Take a deep breath. Try to relax. 

Your house might be haunted, but more likely you are sensing a very low frequency sound that is inaudible to humans.

British scientists have shown in a controlled experiment that the extreme bass sound known as infrasound produces a range of bizarre effects in people including anxiety, extreme sorrow and chills - supporting popular suggestions of a link between infrasound and strange sensations.

"Normally you can't hear it," said Dr Richard Lord, an acoustic scientist at the National Physical Laboratory in England who worked on the project, on Monday.

Lord and his colleagues, who produced infrasound with a seven metre pipe and tested its impact on 750 people at a concert, said infrasound is also generated by natural phenomena.

Feel the bass

"Some scientists have suggested this level of sound may be present at some allegedly haunted sites and so cause people to have odd sensations that they attribute to a ghost - our findings support these ideas," said Professor Richard Wiseman, a psychologist at the University of Hertfordshire in southern England.

In the first controlled experiment of infrasound, Lord and Wiseman played four contemporary pieces of live music, including some laced with infrasound, at a London concert hall and asked the audience to describe their reactions to the music.

Infrasound is produced by storms, seasonal winds and weather patterns and some types of earthquakes.

The audience did not know which pieces included infrasound but 22% reported more unusual experiences when it was present in the music.

Their unusual experiences included feeling uneasy or sorrowful, getting chills down the spine, or nervous feelings of revulsion or fear.

Trunk calls

"These results suggest that low frequency sound can cause people to have unusual experiences even though they cannot consciously detect infrasound," said Wiseman, who presented his findings to the British Association science conference.

Infrasound is also produced by storms, seasonal winds and weather patterns and some types of earthquakes. Animals such as elephants also use infrasound to communicate over long distances, or as weapons to repel foes.

"So much has been said about infrasound - it's been associated with just about everything from beam weapons to bad driving. It's wonderful to be able to examine the evidence," said Sarah Angliss, a composer and engineer who worked on the project.