Leslie cabinet

A Leslie is the traditional amplification for Hammond organs. The cabinet was invented by Don Leslie (1911-2004) and marketed from 1940 in the face of considerable opposition from Laurens Hammond who was of the opinion that it compromised the sound of his own invention.

The Leslie cabinet, although often referred to as a "rotary speaker", usually contains two stationary speakers. The upper one is a "tweeter" fitted with an angled horn (and a non-functioning dummy horn pointing in the opposite direction) which can be rotated about its vertical axis, while the lower one is a midrange unit facing downwards into a rotating wooden drum fitted with a deflector. The horn and drum each have an electric motor to rotate them via pulleys and rubber belts.

The modulation produced by the rotating horn and drum is generally agreed to be a combination of amplitude modulation (tremolo), phase shift and pitch modulation (vibrato or chorus) caused by the Doppler effect.

Each motor can operate at two speeds, and the signature Hammond/Leslie sound is that of the speed being changed, an acceleration or deceleration lasting several seconds due to the inertia of the horn and drum. The two speeds relate to modulation frequencies of approximately 0.5 Hz and 7 Hz, and are labelled on Leslie switch units as "Chorale" and (inaccurately) "Tremolo".

Most clonewheel instruments have an onboard Leslie simulator; this may be analog or digital. Many afficionados insist however that no simulation can replace the true sound of a Leslie.