studio (except for the opening  credits). The clouds that you see out the window are made out of  fiberglass. For the effect of a police siren coming towards the  apartment building at the end, Alfred Hitchcock (I) (qv) had an  ambulance come at full speed, from several blocks away, straight to  the Warner Brothers studio, siren blaring all the way. The sounds  were picked up by a microphone suspended from the studio gate.