From the freewheeling drum and sax rhythms of L.A. Law to the poignant piano ballad of Hill Street Blues, Mike Post's potent TV theme music is well known for its ability to evoke the mood of a high-profile cop show. But on Law & Order, it's not the tune that sets the mood and affects the emotions it's the ominous chung CHUNG sound accompanying the scene changes that chills the blood. Some viewers, in fact, have been known to tune in simply because they want to see those documentary-style scene-ID slides come up (''Apartment of Leonard and Ellen Gorham, 199 East 71st Street, Thursday, Jan. 7'') and get an earful of that resonant, eerie gonging.
''I think of it as the stylized sound of a jail cell locking,'' says the 48-year-old Emmy-winning composer, who also wrote the theme music for Crime & Punishment. ''I wanted to add something that's very distinctive but not a literal sound. What I tried to do was jar a little bit.'' Instead of the electric piano, guitar, and clarinet for which he scored the opening theme, Post synthesized his chung CHUNG electronically, combining six or seven different sounds to get the right dead-bolt effect. One of the eeriest adds: the sound of 500 Japanese men stamping their feet on a wooden floor. ''It was a sort of monstrous Kabuki event,'' he says. ''Probably one of those large dance classes they hold. They did this whole big stamp. Somebody went out and sampled that.''
The result which lasts all of maybe a second and a half does its dark work effectively. ''There's very little music in Law & Order, and very little is needed, '' says Post. ''It's odd, to be honest, when you've written a theme that you think is very musical and what everybody wants to talk about is The Clang.''