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Dr Colvin has analysed recordings of alleged poltergeist knocking obtained from around the world over a 40-year period. The earliest was a recording made by a local physician at Sauchie (Scotland) in 1960 and the most recent was obtained from a poltergeist case at Euston Square, London in 2000. Whilst the two types of rap sound rather similar, they are actually acoustically different, although the effect is only made apparent when the recordings of raps are submitted to detailed analysis.