At 8.4 seconds, the full clip gives the knocking pattern room to land, though you can trim it to the first knocks for alerts. Download the MP3 once to keep it available offline. For a ringtone or notification tone, select the saved file in your phone’s sound settings; menu names vary by device. Keep playback below dialogue until the knock begins, since high volume can mask speech. For suspense, follow it with an evil laugh after a short pause.