Houses, hearing and humming –completing the jigsaw on low frequ

Double glazed houses, hearing, harmonics and all night pubs and clubs a-coming; completing the jigsaw on low frequency noise called ‘humming’. By Dr Chris Barnes Bangor Scientific and Educational Consultants, Wales uk email manager@bsec-wales.co.uk

Homepage http://drchrisbarnes.co.uk

ABSTRACT

Measurements on a specific case of the Hum are made and related to the personal observations of two Hum sufferers. The roles of both acoustic and infrasonic sound are considered with particular reference to the effects of double glazing. 'Hum-like'

effects can also be synthesized using waveform generators and loudspeakers taking into account cochlear non-linearity. The predominant groups of infrasonic and acoustic frequencies for various parts of the county of Gwynedd are identified. If is shown the the Hum can be manifest from local noise sources or far more distance, possibly multiple noise sources. A feasible hypothesis is presented that at least some component of the Hum in Gwynedd can be attributed to the Dinorwig pumped hydro-power station facility. The notion of an additional relationship between hums'

intensity and magnetic fields is also proposed. A possible link between infra sound and gravitational theories of the hums is mentioned. A personal check -list for identification of 'hums-type' is introduced.

Introduction

It would have once been taboo to even consider making any sort of submission to a learned journal on the topic of the ‘hum’. Once the realm of nutcases and crackpots, this ill-understood auditory phenomenon which has made and continues to make many lives a misery is now gaining the recognition it deserves.

It has, very recently indeed, been proposed that people who perceive the hum simply have ‘over sensitive hearing’¹. Whilst not being entirely dismissive of this premise, the present author would suggest that it is poor science to make suggest such a hypothesis without offering experimental evidence or proof of the same. Further he would pose the question why has such over-sensitivity and allied hum hearing has only begin in contemporary times. Anecdotal reports usually refer to the hum as a mainly nocturnal phenomenon. In order to marry up the sensitive hearing hypothesis with such anecdotal observation, one either has to propose that our internal or external night time noise-scape has changed in some way in recent years and/or that for whatever reason a greater and greater portion of the World’s population is getting sensitised hearing. There is sound evidence for both these proposals²and furthermore that sensitizing of hearing causes adaptation of the subjects’ low frequency threshold shift in the presence of new low and even infrasonic sound sources³. This would also tie in with other anecdotal reports of partially deaf people hearing the hum. It would also account for hum hearing peaking in the 50’s age group as hearing loss as a function of ageing usually hits the higher and middle frequency spectrum more he lower end in tact. An important facet of modern living is the fenestration of our modern houses which is today almost exclusively double glazing. This type of window was first introduced in Britain in 1969 coincidentally the era that some of the UK’s earliest hum reports date to.

It is known that although double glazing is very good at screening out high and medium acoustic frequencies, it is very poor at blocking low frequencies⁴. We now live in a country that never sleeps and doubtlessly this has altered, irreversibly, our outdoor soundscape with acoustic contributions from boy racers, pubs, clubs and all night restaurants with their attendant cooling fans, chimneys, pumps and compressors. Add to this increased seismic noise from all night traffic and gas consumption, electricity generation and hydro-pumped storage. Further, add to this the low pass filtering effect of double glazing and the ability of double glazing to transmit cavity wall vibrations and we certainly have the formula for changed internal sound-scapes as well.

The rest of this work will focus on measurements and personal observations of the author and his wife (both hearers of the hum) with regard to acoustic and magnetic spectra measured in the author’s house and at times when the hum has been audible. Further it will be shown how such frequency components might enter as both airborne sound and ground borne vibrations and be modified by fenestration and resonated by houses.

Further experiments are then performed on synthesised frequency components required for perception of the hum.

Because the hum now appears to be an almost World-wide phenomenon, some have sought to suggest it correlates in time and space with military communications aircraft, a sort of single ‘universal’ source⁵. Personal observations will also be presented below to support the notion this is not the case. Whereas it will be shown that usually the hum is caused by several separate acoustic and/or vibratory sources of unknown location and distance that is not to say that in certain houses under certain conditions, a hum –like effect cannot be generated by a single relatively local multi-frequency source such as the bass components of very loud music, a few hundred metres, away exciting several resonant modes of the building.

