Science in search of the low rumble

Infrasound can be used to communicate over thousands of km

The study of low frequency sound is giving science new perspectives on the natural world as well as helping to develop novel weapons technologies. Julian Trick reports.

Sound waves so low in frequency that the human ear cannot detect them may be behind ghostly sightings and haunted buildings.

Human ears detect sounds in the frequency range of about 20 to 20,000 Hertz, or cycles per second. Anything below 20 Hz is defined as infrasound, which although not heard, is experienced in the form of pure vibrations.

This is not a new phenomenon; church organ builders have been using infrasonic pipes for over 250 years

Sarah Angliss

The discovery that infrasonic waves are present in a number of allegedly haunted houses in the UK has prompted scientists to investigate the effect of the waves on human emotions.

The audience at an experimental piano recital, where the music was laced with bursts of infrasound, were asked to record their mood at particular times during the concert.

Some people reported feelings of calmness and euphoria when the infrasonic pipes were switched on. However, others were unable to detect any change in mood.

Starting note

Dr Richard Wiseman, of Hertfordshire University, UK, believes it is the vibrations of low frequency acoustic waves, felt particularly in the stomach, which may cause the recipient to feel uplifted or create unease depending on the environment in which they are experienced.

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"This is not a new phenomenon; church organ builders have been using infrasonic pipes for over 250 years to create a sense of awe in congregations," says Sarah Angliss, an engineer and composer involved in the music project.

The animal kingdom uses infrasound, too.

"The discovery that elephants use low frequency acoustics to communicate over large distances may explain the ability of separated family groups to co-ordinate their pattern of movement for weeks at a time, and the ability of lone males to find a mate in the wildernesses of the African Savannah," says Dr Katy Payne, of Cornell University, New York, US.

Dr Payne has studied elephant communication for 15 years.

She realised that vibrations in the air near an elephant enclosure at a local zoo reminded her of standing near infrasonic church pipes when she sang in a choir as a girl.

Deep burst

Further investigation revealed that elephant calls range from 5 Hz (infrasonic range) to 50 Hz (audible range) and in the right conditions can travel many kilometres.

Blue whales are also known to use infrasonic waves to communicate.

Low frequency sound waves travel huge distances in water, allowing the whales keep in touch over hundreds, if not thousands, of kilometres.

However, the underwater hotline is now under threat from noise pollution created by human sources such as ships, submarines and drilling operations, which create large amounts of infrasound in the depths of the ocean.

Some whales are also thought to use huge bursts of infrasound to stun the mysterious giant squid, which have never been seen alive in their natural habitat, before devouring them.

Sound shot

A more sinister use of infrasonic waves is in the application of non-lethal weaponry.

Elephants are a focus for research

Since the 1950s, scientists have been developing infrasonic devices which produce acoustic waves in the 1 to 10 Hz range.

Sound waves at these frequencies were found to vibrate internal organs, incapacitating opponents for hours, even days and sometimes leading to fatalities.

The main difficulty in developing these types of weapon is the huge amplitudes required to make them effective, and then directing the waves to a specific point.

Early warning

Non-military applications for infrasonic weaponry would include crowd control and personal protection.

The huge destructive power of infrasound is apparent in nuclear explosions.

Waves of infrasound can be detected 2,500-3,500 km away from a blast in under two hours using a monitoring network set up under the Comprehensive Nuclear Test Ban Treaty.

The same network is used to monitor geophysical processes, such as earthquakes, volcanoes, avalanches and severe weather, which also produce the low frequency sound waves.

Monitoring of such activity could give rise to advanced warning of catastrophic events saving thousands of lives across the world.