In 1937, Don Leslie invented tje Leslie speaker. This rotating speaker is used in combination with the Hammond organ, and refines its sound. The effect of the speaker is based on a principle called the “Doppler Effect”. It’s why a train whistle seems to go down in pitch when a train passes you. Don Leslie took a horn speaker and made it go around in a circle: a legend was born. The spinning horn gave the Hammond Organ what is called in the pipe organ world a tremulant. Vibrato, Tremolo, and a little “whoosh” of air all put together. It made the Hammond Organ sound completely different.