The Leslie speaker was originally designed and voiced for the Hammond organ, and almost no Hammond organist worth his salt will be seen traveling or playing without one. We associate the sound of the Leslie with the Hammond so much that when plugging a guitar into a Leslie, a listening layman would most likely say, “Wow! Your guitar kinda sounds like an organ!” The frequencies of the speakers and the design of the cabinet were geared for the Hammond organ, so while plugging a guitar into it sounds really beautiful, it makes the guitar take on organ-like characteristics. This has been used to great effect by many guitarists throughout the years.
The Leslie appears on classic tracks by Jimi Hendrix, David Gilmour, Stevie Ray Vaughan, Jeff Beck, George Harrison, and countless others who used the seductive sounds of spinning speakers to add a unique touch to their tracks. Check out “Cold Shot” by Stevie Ray Vaughan, “Max’s Tune” by the Jeff Beck Group, and “Any Colour You Like” (Wembley 1974) by Pink Floyd for some excellent examples of rotary-speaker guitar tones.
Leslie speakers operate on a principal called the Doppler effect. This is a physical phenomenon that describes the change in frequency as an object changes position relative to a sound source. As a sound source gets closer to you, it will increase in pitch and volume, and the opposite will happen as it moves further away. The Leslie speaker operates on this principal on a relatively small scale, making the effect more subtle then a moving ambulance siren, but all the more beautiful. I always wondered if the Doppler effect could be applied specifically to the guitar. What would happen if a rotating speaker device was designed from the ground up for our 6-string weapon of choice? Could it maintain and embellish the frequencies of our instrument while giving that beautiful, room-filling, three-dimensional effect? Other than the vintage Fender Vibratone and scant offerings from Hammond-Suzuki and Motion Sound—some guitarists might also recall the elusive Mesa Boogie Revolver—nothing really scratched my itch. So, ever a believer in the DIY spirit, I decided to build one.
In approaching this project, my philosophy was to lighten and simplify the original Leslie design as much as possible, fine-tuning it to the guitar while maintaining a tight budget and not compromising on tone or function in any way. This was a tall order, and I had to rethink the Leslie design from square one. I came up with a simple 12" standard guitar speaker firing upward, with a rotating Styrofoam baffle mounted to a motor above it. The Styrofoam would have a sizeable chunk cut out of one of the sides, so the sound could only travel through the open part. The open part spins, therefore shooting the sound around and around to create the Doppler effect. To keep things simple, I decided to make this speaker cabinet passive, except for the power required to run the motor. So it operates like a basic 2x12 or 4x12 cab, and you can use your favorite guitar amp to power this little beast.
Fig. 1 shows a CAD drawing of the cabinet. In this view, the removable rear panel is detached and we’re looking through the back of the cabinet. You can see the upward-firing speaker mounted on a board that’s permanently attached to three walls. The motor, which is fastened to the roof, rotates the Styrofoam cylinder that’s suspended above the speaker. As the cylinder spins, its cutaway pushes sound through “windows” cut into the three walls and rear panel. As the sound passes from one window to another, we hear the shifting tones created by the Doppler effect.
Okay—now that you have all your materials listed and collected, let’s get started! For this project, I enlisted the assistance of a professional woodworker to help build the cabinet while I directed him. (Special thanks to Robert and Monique at Woodworking Specialists in Tucson, Arizona, for helping me out and putting up with my incessant photography.)
Note: The wood I used for this cabinet is pre-finished furniture-grade 9-ply Baltic birch, so I did not need to finish the cabinet. If you are using non-finished wood, apply your favorite stain or finish before mounting any of the electronic components. Follow the instructions on the can to get the best results.
Before you start cutting any wood, study the following three drawings to get a visual sense of where we’re headed. Fig. 2 shows the cabinet dimensions, and Fig. 3 shows the measurement for the four window cutouts. Fig. 4 shows the speaker mount and its cutout; I’ll explain how I arrived at the speaker cutout measurements in a moment—its precise diameter depends on the speaker you’ve selected for this project.