Leslie

The Leslie Tone Cabinet: ``Pipe Voice of the Electric Organ''

_This writeup was exerpted from the OriginalHammondLeslieFaq_

The Leslie Rotating Speaker was named after its inventor, DonLeslie (Here are some patent drawings of DonLeslie's invention. They have been called "incredibly entertaining documents" by some). First models began to appear around 1940. It is designed as a sound modification device, not a hi-fi speaker. The pairing of the Leslie Speaker with another device, usually a Hammond Organ, constitutes a musical instrument. It operates on a simple principle: a directional sound source rotates at a constant speed around a fixed pivot point. The effect at the listening location, some distance removed, is quite pronounced. The characterization of a Leslie Speaker in an acoustically reflective listening area is a complicated proposition at best but at least four effects are in operation: amplitude modulation, frequency modulation, timbre shift, and apparent motion of the sound.

The sound source is directional. Because of that, the intensity of the sound to the listener (or microphone) is dependent upon, at least, the angular position of the rotating sound source. The intensity varies as the sound source rotates and the listener perceives a periodic modulation of the sound as a function of the rotational speed. This is the amplitude modulation (AM) component of the sound and when the listening position or microphone is placed closer to the sound source it will, in general, increase the AM component of the sound.

The sound source when rotating is periodically accelerating toward and decelerating away from the position of the listener. This imparts a doppler shift on the source material and thus a frequency modulation (FM) to the sound. As in other doppler induced pitch shifts the pitch is perceived to rise as the source moves toward the listener and fall when the source moves away from the listener.

The directional pattern of the rotating component is frequency dependent. High frequencies exhibit more beaming than do lower frequencies, which are emanated in a more omni-directional pattern. A shift in timbre is perceived as the angular position of the sound source changes. The treble component is generally strongest when the rotating component is pointed at the listener and weakest when it is pointed away.

Finally, due to the multiple reflections of the listening area and the rotating sound source, the sound appears to emanate from multiple locations imparting a sense of motion to the sound.

The Basic Configuration

The Leslie Loudspeakers company produced many configurations of this speaker. Models came with reverberation, two-piece cabinets, tube and solid-state amplifiers, and more. The most popular Leslie Speaker is probably the Leslie 122. The models 122, 142, 145, and 147 all share a similar configuration. A 40-Watt monophonic amplifier drives two transducers, a 15" woofer and a 3/4" throat diameter Jensen compression driver, through a 16 ohm, 800 Hz passive crossover. The stationary compression driver fires upward into a rotating horn assembly and the stationary woofer fires downward into a rotating drum-like reflector. The rotating assemblies are mechanically belt driven by AC induction motors. Usually, two speeds are available, fast and slow. Models before 1963 were single-speed only, and did not have the "1" in the model number. Also, many older models used a "brake" circuit to bring the rotors to a stop quickly. Today's Leslie cabinets made by Hammond/Suzuki use a single motor with a motor controller card to determine fast, slow or off. ?ScottHampton (Hamptone) and BobSchleicher (tonewheel.com) manufacture aftermarket SolidStateRelays that eliminate relay click and, in the case of the Schleicher relay, allow three-speed operation from the console.

The usual cabinet has three compartments. The upper compartment houses the rotating treble horn assembly. The middle compartment behaves as a vented enclosure for the woofer, contains the crossover, both drivers, and motors for both rotating assemblies. The lower compartment houses the amplifier and the rotating drum. Louvres are located on the three finished sides for upper and lower compartments.

Two basic sizes of this configuration can be found. The 122 and 147 are 41"H, 20.5"D, 29"W, while the 142 and 145 are 8" shorter. The 122 and and 142 have a balanced amplifier input while the 147 and 145 have an un-balanced input. The taller cabinet is said to have a better bass response.

The Treble Rotor

The treble rotor is primarily responsible for the Leslie's sound characteristic. Some organists think the slower acceleration of the lower drum detracts from the sound and disconnect power to the motors driving the drum.

The compression driver fires into a vertical tube that acts as a thrust bearing for the horn, a twin-bell, conical device molded of black Bakelite. The horn starts vertically and flares horizontally. It is belt-driven by a two-speed, AC induction motor, (actually two motors; one for slow, one for fast). Three drive pulley diameters are provided to vary the rotational speed and an idler pulley is used to maintain belt tension. The treble horn, while appearing to be bi-conical, actually has only one operating side. The other side is plugged and exists to provide dynamic balancing to reduce bearing loads and prevent wobble during operation.

A conical diffuser is located at the mouth of the horn. The diffuser plays a large role in defining the sound of the treble horn assembly; the dispersion pattern of the horn is changed from a single, highly directive lobe, to a more omni-directional, multi-lobed pattern. This complicates the doppler pattern and with internal reflections of the cabinet considered, provides a more characteristic sound.

In addition to changing the dispersal pattern, the diffuser performs another function. With the diffuser absent, the distance of the apparent sound source from the rotation center varies inversely with frequency. That is, as the frequency goes up, the emanation point of the sound appears to travel back down the horn toward the throat. The effect of this is that the Doppler shift becomes less as the frequency rises and thus there is less FM effect. With the diffuser in, the emanation point for all frequencies is much closer to the same rotational radius. A trade-off can be made between a higher FM component with the diffuser in, and a higher AM component (especially at high frequencies due to the single-lobed beaming), with the diffuser removed. Removing the diffuser is a common modification. Replacement horns can be purchased both with and without the diffuser.

The Bass Rotor

The lower compartment contains a rotating wooden drum beneath the downward-firing woofer. The drum has an open top, straight sides, and a scoop that starts vertically at the top and rear of the drum and ends up horizontal at the bottom and front of the drum. A shaft runs vertically through the drum's rotational axis. The shaft is supported by a lower bearing beneath the drum that is mounted in the bottom of the cabinet. The upper bearing is mounted in a cross member that is held in place by the secured 15" woofer. The pulley is mounted at the upper end of the shaft between the drum and the woofer.

The primary effect of the bass rotor is to impart AM to the signal. There is very little phase shift of frequencies below 200 Hz due to their wavelength, though some phase shift may occur up around the crossover point of 800 Hz. The result is a low-frequency pulsation or throb that is very effective when used at the slow or chorale speed.

Amplification

The typical unit consists of a 40-Watt monophonic tube amplifier driving the above described components through a 12dB/octave, 800 Hz, 16 ohm crossover. The amplifier uses a pair of ^[6550]s as final amplifiers. The motors that drive each rotor actually consist of a pair of motors, thus four motors exist, each with a pair of wires that plug into the amplifier chassis.

Many believe the classic Hammond/Leslie "sound" depends on keeping things as "stock" as possible, including 16 ohm speakers, drivers and 12db/octave crossovers. The "road" series of Leslies, including the 900 and 800 series, used solid-state amps driving 8, 4 and sometimes 2 ohm configurations.

References

Articles

  * UnearthingTheMysteriesOfTheLeslieCabinet, by Clifford A. Henricksen, April 1981.

Other pages about Leslies

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