Some thoughts on infrasound effects and generation at Loch Ness
Dick Raynor, 12th September 2013
It is documented that people have experienced strange feelings of unease or fear, or a presence, with no apparent cause, and there are also reports of seeing some indefinite object at the edge of vision. These are commonly described as "paranormal" events. When they occur near Loch Ness they can be brought in to the general mixture of phenomena that make up the "Loch Ness monster".
Investigations by the late Vic Tandy http://en.wikipedia.org/wiki/Vic_Tandy found a link between these experiences and infrasound.
Infrasound http://en.wikipedia.org/wiki/Infrasound is "sound" , or pressure waves, passing through the air at a frequency below about 20 Hz - which cannot be "heard" but does have effects on objects including the human body as described in the link above. Infrasound is characterized by an ability to cover long distances and get around obstacles with little dissipation. There are many causes of infrasound in the 15Hz to 20Hz range usually associated with human perception anomalies, often inside buildings, but I have not read of a plausible source outdoors on the shores of Loch Ness. I now believe I may have identified one - the culverts or the drainage channels that run under the roads alongside the loch.
The A82 trunk road runs along the north-west shore and is classified as a single carriageway road and typically has two lanes each 3.65 metres wide, giving a total paved width of 7.3 metres. http://www.dft.gov.uk/ha/standards/dmrb/vol6/section1/td2705.pdf There are grass verges and / or walls on each side of the paved width so the culvert under the road has to extend beyond these to function. I would estimate that the length of the culverts is rarely less than 9 metres.
These culverts are essentially open ended cylinders and can resonate at a variety of frequencies, of which the fundamental "f1" is the lowest. The matter is described at http://en.wikipedia.org/wiki/Acoustic_resonance#Resonance_of_a_tube_of_air
Open cylindrical tubes resonate at the approximate frequencies:
where n is a positive integer (1, 2, 3...) representing the resonance node, L is the length of the tube and v is the speed of sound in air (which is approximately 331 m/s at 0 °C or 343 m/s at 20°C, both at sea level).
A more accurate equation considering an end correction is given below:
where d is the diameter of the resonance tube. This equation compensates for the fact that the exact point at which a sound wave is reflecting at an open end is not perfectly at the end section of the tube, but a small distance outside the tube.
So for a notional 9 metre culvert, 0.6 metres diameter, the resonant frequency will be 331 / 2x (9+0.2), = 18 Hz.
Culverts of slightly different lengths or diameter will have slightly different resonant frequencies and these will vary with temperature.
This is in the range of frequencies that affect humans. The culverts runs directly across the road and could be caused to resonate by the wind blowing at right-angles across their ends, or by other means.
A NASA http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA030476&Location=U2&doc=GetTRDoc.pdf study concluded that the resonant frequency of the human eyeball was also 18Hz.
I hope to measure some real culverts in the near future and to investigate their acoustic properties.
Version1.0, 12th September 2013