Helmut Stettmaier in Bavaria, Germany, originally described the Shepard Tone Vario concept in the March 2018 issue of the now-defunct Radio Controlled Soaring Digest. It's the first article in that issue and the title is "An Innovative Method for Acoustically Rendering Climb Data for Model Gliders Using Shepard Tones."

The idea is that when your glider is circling in and out of lift and alternating between rising and falling, all you'll hear from a conventional variometer are high-pitched beeps while you're rising and lower tones while you're descending. The problem is that it won't tell you whether you're achieving a net profit or loss in altitude with each circle. Helmut offered a solution for that, which I call the Shepard Tone Vario.

The Shepard Tone vario isn't technically a variometer because it doesn't directly indicate vertical speed. Instead it produces a tone with a pitch that varies directly with altitude. It's a sort of audible altimeter that produces rising and falling tones instead of numbers. If you're circling in and out of lift you'll hear the tone rise and fall as you go around and your glider rises and falls, and you'll be able to hear whether the highest pitch you reach during a circle is higher, lower, or the same as the highest pitch you reached during the previous circle.

But the magic is in the Shepard tones. Shepard tones are auditory illusions. They can seem to continuously rise or fall yet never go beyond your range of hearing. There are demos in this blog post and you can find more with a search for "shepard tone" on YouTube, where you'll find some examples of Shepard tones used to good effect in movies.

Helmut wanted someone to help him design special hardware and software to make his concept a reality. His article ends with this:

Perhaps you may wonder why I wrote this article and offered it to RC Soaring Digest when the hardware and software is simply not available. At first there is an invitation: Make it available! I can help with software, making better altimeters, generating Shepard Tones – nearly free of bugs. A very first step could be to equip a flight simulator with this rendering, although thermal climb under difficult conditions is not often simulated perfectly. If someone wants to build something, my email address is at the top of this article.

I thought the idea was brilliant and fascinating, and I realized that Helmut was apparently unaware of the capabilities of FrSky radios and OpenTX, so I contacted him for some advice on the technical properties of Shepard tones and I developed a working version for OpenTX, which I released last year shortly after Helmut's article was published. You can read more about it here and download it here.

I still have it on my Taranis X9D+ and play with it occasionally but I've never actually given it a fair trial, and it doesn't seem to have drawn as much interest as I thought it might. Still, I immensely enjoyed learning what Shepard tones are and figuring out how to create them from scratch with Audacity. As far as I know, the Shepard Tone Vario is the first and only application of Shepard tones for a purpose other than entertainment.

Perhaps someday a well-known soaring champion will give it a try and find it useful, but one obstacle might be that most of the really skilled glider fliers who might be interested in exploiting marginal lift are into F3J, F3B, and F5J, where telemetry isn't even allowed.

Very interesting report and project Mike. I for one, will be interested in the fullness of time, to experiment with this (to the limits of my OpenTx abilities).
I am a strictly non competitive sailplane flyer. However, I don't get to fly often. We have lost our local flying fields and I am currently limited to flying at Mt. Borah which is 5 hours drive away.
However, that won't stop me having a go at it.
More in due course............

Jim.

p.s. And a special thanks to you for your patience with my beginners ignorance and basic questions over on the FrSky forums!

Originally Posted by Miami Mike

Perhaps someday a well-known soaring champion will give it a try and find it useful, but one obstacle might be that most of the really skilled glider fliers who might be interested in exploiting marginal lift are into F3J, F3B, and F5J, where telemetry isn't even allowed.

Mike,
what an amazing project! wow , the video you did to demonstrate it is exceptional at showing what it can do for you and and how it would work in flight. I wish I could actually contribute and be a tester but I haven't got the time motivation or soaring equipment so am simply in awe.. I had no idea competitive thermal fliers cant use telemetry .. and yes I bet it is a set back for a lot of technical innovation as a result.

Hope this project gets real traction and catches on quickly as I believe it will since what better way to learn to be a better thermal pilot than to have the ideal tools to find and stay in them .. Regards. JimS

Mike, I agree with you, brilliant and fascinating, sort of like an aural version of the altitude tape/scale on a HUD.
I'm very interested in human factors engineering and Helmut's explanation makes complete sense to me, so I'm going to give it a try. Since I'm new to sailplanes, I hope this tool will help me learn to see altitude changes and develop a feel for air movements.
Dave