The rotary effect that my teacher told me about in 20241009163931 was originally introduced by Don Leslie, who invented a cabinet with rotating speakers in an attempt to replicate the sound of a pipe organ.
[Don Leslie] was inspired by the experience of listening to a pipe organ. In a pipe organ, the pipes themselves are spread out across a fairly wide range. The base of a bank of pipes can measure up to 12 feet or more across. Therefore, the sources of notes you hear are spread out, and notes emanate from different spots across the stereo field. 1
The Leslie cabinet contained two parts: a rotating deflective drum for low frequencies and a dual horn assembly for the high frequencies.
The most popular Leslie speaker cabinet contains a high-frequency horn driver and a bass woofer, both of which are combined with rotating components: a dual horn assembly (one horn is open, the other acoustically plugged and present only as a counterbalance) for the highs, and a deflective drum for the lows. The rotary components can rotate at high and low speeds, which adjustable ramp-up and -down times. A full-stop braking option lets you stop the rotors completely, which is necessary if you want to tune other instruments such as a guitar to the organ. 1
The Leslie cabinet gets its sound by recreating the Doppler effect. This is because the sound emitted by the rotating sound emitters reach your ears are different times, creating the illusion of a sound repeatedly going to and away from your ears.
Thanks to reflective surfaces in the performance space, as well as the Doppler effect that occurs when a noise-emitting source moves toward, past, and then away from your ears, the Leslie speaker creates a unique effect that greatly enhances the organ sound, especially when the rotation speed is switched at critical points in the performance. 1
The Leslie cabinet was often attached to an electrical organ, such as the Hammond B-3, to make it sound different.
The Hammond B-3 was one of dozens of electric organs manufactured by the Hammond Organ Company from the mid-1930s to the 1970s. […] Since organ tones tend to be rather static, the Hammond engineers developed ways of modulating the sound to make it more interesting. Vibrato and chorus effects where implemented on the B-3, but the best complement to the Hammond sound came from outside the company in the form of a cabinet with rotating speaker components designed by Don Leslie.
You could even attach multiple Leslie cabinets to a single organ:
Connected to a Leslie cabinet or two — or in the case of someone like rock organ god Keith Emerson, a stacked bank of more than a dozen — your Hammond will take on a brilliant new personality. 1
The Leslie cabinet is also called a Leslie speaker.
I tried to get Mom to buy a Leslie speaker because I thought the sound of it was so cool. […] There’s no denying the fact that the B-3 and Leslie speaker were a magical combination. 1