In 99% of the cases where the hum has been audible at the author’s home there only been acoustic activity but during the most pervasive episodes there was also a low frequency magnetic comb spectrum present which to date remains relatively unexplained. Possible natural causes are the aurora, meteoric bolides or seismic earthquake pre-cursor signals⁶.

Apparatus and Materials

For hum acoustic measurement a home made ‘big ear’ type microphone comprising a base loudspeaker was used as the input transducer with a spectral band pass characteristic controlled by direct mass loading of the cone. The big ear was fed directly into the sound card of a Dell Laptop Computer type Latitude D830 and spectrum analysis was performed on this machine using a well known radio and electronics shareware programme entitled ‘Spectrum Lab’. Spectrum lab was used with a frequency window of either 0- 80Hz or 0- 300 Hz and a time per line of 100ms. For magnetic measurement the big ear was substituted for the primary winding of a small ferrite cored mains transformer. The system was so sensitive that a 60 Hz signal, possibly the power grid of other countries could sometimes be detected. For electric field detection a short wire antenna was simply plugged into the sound card. For hum synthesis the same laptop computer running Windows XP and a shareware program known a FG Lite was used together with another laptop running Windows 98 and FG-Lite and a VideoLogic Digi- theatre surround sound system and two additional base speakers.

Methods and Personal Observations

Hum spectra were recoded using the apparatus as described above at times when the hum was and was not present, based on the personal observation of the author and the subjective agreement of his wife, also a hum hearer. The recorded spectra may be displayed directly as image files or the salient points in terms of dominant frequencies may be manually entered and recorded in XL files together with time and date of the event. Hum spectra were recorded in the bedroom of the author’s house where the hum is perceived most loudly and additional measurements were taken with the big ear abutting the wall to check for frequency components entering as direct vibrations in addition to those exciting air column resonance. Hum spectra were also recorded near to a pumped storage hydro-electric power plant, underneath 400kv power lines and in the village of Tregarth where hum has previously been reported by certain residents.

Hum synthesis experiments were performed on the basis of typical frequencies recorded in the aforesaid spectra to test the hypothesis that some or all of the dominant frequencies recorded in whatever combinations might be responsible for the hum perceived. Since it is known that non-linearity in the human cochlea produces 2f1-f2⁷frequency differences examples of such patterns were sought out in the received spectral data and tested in the synthesis experiments.

Results and Discussion

The results of the spectrum analysis experiments at various locations are shown below.

TABLE 1

LOCATION	0-10 HZ	10-20HZ	20-30HZ	30-40HZ	40-50HZ	50-70HZ	70-130HZ	HUM/NO HUM
DINORWIG	3.5,6.5,8.5				50	67.5		Y
LAKE DINORWIG	3.5,10			32	50	67.5		Y
POWER LINES		11,13.5		32	50			Y
TREGARTH		11,19	29	32	50			Y

HOME	3.3,5.5,9			31,34	50			Y
HOME			26,27		50	BROAD	BROAD	N

HOME		17	22	31,33	50		78	Y
DITTO	2,5 MAG		30	30	40,50	60		Y-MAG
DITTO		19		32	50,58	66		Y

HOME		15,18.5	21,23	31	50	61,65,68	73,75	Y-LOCAL LFN

HOME	5,7	12,14,	22,27,	32,40	50	60		Y-STRONG
DITTO	3,9	13,	28	33	50			Y-MAG
DITTO	5,7	13,16.5	22.5	33	50			Y
DITTO	BROAD			33	46,50	60,64	88	WINDOW Y
DITTO	4						122	OUTDOOR
DITTO	4,9,			31	56	65		WALL
DITTO	3,8	10,19			50	67	BROAD	Y-MAG
DITTO	3,7	16		36	50			E-FIELD
								MAINS OFF
DITTO	2,4	10.5,14	23		46,50	60		ROOM Y
TAL Y BONT								N

