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Universal Audio Waterfall Rotary Speaker

Leslie 147 Emulation Plug-in By Hugh Robjohns
Published May 2023

Universal Audio Waterfall Rotary Speaker

This isn’t the first virtual Leslie speaker plug‑in — but it might just be the best one yet!

As countless organists around the world will gladly discuss at considerable length, simulating a real Leslie speaker is just about the most challenging technological feat known to humanity. Okay, so that might be a slight exaggeration, but it’s certainly true that a real Leslie speaker generates a fiendishly complex, yet instantly recognisable form of constantly varying modulation.

Invented by Don Leslie in the late 1930s, the eponymous speaker cabinets were intended originally to enhance the sound of Laurens Hammond’s first console organs, making them sound more like the real church and theatre pipe organs that Hammond was trying to emulate. Although the Hammond company wasn’t interested in using Leslie’s originally designs, his speakers quickly became a must‑have accessory for Hammond organ players, and went on to define a sound that we all know and love today. Moreover, through the 1960s Leslie speakers became very popular effects for electric guitars, electric pianos, and even vocals.

There have been well over 50 different models of Leslie speaker over the years, with very different cabinet sizes and shapes, different amplifier designs, different speaker configurations, and even different speed options. However, the ‘classic’ Leslie arrangement involves a large bass speaker firing downwards into a rotating drum with an angled baffle, which throws the sound horizontally into the room. The high end is projected via a compression driver firing upwards into a rotating horn (there is only one working horn — the second is a dummy for balance only!). Early Leslie speakers offered only the fast Tremolo speed option (or stopped), but later popular models all provide slow (Chorale) and fast (Tremolo) modes, and some also included a stationary option (Brake), too.

The rotating bass rotor and treble horn create both cyclical amplitude and pitch (Doppler) modulations, but the cyclical effect is surprisingly intricate due to myriad internal cabinet reflections. Complicating things even further, the bass rotor spins in the opposite direction to the treble horn, and the lightweight Bakelite horn speeds up and slows down much more quickly during speed changes than the relatively heavy plywood bass rotor — another complex artefact which plays a critical role in the characteristic Leslie sound.

Early analogue Leslie simulators were, it must be said, profoundly unrealistic. However, with the availability of ever‑increasing amounts of digital processing power more modern Leslie emulations have become remarkably realistic. For example, the digital emulations built into Suzuki‑Hammond’s last few series of digital tonewheel organs have actually been remarkably good, and Neo Instruments’ Ventilator series of effects pedals are widely acknowledged as setting the standard for compact hardware Leslie emulators.

However, for such a phenomenally complicated form of modulation, software plug‑ins can afford even greater digital processing resources and consequently it is in this form that the most realistic emulations can be found. Several virtual Hammond organ VSTi plug‑ins now have really impressive Leslie simulators and, joining an already impressive cohort, we now welcome a brand‑new offering from Universal Audio in the form of a plug‑in called Waterfall Rotary Speaker. This new plug‑in is derived directly from UA’s new Waterfall B3 Organ VSTi plug‑in but, whereas that Leslie simulator can only be used as a part of the whole organ instrument, the Waterfall Rotary plug‑in is a stand‑alone effect which can be used with myriad instrument sources, not just organ.

To the undoubted chagrin of UAD fanatics everywhere, this new Waterfall Rotary Speaker plug‑in is currently only available in the Native format, for all major Mac and PC platform DAWs and secured via an iLok account. (A 14‑day free trial is available.) The Native plug‑in is compatible with VST3, AU, AAX64 and LUNA host applications, and is also available within the UA Spark subscription portfolio. Unfortunately, though, it is not compatible with any of UAD’s hardware accelerators or any of the Apollo interfaces — although I imagine UA must be considering developing a SHARC‑compatible version for the hardware accelerators, since it would undoubtedly be a very popular and powerful effect in the Apollo console, for example.

While modelling the cabinet, speakers and mic techniques are critically important, there’s more to the full Leslie sound story, and UA have meticulously modelled the unique characteristics of the valve amp employed in the Leslie 147 (and 122)...

Waterfall Rotary Overview

Under the hood, Waterfall Rotary is built on a phenomenally detailed model of a Leslie 147 cabinet in near‑perfect condition (see 'What Is A Leslie 147?' box). The plug‑in window shows a beautifully detailed photo‑realistic rear‑quarter view of a Leslie speaker setup on a carpet in a virtual studio. There’s an array of mics on stands positioned around the cabinet, with the top and bottom cover panels removed to reveal the rotating horn and drum, along with the valve amp chassis tucked alongside the bass rotor. These superb graphics show the horn and rotor turning at different speeds and in the correct contra directions, too! We shouldn’t be swayed by pretty graphics, of course, but the attention to detail here is genuinely impressive and almost mesmerising in much the same way as a real Leslie.

Various user configuration options are arrayed in two control panels, one above and one below the graphical representation. The top control section features classic chicken‑head rotary controls for setting the input volume and drive level. (Clicking the identifying labels below the volume and drive controls recalls their default settings.) There are also separate trimmers for adjusting the nominal fast and slow rotation speeds of the horn and drum individually, along with their respective acceleration/deceleration rates.

To the right‑hand side, two vertical slide switches change the rotation speed (between fast, brake, and slow modes), and turn the whole Leslie effect on/off. Adding immeasurably to the sense of realism, switching to...

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