By Stephen HutcheonA New Zealand scientist believes he's captured a recording of the mystery hum that has been heard by scores of people living and in and around the city of Auckland.
Dr Tom Moir, a computer engineer at Massey University's Institute of Information and Mathematical Sciences, made the recording at a house in Auckland's North Shore suburb of Glenfield earlier this week.
Dr Moir and his colleague Dr Fakhrul Alam have dubbed the sound an unidentified acoustic phenomena.
Four people who previously reported hearing the low-level hum have confirmed that this is the sound they can hear in their homes.
"If this is indeed the hum, then it's acoustical and not electromagnetic," Dr Moir said.
Dr Moir previously pinpointed the low-level drone at a frequency of 56Hz, which is very close to the 50Hz frequency produced by the 240 volt AC main electricity supply delivered to homes in New Zealand (and Australia).
Although 56Hz is within the standard range of human hearing - which can range from 20 to 20,000Hz - it is too low for many people to pick up.
One of Dr Moir's students, Ms Nair Tsuji, who is able to hear the sound, has acted as Dr Moir's "ears". She also confirmed that the sound they heard in the Glenfield home was the same as the one she hears in her home in Whangaparaoa, about 30 minutes' drive north of Auckland.
All the 30-plus cases reported to Dr Moir are occurring in Auckland's north.
Dr Moir said the next step was to triangulate the sound in the hope of pinpointing the source.
He said that although there was a "high probability" that this was the sound, he's doubtful that he would ever be able to track its source.
According to a theory put forward by Professor Rod Cross, at Sydney University's department of Physics, the sound could be the humming of sand dunes, as described in the latest issue of Physics World, a monthly academic journal.
Professor Cross said he hadn't yet read the article, only heard about it. But it is available online here.
"The sound file on the web sounds like someone blowing over the top of an empty bottle," Professor Cross wrote in an email. "The New Zealand hum sound might therefore be due to wind blowing over hills and valleys. It may not actually require strong winds to cause the effect. Perhaps slowly moving air could do it."
Taking another informed guess, Professor Cross said the sound could be due to the motion under the earth. For example, hot gases or liquids rising through cavities could cause an organ pipe effect.
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