The most striking commonality in the above table is the presence of infrasound below 20Hz in all locations and at all times when the hum is subjectively observed by both the author and his wife. There is also a considerable number of geometric and harmonic type relationships with the frequencies observed. There are also a substantial number of discrete ‘carrier’ frequencies observed. In the cases where the hum was not observed, the frequency spectrum was very quiet below 26 Hz. Also the frequency spectrum in the range 50 Hz-130 Hz was either totally quiet as a Tal y Bont or very disturbed mainly with broad band constantly frequency shifting noise due to the Doppler shifts of almost continuous moving traffic as at the author’s home mid- morning. The notion of involvement of infrasound and acoustic frequencies operating together to produce the hum is not new but the author believes far too little notice has been taken of it. One of the cases where infrasound and acoustic frequencies have acted together to produce the hum, is the Kokomo Hum⁸, 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 distrubances at Salbu in Norway due to industrial fans ( again airborne LFN)⁹, 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 and airborne infrasound below 10 Hz from its chimney¹⁰.

In the present study the most common infrasonic frequencies present with other higher acoustic frequencies seem to be in the range 14-19 Hz. Frequencies in this range are present in 80% of hum cases observed by the present author. If these frequencies are not present then frequencies in the range 7-10 Hz seem to be required instead. Along with these infrasonic frequencies in the range 31-34 were present in all but case of the hum. 5oHz is ubiquitous even in houses with the mains switched off and it is difficult to ascertain whether this is a true audio frequency or simply picked up by electromagnetic induction. Frequencies in the range 60-68 Hz are also very common in the results which were ‘hum positive’.

Taking the hypothesis that some infrasound and acoustic sound are somehow required together for hum perception the hum synthesis experiment described above was performed. It decided to use a frequency pairs in the regions most prevalent from the above results. Frequencies of in the region of 17 or 24 Hz were utilised for infrasound (the low frequency- see table 2) together with a single higher frequency. Separate speakers and a surround sound system were used to output the frequencies and a bass speaker with an additionally mass loaded cone was used to output the infrasound. Most of the time ordinary sounding beat notes could be heard. Sometimes, however, when frequency adjustments with a minuteness of only a few tens millihertz of frequency were made to certain pairs in these would produce the perception of remarkable hum like effects in the tow subjects’ ears. This was not very reproducible, possibly because the room resonances were shifting as the experiment was performed in day –time when there was a lot of outside noise. A frequency pair of 17 and 51 Hz is consistent with 3^rd harmonic generation in the cochlea, whereas frequency pairs of 25 and 50 Hz and 16 and 33 Hz would be consistent with 2f1 –f2 generation in the cochlea. The observation of slow beats at 17 and 25Hz represents a hitherto unmentioned human cochlea non –linearity of the form 3f1-2f2, although this has been mentioned in guinea pig studies¹¹and gerbil studies¹².

TABLE 2

low frequency	high frequency	comments
17 Hz	25 Hz	slow beats
24.92 Hz	48.544 Hz	fast beats
16.55 Hz	49.731 Hz	Hum-like effect
16.689 Hz	33.092 Hz	Hum-like effect
16.689 Hz	50.261 HZ	Hum-like effect

It is fascinating that such a simple experiment can generate hum like effects and 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?⁵. So yes the hum is in people’s heads but only because of the external frequencies applied.

However, in reality the hum is a far more annoying and complex frequency phenomenon. For vibratory excitation, it is known that people find vibrations with more frequency components more annoying^13. Indeed, some have described the hum as more of a vibration. Vibrations in building might be conducted seismically or acoustically and in order to test more of this the author made specific measurements of the acoustic spectra at his house internal walls, windows and outside air in a particular case when the hum was present indoors, yet there was no obvious outdoor LFN (low frequency noise source).

The result from table 1 shows that the only obviously measurable outdoor sources were at 4 and 122 Hz respectively. Neither of which are frequencies which could induce perception of the hum in either test subject either alone or as a pair. However many more frequencies were measurable with the big ear against a load bearing wall of the property with the mains electricity turned off to minimise vibration from freezers and the like and to minimise 50 Hz direct induction. The frequencies present on the wall of the property were 4, 9, 31, 56 and 65 Hz. The logical deduction must be at least some of these frequencies must be borne into the property via ground vibration i.e. seismically.

Seismic frequencies in these ranges have been measured at various places throughout the world.

