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The Hum is an enigmatic acoustic phenomenon first reported in Britain in the 1970’s and later in the USA in the early 1990’s. Hearers of the Hum usually report hearing a noise like a distant idling engine and usually although not exclusively in the night when it is extremely quiet. Some also report the noise as being difficult to screen out even using ear plugs and therefore capable of causing sleep deprivation, general annoyance and anxiety. There would appear to more outbreaks of the Hum being reported throughout the World in recent times so its emergence would appear in some way to map infrastructure development.
The paper discusses the background to the phenomenon known as the Hum. Spectral analysis plots of the acoustic spectra with and without the author’s subjective experience of the Hum are presented. In most cases the Hum in Bangor is manifest as a result of infrasound and low frequency acoustic sound, occasionally exacerbated by a magnetic component. Very subtle variations in the spectra such as tonal pulsation of selected components and wind and traffic noise can make a vast difference to subjective Hum perception even when all or most of the tonal components needed are present most of the time. An approximate acoustic calibration using a signal from a central heating pump is included.
Anthropogenic infrasound from infrastructure is usually be expected to be generated at or close to ground level and is a possible candidate also allowing propagation into multiple transmission media, so would seem to be a more acceptable cause for infrasonic Hum, but does not readily explain anecdotal reports of the Hum being momentarily altered by the passage of passenger planes. Unless, that is, some of this infrasound is being propagated very large upward distances and is then re-reflected which has been shown to be the case (Mutschlecner and Whitaker 1990). This fact has recently been re-asserted by Krasnov et al (2005) and has been proven experimentally by Koshovvyy et al (2007). The propagation of infrasound to earth from a natural source, the aurora, is known to be affected by the jet stream (Johnson 1976). Indeed some are intentionally experimenting by projecting high power synthetic and monochromatic infrasound into the ionosphere (Rapoport et al 2003). The present frequency of such experiments and how they might affect Hummers is not readily known. Whatever the sound source injected, effectively several channels exist for propagating infrasound from the Hum. These are seismic (refs) and airborne. Of the airborne channels, one can exist in the night-time boundary layer (Waxler) and it is also known that a further two higher channels for atmospheric ducting of infrasound exist, one in the stratosphere and one in the thermosphere (Gibson and Drob). It may be that to produce the pulsating effect of the Hum propagation between the source and the hearer has to be through more than one of these channels, with associated phase delays between them.
Originally posted by CLPrime
reply to post by don rumsfeld
That possibility has been discussed. It seems more likely that it's an atmospheric condition that usually causes clouds, such as a relatively large temperature difference with altitude. It doesn't always happen with an overcast/cloudy sky. Most of the time, but not always.
Originally posted by StealthyKat
reply to post by CLPrime
Thanks CL! Yes, it does sound similar. The only thing is that what I heard started suddenly, lasted fro 30 to 45 minutes or so, and stopped abruptly, like a switch had been turned of (which BTW made me call airports and naval bases to see if they were testing engines etc....they said no) A flying aircraft wouldn't behave that way would it? Also, we did look at the whole sky searching for planes or jets, but didn't see any. If I ever hear it again though, I will be sure to include the horizon, because you never know. Spectacular, or mundane, I just want to know what it is