Some short comments on a particular case of the Hum and room resonance
Introduction
Until recently, the highly subjective acoustic phenomena and affliction
known as the Hum was a poorly understood phenomenon fitting almost within the
realms of Science Fiction. Sufferers hear, usually in the dead of night, a
pseudo randomly pulsating sound rather
like a distant idling engine or wasp trapped in a bottle. Due to the efforts of Hum researchers such
as Barnes (the present author), Moir (
Also cited by Barnes is the notion that certain locations are more prone to the Hum on the basis of their distance from certain transmitting antennas, http://drchrisbarnes.co.uk/HUMPRED.htm and that their frequencies may be Bragg matched with incoming infrasonic and/or acoustic frequencies, http://drchrisbarnes.co.uk/HUM.htm and http://drchrisbarnes.co.uk/HUM.html.
Moir has recorded a predominant acoustic frequency of 56 Hz in the Aukland Hum,
But with a significant peak also at 32 Hz, http://www.speechresearch.co.nz/humspec.jpg
Kohlase
(personal communication) has recorded a range of infrasonic and acoustic
frequencies from the
Sadly Dawes’ present website no longer seems to contain his early research. Whereas space weather is known to influence the power grid, few people realise the power grid also modulates the electro-jet! http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6VSV-4893XYX-214&_user=10&_coverDate=12%2F31%2F1985&_rdoc=1&_fmt=high&_orig=search&_origin=search&_sort=d&_docanchor=&view=c&_searchStrId=1655822981&_rerunOrigin=scholar.google&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=a7462bd5f7c5240d8fe75fed869a8531&searchtype=a and
http://www.agu.org/pubs/crossref/2007/2006JA012134.shtml
The resultant ion acoustic waves may well be involved in the Hum! Indeed the Earth – Ionosphere cavity which accommodates the first Schumann resonance at about 8.3 Hz may accommodate 10Hz as a non-linear mixing product between incident signals from the US and UK power grids.
The rest of this paper will concentrate on infrasonic and acoustic frequencies. In his paper ‘Search for the Hum, the case for Infrasound’ the present author has shown that frequencies arriving of approximately 9 Hz and 28 Hz are significant players in the Hum at his residence. Particularly intense Hum-like effects could also be synthesised from these frequencies using acoustic generators.
The largest room in the residence is approximately 6 metres long. The house has three floors openly interconnected by airspace and a chimney approximately the same height i.e. some 9 metres. Assuming normal compression waves and normal sound speed in air, these distances yield a half wave resonance of approximately 28 Hz and a Helmholtz type resonance of approximately 9 Hz.
Clearly we have a case wherein the incoming frequencies are driving room resonance, no wonder then the Hum can appear so bad. The author has also shown elsewhere that at least one component of the Hum is either vertical or has a strong vertical component, http://www.drchrisbarnes.co.uk/HUMVERT.htm.
By simple association this is likely to be the 9 Hz component as it can only drive the vertical 9 Hz resonance according to the dimensions of the house.
The lowest frequency component in the Hum as perceived by many is its amplitude pulsation usually reckoned to be between 0.5 –5 Hz. Although a separate frequencies in the region of 3-5 Hz has been recorded (refs) lower frequencies have not. However the spectra recorded in studies by the author at his residence http://www.drchrisbarnes.co.uk/HUMWALL.htm and http://drchrisbarnes.co.uk/HUM.htm
have shown what appears to be pseudo-randomly pulsating infrasound (approximately 1 pulse per second). There are actually four equally possible hypotheses for this! Firstly, there could be a pulsating incoming signal. Secondly, there could be beats due to asymmetries in the room geometries. Thirdly, there could be pressure flow oscillations due to atmospheric turbulence. Some people have been shown to be ‘meteo-sensitive, http://aje.oxfordjournals.org/content/160/11/1047.full to such ‘fast’ atmospheric pressure fluctuations, http://www.springerlink.com/content/pqtwg51wt4cctd6m/.
Such fluctuations would be expected to affect the 9Hz Hum component the most. It is hoped to report further on this in the future. Lastly it is possible the Hum is arriving via two or more different acoustic channels with propagation delay. For example, there could be sky and surface waves or airborne and ground borne waves travelling at different velocities, themselves susceptible to pseudo - random variation.
The author has recorded clear acoustic spectra using a
Laptop and spectrum analysis program; see all of above references of
author. In cases when the Hum is ‘off’
the infrasonic components in these spectra including that at about 9 Hz is
totally absent. One conclusion is that
the 9 Hz signal switches on and off. Within the limits of experimental error a
local source known to generate a frequency of 8.33 Hz, namely the Dinorwig Pumped
Storage power plant, does switch on and off.
Given its location and distance,
such a signal would also satisfy the premise of being able to propagate by both airborne and
ground borne channels, see also ‘Solving a case of the
A second equally possible and quite tantalising solution is that merely the conditions for resonance have changed and that without resonance the incoming 9 Hz signal is too weak to record. It is often said that the Hum alters with the weather. Indeed the author has shown involvement of the 300mb jet stream for the ‘Bangor Hum’ http://www.drchrisbarnes.co.uk/HUM.htm.
But what if the ‘weather’ effects are far more local? For example, local variations in both temperature and humidity will affect the speed of sound within the property. It is hoped to test this hypothesis in the very near future.