At the same time the frequencies measurable adjacent to the double glazed window in the main bedroom of the property affected by hum contained a broad infrasonic component between 0 and 10 Hz, and discrete frequency components at 33Hz, 46 Hz, 50 Hz, 60 Hz, 64 Hz and 88 Hz. In other words the double glazed window was emitting some frequency components neither contained within the outside air or the seismic input to the building. It would seem either the double glazed window has its own resonances excited by forced vibration or is somehow acting as a seismo- acoustic mixer. There is an enormous set of sum and difference possibilities here and some can definitely be seen. For instance the output at 88 Hz could arise from the difference between the airborne frequency of 122 Hz and the seismic frequency of 31 Hz to within a few Hz. Sum and difference products involving the seismic and airborne frequency of 4 Hz can also be seen e.g. 56+4 = 60Hz. Is there any wonder then that anecdotal reports speak of double glazing making the hum worse. Personal observation confirms the same. Neither the author nor his wife heard the hum until they had double glazing fitted in their present residence. The hum is not perceived louder when one puts one’s ear directly against a double glazed window because the amplification of outside sound and ground borne vibration overwhelms the ear. The clue as to seismic or surface wave involvement is in listening to approaching vehicles. One can hear the rumble of a vehicle on the road long in this manner long before it is visible or by conventional listening. The conclusion here is that further research needs to be done on to the modes of seismic and acoustic coupling from building cavity walls into double glazed units. The stiffness of the house walls may also be very relevant. The stiffer the walls the higher the frequencies that should be transmitted rather as a band pass filter. Some years ago the author before his present interest in the hum lived in a solid walled single storey building and used to hear a noise like the hum only several octaves higher rather like distant water tricking in an iron pipe or very high pitched Morse code. The property had double glazing in some of the rooms and in retrospect this in combination with the wall may have been transmitting higher frequency components than the more commonly accepted ones giving rise to the hum.

The question further arises, posed by many, what is the source of the hum? Some have sought to suggest that there ought to be a ‘World-wide’ universal source since the hum is now reported throughout the world. The evidence presented here suggests this is not the case. So could the hum even have more than one cause when perceived at the same property? The answer it would seem is yes, if the correct resonances can be excited. One particular case of a hum like experience at the author’s home confirmed this. It was about 11-30 pm and the hum seemed far more impulsive and penetrating than usual. Opening the double glazed bedroom windows revealed a LFN source in the form of very loud music a few hundred metres away. The frequencies contained in the room spectra are in table 3 shown below.

TABLE 3

15,18.5 Hz

21,23 Hz

31 Hz

50 Hz

61,65,68 Hz

73,75Hz

These are similar frequencies but not identical to cases when the usual hum is experienced.

It would seem that within sufficient external power window, room or even whole house resonances may be excited with appropriate frequency components for hum like perception. Other times as with the classic hum one can open the window and hear virtually nothing because some of the components are at subliminal airborne acoustic strengths and others being transmitted seismically. In these cases there is no reason to suppose all the frequency components need to be generated by a common source but neither does anything rule that possibility out. The former example was the case for the Kokomo Hum.

Possible source of the hum or a component of the hum in North Wales

A possible source of the hum or one or more of its components in Gwynedd North Wales is the Dinowig Hydroelectric Pumped Storage Power plant. There a six synchronous pump-motor-generator units weighing some 520 tonnes each¹⁴when spinning there is a tremendous amount of energy and angular momentum. The natural vibration frequency of each unit is 8.33 Hz although other modes may be possible. Other vibration frequencies in hydropower plants are known to be of form of pressure oscillations ¹⁵. There is considerable literature on equivalent However; it is known there have been bearing noise problems at Dinorwig in the past^{16 .} Indeed reference to NETA, the electricity trading websites shows that one motor –generator unit, namely T_DINO-1 is not being used at present, so one may ask the question is it being serviced or repaired. Possible harmonic frequencies of Dinrowig could give rise to the hum and indeed the same or similar frequencies are observed at hum locations. This is seen in Table 4 below where the data from table 1 has been reproduced with the relevant harmonics highlighted in bold.

