Rotary speaker origins


The rotary speaker is also known as Leslie speaker, due to his inventor Donald Leslie. He created and manufactured the Leslie speaker which refined the sound of Hammond organ and popularized electronic music. Donald Lesslie was impressed by the sound of Hammond organ. However, he was not satisfied with how the instrument was resonating when taken out of the church hall to some other confined space. Later, in 1937 he invented a special speaker which rotated inside of its cabinet. This is because he wanted to improve the sound of the Hammond organ.


Doppler effect


This system produced a Doppler effect which modulated the sound and by this imitated the resonance of the organ in a large 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 music and jazz. It wasn’t until the 1980s that Hammond bought Leslie’s product to include with their organs.


Organ instruments reproduction


As said above the rotary speaker was first used for the electronic reproduction of organ instruments. The sound in the listener’s ear is altered by the Doppler effect, 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 the speaker A and in the opposite direction by the speaker B.


Loudspeaker effect simulation


A combination of modulation and the delay line modulation can be used for a rotary loudspeaker effect simulation. The simulation makes use of the modulated delay line for pitch modifications and amplitude modulation for intensity modifications. The simulation of the Doppler effect of two opposite horns is done by the use of two delay lines. They are modulated with 180 degrees phase shifted signals in 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.

By imparting amplitude and the pitch modulations as well as some spatialization, this effect makes the sounds more lively. 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 flavour. This effect can also be interpreted as a rotating microphone between two loudspeakers.


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