HammondWiki - Rotosonic Drum

The first Leslies achieved swirliness by using a fixed driver firing into a rotating horn. Most of these leslies have the distinctive dual-horn shaped horn (of which one was a fake, there only for balance) on the treble driver, and a wooden (later styrofoam) drum shaped horn (called the "rotor") on the bass driver.

Rotosonic Leslies, on the other hand, spin the drivers directly. One (and in rare models two, there may even be one with three) car-audio type 6"x9" speakers are mounted in a spinning drum. To surmount the problem of how to make good, low-impedance, electrical connections to the spinning speakers, DonLeslie invented the Mercotac slip-ring (which I'm guessing made him quite a bit richer than Leslie speakers ever did.)

Most Hammond players do not consider the Rotosonic Leslies to be to their liking, and many Hammond players, especially in jazz and rock music, roundly condemn the units. However, other organists disagree, finding the Rotosonic a good fit, often for those who play music in a theatre-organ style. The only sure way to decide whether or not a Leslie with a Rotosonic drum is worth adding to one's array of tone equipment is for the organist to play through one and see if the sound and "feel" of the Rotosonic suits their taste and preferred style of organ music. The Rotosonic tremulant is a bit more pronounced than that of the standard wood or foam rotor with a more distinct "pulse" as the drum revolves. Moreover the increased spinning mass takes longer to reach tremolo speed or to spin down to chorale. However, in a standard single-driver Rotosonic drum the longer ramp times can be kept somewhat in check by correctly adjusting the belt tension and ensuring that the belt is not worn or becoming slippery.