TABLE 4

LOCATION	0-10 HZ	10-20HZ	20-30HZ	30-40HZ	40-50HZ	50-70HZ	70-130HZ	HUM/NO HUM
DINORWIG	3.5,6.5,8.5				50	67.5		Y-CAR
LAKE DINORWIG	3.5,10			32	50	67.5		Y-CAR
POWER LINES		11,13.5		32	50			Y-CAR
TREGARTH		11,19	29	32	50			Y-CAR

HOME	3.3,5.5,9			31,34	50			Y
HOME			26,27		50	BROAD	BROAD	N

HOME		17	22	31,33	50		78	Y
DITTO	2,5 MAG		30	30	40,50	60		Y-MAG
DITTO		19		32	50,58	66		Y

HOME		15,18.5	21,23	31	50	61,65,68	73,75	Y-LOCAL LFN

HOME	5,7	12,14,	22,27,	32,40	50	60		Y-STRONG
DITTO	3,9	13,	28	33	50			Y-MAG
DITTO	5,7	13,16.5	22.5	33	50			Y
DITTO	BROAD			33	46,50	60,64	88	WINDOW Y
DITTO	4						122	OUTDOOR
DITTO	4,9,			31	56	65		WALL
DITTO	3,8	10,19			50	67	BROAD	Y-MAG
DITTO	3,7	16		36	50			E-FIELD
								MAINS OFF
DITTO	2,4	10.5,14	23		46,50	60		ROOM Y
TAL Y BONT								N-CAR

At Dinowig itself, the vibration fundamental frequency can be seen as can harmonic content which varies according to precise location. This could be influenced by the underlying rock strata. So in Dinorwig it is possible that the power station could be the sole cause of the hum. The question arises could signals from Dinorwig propagate to Bangor? There is evidence in the literature for anthropogenic seismic signals propagating hundreds or even thousands of kilometres ¹⁷.This supports the hypothesis. Airborne low frequency sound and infrasound propagates much further at night ¹⁸in a stable boundary layer¹⁹ so this too would support the notion of a hum in Bangor due to Dinorwig. Dinorwig need not, of course, be the only or unique source of the hum. It might be that it contributes to the lowest frequencies and the higher frequencies are available more locally.

Observations are presently being made on Dinorwig as both a motor (pump) and generator in relation to the hum. This is by no means trivial and will have to form the subject of another full scientific publication. This is because with six sets of motor-generators there are sixty-four possible combinations of operation for both electricity generation and water pumping. Data for power station operations is freely available in the public domain at the electricity trading website known as NETA²⁰. Electricity generating units are given a specific code name and the amount of electricity absorbed or produced by the unit on given dates and time periods can be seen. This is very instructive for it can be seen that with particular combinations of motor or generator in operation the hum is much worse than for others. For instance during the early morning hours of June 2^nd 2009 when the motor pump units known as t_dino-2-6 were all operating together without t_dino-1 the hum was almost unbearable at the author’s home. Later that morning at about 7-25 am the hum was actually heard to ‘switch off’ and switch back on again at 7-45 am! Reference to NETA showed that the hum was present when just two of the units, namely t_dino-2 and t_ dino-3 were being used as generators, yet during the precise period of perceived switch off they had actually been joined by a third unit know as t_dino-4. The mechanism for these effects will clearly need significantly more research but one can postulate about things like interfering frequencies or phase cancellation. Clearly it will be very time consuming to gather comprehensive information about the Dinorwig system but may be worth while and revealing for a fuller explanation of the hum. There are more and more large underground rotary machines in the world now not just generators but flywheel electricity converters, mining machines, salt cutters, oil exploration and drilling etc. etc. and many of these could be potential seismic sources for hum components.

