Moir and his co-worker in New Zealand, see Chapman (2006) [18].  On  the other hand, Ruprecht et al [19]in noise reduction tests for a small Francis turbine conclude that a higher frequency in the region of 166 Hz is more disturbing. Of course it should be pointed out that Ruprecht was dealing with plant operatives and not Hummers (people sensitive to LFN or the Hum).

Considering the frequency spectrum involved, it is actually the lower of the two frequencies i.e. the infrasound which  would be expected to propagate considerably further in both the ground (seismically)  and air (acoustically) than the 30 Hz frequency.  In other words other than as a locally experienced noise i.e. local to the power stations themselves, the present author feels that the observation of 30 Hz is probably more symptomatic than cause.   There is no reason to suppose that two or more dependent or independent sources, local and distant could not be responsible for the Hum. For example two separate sources were responsible for some of the cases of the Kokomo Hum [6].

In the UK some Hums heard at considerably larger distances from hydro-plants than those observed by Rizzardi. Such plants are, nevertheless, suspected as being at least in part as being involved [8-10].

It may be necessary to develop additional or more complex models to explain how this can be and how the Hum can be such a problem.  To a certain extent this has been done elsewhere by the present author but the sake of completeness, will be revisited here.      

Nevertheless  the contact by Rizzardi and the finding of his work [14,15] is indeed an  exciting development and it adds considerable credulity   to the present authors’ previous works. The author views it of imperative importance to explain how the quasi-periodicity of the Hum arises. In this respect an investigation into pumped storage provides some interesting potential answers. 





Hypothesis:  Why is pumped storage is such a problem with the Hum?

 Pumped storage power stations in general are capable not only  of providing airborne sound and infrasound and coherent seismic vibration, but also in line with other renewable energy systems such as wind power their output waveform is notoriously unstable and may contain voltage and phase flicker.   The resultant   electromagnetic signals will also have a degree of coherence with the acoustic, infrasonic and  seismic signals and may    be capable of reinforcing such signals i.e. that which is the Hum  at a distance either by interaction with the  atmosphere, the ionosphere or  lithosphere or all three.  The resultant infrasonic signals may also be capable of exciting pulsed room resonance directly in buildings or as a result of Helmholtz type resonance  in underground  pipes or over ground chimneys attached thereto [20].

Most pumped storage power stations with a high head employ Francis turbines [21-23].   Francis Turbines are notorious for pressure and power fluctuations due to hydrodynamic hydro-acoustic oscillations  of the vortex rope often  at infrasonic frequencies and often with complications due subject to cavitations and or elbows or bends in the draft tube, see Fanelli (1989) [24].     In some cases severe self oscillatory hydro-acoustic   resonance, see Susan-Resiga et al (2006) [25] and Nicolet and Herou (2006) [26] or a similar condition with   narrow band acoustic emission can occur, see Karavosov et al.  Sometimes this can lead to catastrophic failure of the turbines, see Bashnin (2013) [27].   Francis turbines can also suffer from transient effects, see Nicolet et al. (2002) [28].


An almost unique feature of the Francis turbine is that oscillations of the penstock and discharge pressure can be excited with a rope  frequency close to one of the eigen frequencies of the electric system, and particularly at around half load and due to effect in the draft tube they are capable of acting non-linear  oscillators with dynamic hysteresis, see Fanelli 1996 .  Dorfler (PhD Thesis 1982) [29] has also confirmed that pressure fluctuations at partial load in reversible Francis Turbines, i.e. of the type used in pumped storage hydropower, are irregular.  The power plant close to the author's home has all these features, it has been shown it is non-linear, multivariate and time varying, see Mansoor et al (2000) [30].   There can under some instances be increasing instability in the reverse pump mode, see Gong et al (2012) [31].  Such a mode is usually used at night-time to make use of cheap electricity. Night-time is when most Hum is experienced.


Some attempts have been made to control  Francis turbines with either jet control,             see   Susan-Resiga et al ( 2006)[32]  or artificial neural network (ANN) systems, see Kishor et al 2007 [33].    In China, frequency converters have been used at the terminals of pumped storage plants to minimise these sorts of problems, see Galasso (1991) [34].  In this respect it is interesting to note that until the advent of wind power, another potential cause of the Hum, China had no Hum reports, [35].     Kishor (2007) [33] concludes that these problems have still not been adequately solved.


Thus when large pumped storage power plants are connected into electricity grid systems, their speed governor characteristic can dominate low frequency inter-area oscillations,    see for example Gencoglu et al (2010) [36].   Power system oscillations generally lie in the range 0.1-0.8 Hz, see Klein et al (1991) [37].   The present author sees this as crucial to explaining the pervasiveness of the Hum, in that all transformers and synchronous machines on a network will be subjected to the effects of these, as will electrical  ground currents and power line harmonics radiated into space.      


In addition to pumped storage systems,  wind farms are now been shown to provide huge inter area oscillations, see Brownlees (2006 and 2007)   and Wilson et al (2011) [38]. Perhaps then not surprisingly, the present author has shown contemporary Hums to be more or less exclusively limited to parts of the world where renewable energy systems are employed.    


The frequency range of seismic signals from hydro-plants and the frequency range of power-systems inter-area oscillations overlaps.  This range is also the range of reported quasi-periodicity of the Hum.  Wilson et al  [38] show that in a power system with such modes, un-damped oscillations can continue for periods of several hours at a time.  Such time scales are  exactly what is experienced with outbreaks  of the Hum.   The situation is further complicated because power systems may also have an effect on natural ULF pulsations of the earth's magnetosphere, see Fraser Smith (1981) [39].   The precise mechanism is unclear but somehow by altered mechanism in the ionosphere, the atmosphere, the lithosphere or all three, the present author has noted that there is nearly always an increase in the amplitude of the Hum when the IMF ( interplanetary magnetic field Bz) is southward pointing [40].   This not only may add to the quasi- periodicity and geo and temporal sporadic behaviour of the Hum but also    opens up the possibly that some individuals may perceive the Hum by senses in addition to audition.  Such findings may also account for why some facets of the Hum behave as    an anthropogenic phenomena e.g. the weekend effect, and some facets are more like a natural phenomenon.                   


Besides problems with the Hum highlighted above  there is clearly separate concern amongst the electricity industry community  regarding the dynamic behaviour of generating stations in general, see  Hung (2007) [41].  For instance GPS synchronised phasor measurement between areas is now possible to monitor inter area oscillation, see Naoto 2006 [42].   Let us hope better power systems control features will follow soon as a result, with potentially a consequential reduction of or elimination of the Hum. In the meantime, the Hum clearly remains a highly complex and enigmatic phenomenon or group of phenomena.  The author's hypotheses on the Hum, although complex, are perfectly logical and let us hope they will soon be independently validated.     One  proof of their validity would be the automatic reduction of or abatement of many world Hums coincident with the development of such improved features which will simultaneously  benefit the power generators with improved safety and efficiency.    At least with regard to Francis Turbines  Rizzardi [14,15] has provided the first element of validation.

































30.  S.P.  Mansoor,  “Behaviour  and  Operation  of  Pumped  St orage  Hydro  Plants”,  Thesis,  University  of  Wales,  Bangor, April 2000.