The THX ‘deep note’ sound that plays under the title card was computer generated by a man named Andy Moorer:

I was working in what was then called the “Lucasfilm Computer Division” that existed from roughly 1980 to 1987 or so. It spawned several companies, including Pixar and Sonic Solutions. I was head of the audio group. In about 1982, we built a large-scale audio processor. This was in the days before DSP chips, so it was quite a massive thing. We called it the ASP (Audio Signal Processor).

Music thing: TINY MUSIC MAKERS: Pt 3: The THX Sound