Sometimes it is not only misperception that can make a place feel weird or sinister; physical environmental factors can play a part. One of these is ‘infrasound’: sonic vibrations below the level of hearing, which nevertheless exert an effect on the body.

In 1998, lecturer Vic Tandy, working in a reputedly haunted laboratory, experienced cold sweats, depression, a feeling of presence, and the materialisation of a grey apparition. He was able to trace these effects to a standing sonic wave of 19 Hz, produced by a nearby extractor fan.

Tandy went on to demonstrate the presence of infrasound at other ‘haunted’ locations, including Warwick Castle.

However, more recent studies of infrasound have failed to re-create Tandy’s findings. No other researcher has yet replicated Tandy’s ‘grey apparition’ or been able to confirm that there is anything special about the 19 Hz frequency. Some have pointed out problems in Tandy’s calculations concerning his standing wave.

Although it seems possible that infrasound may give rise to psycho-physical effects similar to those reported in ‘hauntings’, the conditions required and the precise nature of those effects have not yet been confirmed.

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