AN-9: Rotary Speaker Modeling Plug-In
by Ross Penniman
This App Note describes a Leslie Rotary Speaker Modeling Plug-In that models both the rotating HF horn and rotating LF bass scoop. Here is an excerpt: “The pick-up of sound is assumed to be two virtual microphones placed at the corners of a square which inscribes the path of the rotating horn (as shown in Fig. 2). These microphones are assumed to be omnidirectional with flat frequency response and are placed in anechoic surroundings (not very realistic, but it saves quite a bit of additional DSP).
1) A circular delay buffer is used to simulate the propagation time from the mouth of the horn to the left and right microphones. As the delay time shortens and lengthens, the Doppler effect will happen automatically. Only one delay line is needed since there is only one source. The left and right signals each have their own read index which moves independently through the buffer.
2) The relative distance between the horn and microphones is used as the basis for the amplitude (tremolo) effect. This will be applied as a simple modulated gain.
3) A second order Butterworth low-pass filter with changing cutoff frequency is used to simulate the tonal modulation effect.”
This is the first App Note that Will Pirkle did not write. Thanks to my colleague and former grad student Ross Penniman for submitting it! NOTE: because it uses geometry for several calculations, you can implement impossible (or at least very impractical) Leslie simualtions, for example the pair of horns could be 20 feet apart, or the bass scoop rotational rate could be ridiculous, like 10kHz…