Hum and Power lines

One of the locations referred to in Table 1 is underneath 400kv power lines. The hum could only be observed inside the car parked at this location. To some extent a car may act as a band pass acoustic filter rather like double glazing. A car’s suspension has natural resonances in the infrasound region²¹and its interior resonances at 100-200 Hz ²². It may be that the hum produced here is not of seismic and acoustic origin but is purely of all acoustic origin. It may be that the magnetic field from the power lines induces eddy currents in the car structure and wiring such that vibration occurs. Power lines may generate sub harmonic frequencies electrically ²³ and power lines may also generate acoustic frequencies due to Aeolian vibration²⁴. It may be that the author and his wife have become sensitised to certain frequency components infra-acoustically at their main residence and therefore perceive the hum near power lines if these same components are present in the electromagnetic field. Some have said the electromagnetic field perturbs gravity itself ²⁵ and tried to account for the hum on this basis. An alternating gravity field would of course generate an alternating acoustic pressure field and vice versa. For example it is known there is a relationship between atmospheric pressure and local gravity. ²⁶

The observation of the hum at Tregarth in a car is perhaps the most difficult to explain. The site did how power lines but only overhead 3-phase low voltage lines. The author is aware of a report of a strange humming noise in this village some 20 years ago as one of colleagues was called to investigate but was never successful in tracing the noise²⁷. Maybe the hum was a combination of magnetic induction and acoustic or seismic power. A main high pressure gas pipeline is known to cross the village of Tregarth and such pipelines have been associated with the hum in the past²⁸ and can, under some circumstances, output infrasound.

Electric and magnetic fields at author’s home

During periods of hum there are also often strong magnetic and electric field signals present even with the mains electricity switched off. It is difficult to read too much into this because the author lives on the edge of a town which has a very noisy electrical environment anyway. It has been noted that the hum appears far more intense when there are magnetic comb spectra present. Such spectra may be generated naturally as a result of ionosphere pulsatations²⁹ which often accompany natural atmospheric infrasound production or as a result of seismic Q spikes. An experiment was done to see if the author had direct magnetic sensitivity by sweeping an amplified audio generator into a magnetic choke placed near his head. No noise or hum could be perceived. There are various possible inferences to be drawn. Firstly, that during natural magnetic signal production atmospheric infrasound either propagates better or natural infrasound enhances the hum. Secondly that seismic components travel more effectively. Finally it is possible that acousto-magnetic perception is a quantum biological effect and that the correct conditions were simply not matched in the experiment with the low frequency choke. Such effects have been postulated on ³⁰.

The hum outdoors

The author and his wife have only twice heard the hum outdoors. Both were very coastal, locations within kilometres of a busy fast road, few meters of a beach and also close to woods, piped water outflow and within line of sight of a couple of powerful multi-frequency broadcast facilities. Unfortunately spectrum analysis equipment was not present so one can only give a tentative hypothesis as to the possible cause and frequency sources involved.

The woods may be significant because collections of trees can provide acoustic low pass filtering ³¹. The water outfall pipes may be significant because there could be a low frequency resonance and/or conduction of seismic sound. Finally the presence of seawater and broadcasting transmitters may be significant as wave-wave mixing and /or demodulation effects may occur ³². Finally filtered frequency traffic noise from the fast road may also play a part. Traffic has been blamed for the hum in the past²⁸.

The hum and travelling

The author always notices that after travelling any distance greater than 200 km by car he gets desensitized or looses hum perception for at least 48 hours. Moving cars generate considerable infrasound and low frequency booming noise in the range 10-200 Hz encompassing the hum frequency band ³³. They also emit varying magnetic fields from the engine electronic systems and tyres³⁴. The author believes the car produces a sort of temporary threshold shift in hum hearing a very useful facet!

