The sound is heard at the beginning of the show then again whenever a major scene change occurs, accompanied by a black title card with white text describing the new scene.
I wondered who was the genius who created such a memorable, iconic sound.
One source said the sound was created to be an audible symbol of a jail cell door slamming shut.
IMDb.com posted that the sound, created by Mike Post, was made "by combining close to a dozen sounds, including that of a group of monks stamping on a floor."
Mike Post is an American multi-Grammy and Emmy-winning composer. He's best known for his TV theme songs for series such as Law & Order, Law & Order: SVU, The A-Team, NYPD Blue, Renegade, The Rockford Files, L.A. Law, Quantum Leap, Magnum, P.I., and Hill Street Blues.
Although I read various articles, I couldn't find a specific statement from Mike Post, who made the sound, or from Dick Wolf, the show runner who went to the well-known Mr. Post, about exactly how the "Donk Donk" sound was made.
However, there were comments on many of the articles from sound engineers who offered their own explanations of what was used to make the sound.
In the meantime, enjoy the sound of the "Donk Donk" in this very short video.
Music brands TV shows into our subconscious.