This is the most successful sound in television history. Let’s consider the art form it represents, and why this is the finest example.

It is, of course, the signature bleat from “Law & Order,” something I surely did not need to tell you, which by itself is a measure of its greatness. Not a lot of shows employ this sort of aural calling card, perhaps because coming up with the perfect sound is not easy. Imagine invoking an entire television series and its themes with an audio flourish that lasts no more than a few seconds. Not a theme song — that’s a different matter, and far easier to create — just a fleeting tone, honk, clang or other noise.

The Hall of Fame for such sounds would include the eerie four-tone introduction to “The Twilight Zone” — distinctive and evocative, setting the stage for weird, supernatural goings-on. And the ticking timepiece of “60 Minutes” — an urgent, attention-must-be-paid sound perfect for a newsmagazine. And the shutter-click of “NCIS,” with its suggestion of “pause and examine closely” — the show’s dominant law-enforcement tool.

Towering over them all, though, is the “Law & Order” dun-dun. Or chung-chung. Or bah-bonk. Or DA-doink. Or however you want to describe it; everyone who tries seems to do it differently.

You’ll know it if you hear it, which is the very point: This two-beat metallic sort of thunk is instantly recognizable all over the world, so much so that it has become an object of parody.

What makes it so right? Well, it helps that the show that gave birth to it is one of the most successful series in TV history, rerun and syndicated and spun off to the point that the sound has been inescapable for 27 years.

But mere ubiquity doesn’t crown you king of the TV mnemonics; you have to make a statement, and the right statement. Or, in the case of this particular noise, several statements. It’s the sound of a jail door shutting and locking. Of a judge double-pumping to gavel a courtroom to order. Of all the apprehensions and tensions of an urban night condensed into the length of a heartbeat. Of a scripted TV show with the gravity and aspirations of a well-made documentary. (Mike Post, who is credited with creating the dun-dun, has said it’s actually a composite of various sounds, including a bunch of men stomping their feet.)