Art Engineering of the Leslie rotary Speakers

Stand Out Art Engineering

Introduction

The Leslie rotary speaker is more than a piece of audio equipment; it’s a piece of engineering art that set new iconic musical expression. Born in the 1940s, its swirling, Doppler-infused sound became a signature for many artists and producers. Let’s dive.

Man & Creation

Don Leslie was a young man working as radio service engineer at a department store in LA that fixed and sold parts for the newly released Hammond Organs. He got very excited to acquire one. And soon when he did, he was disappointed by the sound that didn’t match that of pipe organs and theatre organs. It was lacking a sort of magnifique, which set Leslie on a mission to create and refine an audio system that’s adds the missing characteristic sound.

In the process, he figured that having a rotating contour would compensate for distance-placement effect accounted for in pipes on the real thing. Naturally, the doppler effect was considered in procedure. Amplitude modulation, frequency modulation, and spatial modulation would become properties of the final model. His invention combined two delicate rotors, rotating horn for higher frequency bands and spinning drum for low frequencies, plus a crossover middle section. The feeling of rotating motion became the signature sound of Leslie superseding a classical vibrato or tremolo effect, in its dynamic range and overall sonic impact. It was a new force of nature.

Excited by his creation, he went on to try to sell it Hammond so it can get into as much hands as possible. The at thew time commercially growing Hammond company rejected it unreasonably. Notwithstanding, it went on to become a hit with no advertising in a very short frame. The speaker had a life of its own from there on and is still one of the most exciting sounds today, with its ever ebbing and flowing remodels and reinterpretations today.

Leslie Model

Signature Leslie Sound

Leslie systems have directional speaker output rotated around variable speed at a fixed point, creating the rotary effect. Leslie system can create amplitude modulation (AM) as a tremolo or vibrato feature since it rotates with higher intensity at center then diverts away with lower amplitude creating dynamic amplitude range that can be manipulated.

It can also create Frequency modulation, following the principals of the doppler effect whereby the frequency pitch of the tone will increase at center (detunes upward) at its higher amplitude and decrease in pitch (detunes downward) as it rotates away creating a dynamic pitch.

Moreover, space modulation could take place with regards to the room, causing fluctuant harmonic content and effect due to multiple reflections present.

The higher the amplitude modulation the higher the frequency modulation and vice, true to distance and speed variables. The characteristic sound feels like an ebb and flow, sounds like a rise and fall, is very natural.

Also, Leslie Cabinets have a specific structural resonance in the old wood and specifically give a very maculate tone to percussive notes.

Different models came with different speeds, sizes, induced noise, and connectors.

Doppler Effect

The Leslie Mechanism

The Leslie tube amplifier drives the signal to the crossover middle section where low frequencies below 800hz are sent via a woofer to the bass (bottom) drum spinner and high frequencies are boosted through a tweeter to the treble (top) horn rotor. Each rotor is driven by two separate motors (fast and slow) mounted in one package, which makes total of four motors. The motors are connected proximate to the rotors via belt drives.

The main speed variable control is slow or fast, controlling the rotating power of the treble horn and bass speaker. The treble rotor and bass spinner behave like an acoustic projector. The treble rotor which looks like a double horn, one is operable, the other just a counterweight for balance provide symmetric air drag without eccentric wobble forces. The treble rotor has a high-speed response and is largely responsible for the ‘’Leslie Sound’’. This where most of the frequency modulation is accented.

The bass rotor has a slower response, creating a characteristic time pronounced effect. Direct current is sometimes applied to create a faster motor transition. Not much frequency modulation will be detected in the bottom rotor ranges of 200 to 800z; more of the amplitude modulation will be felt there. Minor frequency modulation however can be present at the crossover of 800hz. Below 200hz is hardly affected in terms of modulation, as the wavelength of those ranges are too wide to be disturbed.

Schematic Diagram

Miking Applications

Since the Leslie cabinets volume can’t cut through too hard in a live stage setting, usually miking goes hand in hand especially when using only one Leslie. Some players found the solution by stacking multiple Leslie to drive the sound and cut over the guitar!

Most however resorted to miking solutions and application. The miking solution for stage often resembled that for the recording studio, with the most common set up presented here.

One common setup uses two mono mics, one for top horn and one for low drum spinner, for an overall defined signal, one that could sit at back or mid track of performance. The closer the mics in this setup, the stronger the presence.

Triplet mic setup, two microphones on treble (XY) and one on the bass is a common set up among producers, as it brings the whole body of character of the instrument and effect.

Achieving stereo imaging with two microphones on two channels panned left and right is also popular among recording engineers who appreciate the spatial information Leslies have to offer, however this one can be executed with very fine discretion, as too much stereo could cause some types of bleeds and nasty biproduct. It is trickier to achieve very wide stereo imaging with just miking. Most usually console is needed to strike the right balance.

Miking from the back side a favorable option to many producers too. Choice of microphones and choice of longitudinal and latitudinal position, and placement is a matter of taste to model and producer. Always healthy to try and test what works for your model and idea.

Crazy miking scenario

Rotating Legacy

The legacy of rotary sound lives till today with the countless emulations digitally and in pedal form. We can see examples like Lestyr models, neo instruments models, Boss, and more.

The fact that Leslie speakers took off with zero advertising proves the point that great inventions placed in the right hands in the right time frame, take on their own life. Leslie was destined to reach heights. Initially promoted by radio stations that Don Leslie worked with directly. The sound reached masses via radio broadcasts which in turn steamed curiosity and created natural demand. Beyond the radio stations that promoted it, major artists and producers took on the sound and brought it to wider masses and even made hits with it. The artist list includes John Lennon who used it on his vocals, David Gilmour and Jimi Hendrix who used the first uni-vibe pedal, Billy Preston, and many more.

Another legacy is the business life of the invention. Initially rejected by Hammond, only to be bought up by it 40 or so years later. after Hammond was bought by Suzuki and Leslie was bought by CBS, Suzuki bought up Leslie from CBS.

forever animative