Rotary Speaker

Rotary Speaker Origins

The rotary speaker is also known as the Leslie speaker due to its inventor, Donald Leslie. He created and manufactured the Leslie speaker, which refined the sound of the Hammond organ and popularized electronic music. Donald Lesslie was impressed by the sound of the Hammond organ. However, he was unsatisfied with how the instrument resonated when taken from the church hall to some other confined space. Later 1937, he invented a special speaker that rotated inside its cabinet. This is because he wanted to improve the sound of the Hammond organ.

Doppler Effect

This system produced a Doppler effect, modulating the sound and imitating the organ's resonance in an ample hall space by projecting it in 360 degrees. Lesslie's invention was primarily used for gospel and church organs, creating a Theatre Organ Tremulant effect. The final version of the Leslie speaker is the Rotosonic drum, wherein a loudspeaker is physically mounted in the spinning rotor with a narrow aperture (opening) to produce a very authentic Theatre Organ tremulant sound. It was also used in psychedelic and rock music of the 1960s and 1970s. Furthermore, it has since made its way into many genres of music, including pop and jazz. It wasn't until the 1980s that Hammond bought Leslie's product to include with their organs.

Organ Instruments Reproduction

As mentioned above, the rotary speaker was first used for the electronic reproduction of organ instruments. The Doppler effect alters the sound in the listener's ear, the directional characteristics of the speaker, and the phase effects due to air turbulence. The Doppler effect raises and lowers the pitch according to the rotation speed. The directional characteristics of the opposite horn arrangements perform an intensity variation in the listener's ear. Both the pitch modification and the intensity variation are performed by speaker A and in the opposite direction by speaker B.  

Loudspeaker Effect Simulation

Modulation and delay line modulation can be used for a rotary loudspeaker effect simulation. The simulation uses the modulated delay line for pitch and amplitude modulation for intensity modifications. Using two delay lines simulates the Doppler effect of two opposite horns. They are modulated with 180-degree phase-shifted signals in a vibrato configuration. A directional sound characteristic similar to rotating speakers can be achieved by amplitude-modulating the output signal of the delay lines. The modulation is synchronous to the delay modulation in a manner that the back-moving horn has a lower pitch and decreasing amplitude. At the return point, the pitch is unaltered, and the amplitude is minimum. The movement in a direction to the listener causes a raised pitch and increasing amplitude. A stereo rotary speaker is perceived due to an unequal mixing of the two delay lines to the left and the right channel output. This effect makes the sounds more lively by imparting amplitude, pitch modulations, and some spatialization. At the lower rotation speeds, it is reminiscent of the echoes in a cathedral, whereas at the higher rotation speeds, it gets a ring-modulation flavor. This effect can also be interpreted as a rotating microphone between two loudspeakers.

Additional Resources & Source Texts

• Hearing Effects: Modulation (Rotary Speaker, Chorus, Flanger, Phaser)
• Donald Leslie