Helicopters, infrasound and the fear frequency

While the sound of a helicopter may have many rushing to  their windows to  identify the make of the aircraft,  the majority of  people  feel an intense sense of annoyance, or more  accurately, disruption or anxiety. 
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So, what is  it about helicopters that  is so intrusive, placing  people  and animals outside their comfort zone?​​

The answer is    Infrasound    otherwise known as  the 'fear frequency'.

It's all explained   in the following article   'The Fear Frequency'  by Mark  Pilkington ( The Guardian 6 Oct 2003) 

​​  "Have you ever wondered what a ghost sounds like?"  Engineer Vic Tandy may already know. ​"The key here is frequency: 19hz is in the range known as infrasound, below the range of human hearing, which begins at 20hz. Engineer Vic Tandy  learned that low frequencies in this region can affect humans and animals in several ways, causing discomfort, dizziness, blurred vision (by vibrating your eyeballs), hyperventilation and fear, possibly leading to panic attacks".​

 In the early 1980s, Tandy was working in a laboratory designing medical equipment. Word began to spread among the staff that the labs might be haunted, something Tandy put down to the constant wheeze of life-support machines operating in the building.

One evening he was working on his own in the lab when he began to feel distinctly uncomfortable, breaking into a cold sweat as the hairs on the back of his neck stood on end. He was convinced that he was being watched. Then, out of the corner of his eye, Tandy noticed an ominous grey shape drifting slowly into view, but when he turned around to face it, it was gone. Terrified, he went straight home.

The next day Tandy, a keen fencer, noticed that a foil blade clamped in a vice was vibrating up and down very fast. He found that the vibrations were caused by a standing sound wave that was bouncing between the end walls of the laboratory and reached a peak of intensity in the centre of the room. He calculated that the frequency of the standing wave was about 19hz (cycles per second) and soon discovered that it was produced by a newly installed extractor fan. When the fan was turned off, the sound wave disappeared.

"Can affect humans and animals in several ways, causing discomfort, dizziness, blurred vision (by vibrating your eyeballs), hyperventilation
​and fear, possibly leading to panic attacks

A more recent investigation took place in an allegedly haunted 14th-century pub cellar in Coventry, where people have reported terrifying experiences for many years, including seeing a spectral grey lady. Here Tandy also uncovered a 19hz standing wave, adding further evidential weight to his theory.
In an interesting parallel, researchers have recorded that, prior to an attack, a tiger's roar contains frequencies of about 18hz, which might disorientate and paralyse their intended victim. Is this the sound of fear itself?"

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We have uploaded  some articles of interest  on the subject here.  You can  listen to  the sound  of 20 Hertz,  the theoretical minimum frequency a human can hear.

An article     in Vertical magazine 'The science behind helicopter noise — and how the industry is working to reduce it '     identifies  "... two locales where helicopter noise seems to be a major issue. In urban areas, you have a multitude of helicopter operations: law enforcement, medical transport, local news, some business transportation, and tourism. And then there’s the more remote, scenic tourist destinations, where those on the ground object to aircraft interrupting their experience in an otherwise serene wilderness. Typically, helicopter tours are the main source of annoyance in these places.

The  majority of  this  information can be logically applied to  what is anticipated from  heliport activities on 2224 Tully  Mission Beach Road. The  Kestral Aviation  information sheet   states  " We will be predominately using our Bell 206L3 Long Ranger and Sikorsky S-76 helicopters, and on rare occasions, our Bell Medium (212 or 412) fleet when need requires (such as in response to natural disasters).  Vertical magazine informs   "...The  Bell 212 generates high levels of impulsive noise..."

"To reduce the number of noise complaints, HAI’s Fly Neighborly Guide recommends flying higher and further away from populated areas when possible".

The article  Dynamics of rock arches  shows  a plot of the sound spectrum generated by a two-blade Bell 206 helicopter (below).  In it you see the first frequency peak at 13 Hz with a series of overtones at integer multiples

​The sound spectrum generated by a Bell 206

​Thank you to Margaret Moorhouse  from ASH for sharing this information.