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What causes a sonic boom?

13 April 2012 Last updated at 11:19 BST

The MoD has said a loud bang heard across much of England on Thursday night was a sonic boom caused by one of two Typhoon aircraft responding to an emergency call.

The jets were scrambled and authorised to go supersonic after a small civilian helicopter had emitted an emergency signal.

Dr Jim Wild from Lancaster University explained to the BBC how and why a sonic boom happens.