The Burning - S9-E16
Continuity mistake: At the beginning of the episode, when Puddy is farewelling Elaine on the street, he is standing on the sidewalk and leaning through the driver's window. The following shot when she pulls out quickly, you can see through the windows of the car that Puddy is nowhere to be seen.
Answer: Composer Jonathan Wolff used a synthesizer, although in seasons 7-9, a real bass is used in addition. Wolff also recorded himself making hundreds of mouth noises, pops, and slaps to add to the synthesized bass licks so that each episode has a different theme. The only real "back-story" is Jerry Seinfeld was having trouble coming up with a theme song and talked to a friend who happened to know Wolff. They wanted to avoid that cheesy late 80's sit-com theme song and Wolff came up with what we enjoy now. Jonathan Wolff has also talked about this further in interviews, recently Reed Dunela interviewed him, so for a fuller account of his story; check out "The Wolff of 116th street".
Bishop73