RFC, FAQ Section 2.0

[bpb@bpbsn.cca.rockwell.com (Brad Baker)]

All,

What follows is section 2.0 of the FAQ, as it currently exists. I
would like confirmation of some things:

In section 2.1.1, i make reference to the "Memphis sound". I got this
>from a VOCE blurb on their SPIN module and have never seen it anywhere
else. Can anyone confirm this?

In the last paragraph of section 2.1.1, i mention four particular
models of Leslie. Is it correct that the 122 and 142 are tall, 147 and
145 are short, 122 and 147 are balanced input, 142 and 145 are
un-balanced input?

In addition, please review the mic'ing techniques. I have little
experience in this area and got these recommendations from a request
to the rec.audio.pro newsgroup, only this week.

Regards,

Bradley Baker
bpb@mlb.cca.rockwell.com

______________________________________________________________________

(from the Table of Contents)

2.0     The Leslie Tone Cabinet
2.1     The Basic Configuration
2.1.1   The Treble Rotor
2.1.2   The Bass Rotor
2.1.3   Amplification, etc.
2.2     Model Specific Information
2.3     Player technique (modifications, celeb configs, etc)
2.4     Cabinet miking technique (live and studio!)

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2.0 The Leslie Speaker

The Leslie Rotating Speaker, named after its inventor, Don Leslie, 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 constant (or
variable) 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 three
effects are in operation.

Since the sound source is directional, 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.

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.


2.1 The Basic Configuration

The Leslie Loudspeakers company produced many configurations of this
speaker. Models came with reverberation, speakers in drums, two-piece
cabinets, tube and solid-state amplifiers, and more.  The most popular
Leslie Speaker is probably the model 122. The models 122, 142, 145,
and 147 all share a similar configuration.  A 40-Watt monophonic
amplifier drives two transducers, a 15 inch woofer and a 3/4" throat
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. In general, two speeds are
available, fast and slow.

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 142
are 41"H, 20.5"D, 29"W. The 147/145 are 8" shorter. The 122 and 147
have a balanced amplifier input while the 142 and 145 have an
un-balanced input. The taller cabinet is alleged to have a better bass
response.

2.1.1 The Treble Rotor

The treble rotor is primarily responsible for the Leslie's sound
characteristic. Some organists think that the slower acceleration of
the lower drum detracts from the sound and disconncect power to the
motors driving the drum. This is the so-called "Memphis" sound.

The compression driver fires into a vertical tube 3" long that acts as
a thrust bearing for the horn, a twin-bell, conical horn 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 and
replacement horns can be purchased without the diffuser.


2.1.2 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 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.


2.1.3 Amplification, etc.

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 6550s 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.


2.2 Model Specific Information

    (Models, productions years, pinouts,
    (Leslie Combo Preamp)
    (Hammond Adaptor Kits)

2.3 Player configurations and Modifications

    (What K.Emerson used for a setup)
    (Driver/amplifier mods)

2.4 Leslie Cabinet Mic'ing Technique

The following methods have been recommended:

2.4.1 Use two microphones, a U-87 for the top rotor, and a FET 47 on
the lower rotor. Place the 87 on the opposite side from the 47. If you
love the sound of the motor, and the extra presence of the keys
clicking, the closer you get the better.

2.4.2 Use two 87s (same side of the closed cabinet), or one M49 (in
the center of the open side of the cabinet, far away to add room...
use this method in omni) but make sure it is monophonic.

2.4.3 (live technique) Use two MD421s on the upper rotor and 1 RE20 or
M88 on the lower rotor. Pan the two 421s to L & R. Add the low mic
panned mid way.

To reduce wind noise while close mic'ing wrap the microphones in 6" of
open-cell foam.


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