Clues as to your personal hum

It would seem from this study and reference to some of the more obscure data available is that what is crucial for the perception of the hum are certain types of discrete frequency spectra, always involving at least some infrasound and some acoustic sound and in the absence of random noise which tends to mask the hum or in the absence of wildly varying frequencies such as Doppler shifted traffic noise. What makes the hum so interesting for an acoustical scientist? It is simply the fact that there are several alternative channels for the conduction of the frequency components of the hum into houses and cars for that matter. Anecdotal reports attach so many variables to the hum this has hindered hum researchers in trying to tie down causes. Logically such variations are expected in that the hum is subjective and in the physics of the hum. It is this latter facet which should give people clues about their own hum. The first question to ask is, have you ever being conscious of the hum switching directly on or off? If this is the case then this is most likely representative of some sort of anthropogenic source, heating plant or rotating machinery for example. The second question to ask is do you hear the sound in various houses throughout your neighbourhood, if so then the source of hum or a component(s) thereof may be several kilometres away or even further. There are some anecdotal reports of the hum fading out or fading in rather like radio propagation. Undoubtedly if this is your hum your hum is propagating much further distances through the atmosphere and will appear to be more affected by things like the weather. Traffic noise has been blamed for the hum in the past²⁸. Traffic on motorways is of course continuous and does not switch on and off but could potentially fade in and out in some areas. In coastal areas there remains the possibility that air-gun type seismic surveys may produce impulsive seismic noise^35,36. Seismic waves also cause local gravity perturbation leading to other possibilities for their perception³⁷. Other coastal sources of seismic noise in the 5-90 Hz band can be oil rig drilling and sophisticated bit control systems could give rise to quasi-periodic effects³⁸. A very useful reference in this area is the International Handbook on earthquake and engineering, chapter 19, which also makes reference to seismic sub-harmonics from power generating facilities³⁹. Beware, however, because even seismic propagation may appear to be seasonal and affected by the moon and tides (ref).

Conclusions

The following has been shown;

· For the perception of the hum in the two subjects concerned, the presence of infrasound below 26 Hz in addition to some low acoustic frequencies in the range 30-90 Hz appears to be a crucial pre-requisite. This concurs with findings for the Kokomo Hum and other lesser known cases.

· Infrasound seems to need to be present in the ranges either 7-10 Hz or 14-19 Hz.

· The hum appears to be related to cochlear non –linearity and hum- like effects can be synthesised by appropriate frequency pairs played from an appropriate sound system. A new low frequency acoustic non- linearity function of the cochlea at frequencies 3f1-2f2 has been observed. So the hum is both within us and outside but can only be properly perceived because of the biology and physics of the ear.

· Double glazing and cavity walls in houses enhances the hum.

· Double glazing seems to act as a seismic-acoustic mixer producing more low frequencies which equates to more annoyance by the hum.

· The hum can sometimes arise as a result of a single local multi-frequency LFN source such as loud music or communal heating plant.

· The hum in Bangor North Wales might predominantly be caused by Dinorwig power station and in Bangor the hum can sometimes be heard to distinctly switch on and off.

· The hum can be caused by distant seismic sources, distant infrasonic and acoustic sources or both.

· The frequency components for the hum do not necessarily have all to arise from the same or a single source.

· The hum seems to be enhanced when low frequency magnetic comb spectra are detectable.

· The hum heard in cars under power lines might have different causes from the hum heard indoors.

· The hum can under rare circumstances be heard outdoors, again with potentially different causes.

· Although the hum as experienced by the experimental subjects in this paper is predominantly an infra-acoustic effect nothing here reported rules out the possibility that certain or all individuals may be sensitive magneto-acoustically due to quantum biology. An exploration of hum sites in relation to their magnetic environment will be made in another study.

· Long journeys in moving cars deaden hum perception in some individuals.

· An feasible alternative to the perception of the hum by infrasound and acoustic sound is in local gravity perturbation and coincident sensitivity of the vestibular apparatus.

References

1. http://www.telegraph.co.uk/scienceandtechnology/science/sciencenews/5347896/X-Files-hum-phenomenon-could-be-over-sensitive-hearing.html

2. http://www.dailymail.co.uk/news/article-447781/Warning-bad-tempered-Britain-noise-complaints-treble.html

3. http://intellagence.eu.com/acoustics2008/acoustics2008/cd1/data/articles/002964.pdf

4. http://sound.sial.rmit.edu.au/ADR/FactSheets/GlazedDoors.pdf

5. http://www.scientificexploration.org/journal/jse_18_4_deming.pdf

6. http://geomaps.wr.usgs.gov/gump/people/mcphee/pdf/SRLpaper_2000.pdf

7. http://www.springerlink.com/content/w75l83n54338qkh8/

8. http://www.milieuziektes.nl/ELF/KokomoHumFinalReport.pdf

9. http://home.intekom.com/salbu/apollo/apollo2.html

10. http://www.noiseandhealth.org/article.asp?issn=1463-1741;year=2004;volume=7;issue=25;spage=23;epage=28;aulast=Feldmann.

11. http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=JASMAN000109000006002852000001&idtype=cvips&gifs=yes