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.

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.