The Hammond organ owes much of it's sound to a speaker system called a Leslie. Named after it's ingenious inventor, Don Leslie. While working on 3-D movies, he wanted a way to make the soundtrack more three dimensional. Hammond had developed a rotating drum speaker, but Leslie really developed and perfected it.
Leslie did this by splitting the audio input in two. Sending the high frequencies to a tweeter speaker and the low frequencies to the woofer speaker. Both speakers in turn blast into a rotating horn. These two horns are hooked to two-speed electric motors that rotate the horns in opposite directions. The sound is sprayed around a 360 degree circle giving the organ a tremolo effect. There is a slow speed and a fast speed with a switch on the organ that the player can use to make the speed change.
This experiment in 3-D sound failed but theater organ players tapped into the Leslies and a new sound was born. It is a marriage made in heaven.
With the combination of the Hammond tone generator, drawbars, tremolo effect and doppler effect (the reason a sound from an object is higher pitched as it travels toward you, and lower pitched when it's traveling away from you), one of the most unique musical instruments man has contrived comes to life.
Keep and ear out for the Hammond.
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