Don Cross / CosineKitty / Sapphire

“Midnight Samadhi” - space music for late night contemplation.


I’m not sure I think space music… But I like it!

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Very cool, definitely gives me some Dune vibes!

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As Harrison said … we were talking about the space between us all … :headphones:

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The state of enlightenment by a star just as it passes beyond the event horizon into the midnight of the black hole.


Nice! Some great sounds in this.

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Sounds great, any chance of the patch been available to mess with/play?

Yes, it is at: Midnight Samadhi | Patchstorage

Thanks, just rooted on there and found it. Contrary or what? That’s me. Enjoying your drum circle patch as well. Great work!

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This is an early peek at a new VCV Rack module I’m working on, called Tube Unit. As you can see, it’s totally not ready to be released yet, but I’m already having fun with it.


Oh Yes! I am very intrigued!

Is there any reverb added post processing? Or is that just the dry sound? It sounds quite lush.

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This sounds amazing!


Are those n-foil knots? Looks and sounds very intriguing!

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There is reverb inherent in the physical model, so it’s part of the processing, not post-processing. As a flutist, you might appreciate that this physical model (as the name Tube Unit hints) is based on the resonance inherent in a tube-shaped waveguide.

A flute is approximated by a tube that is open at both ends, causing a negative pressure reflection at both sides. That means a high pressure wave traveling down the tube causes some air to “coast” out the end (air has mass, and thus momentum), pulling a partial vacuum behind it. The result is for a low pressure wave to reflect back the other direction. Likewise, that low pressure wave “sucks” excess air into the tube when it reaches the other end of the tube, causing a high pressure wave to move back the first direction.

The result is, the pressure wave has to reflect twice and to travel the length of the tube twice for the system to complete a full wavelength. Divide twice the tube length by the speed of sound and you get its fundamental resonance frequency.

There are also instruments that act more like a tube that is open on one end and closed at the other, for example trumpets and clarinets. In these instruments, you still get a negative reflection at the open end, but a positive reflection at the closed end (the mouthpiece). This drops the fundamental frequency by an octave because now the pressure wave has to travel the length of the tube four times to come back to the same phase. (Fun exercise: see if you can imagine this in your head to see why.)

If you have ever held a length of PVC pipe and smacked one end of it with the palm of your hand, you can confirm these effects. If you smack the end but hold your hand firmly to keep that end closed, you will hear a lower note. If you smack the end but quickly remove your hand, you will hear a note one octave higher.

I started thinking about the difference between a flute and a clarinet (open/closed tubes versus open/open tubes) and generalized the concept as a pair of reflection coefficients like [+1, +1] and [+1, -1]. I also love stereo field, so I’m attracted to instruments that inherently produce stereo output. This got me wondering if I could use complex numbers, so the real part becomes the left channel and the imaginary part becomes the right channel.

Putting these together, I started wondering, what would happen if air pressure could be a complex number instead of a real number. This led to the idea that the reflection coefficient at the end of the tube could be a complex number e^{i \theta}, where 0 \le \theta \le \pi. This means the reflection can be +1, -1, or any complex number around the complex unit circle between them. The idea was so interesting that it became the inspiration for Tube Unit.

That’s an interesting take on this, because as soon as I thought about adding and multiplying complex numbers iteratively, naturally this led me to start thinking of Mandelbrot sets and Julia sets, with their inherently chaotic twisty shapes. So that’s why I added those scopes to plot the left versus right output channels (imaginary versus real, just like in a Mandelbrot image). My intention is that Tube Unit should be capable of fractal-like complex behavior by the time I release it.

So I’m going to keep experimenting and adding more parameters. I want more degrees of freedom to control the timbre for the way the vibrations are injected into the tube, as well as more control over the way the reflection works. Basically, there will be a lot more knobs, CV inputs, and attenuverters before it’s done.


I might have a wee bit of experience playing with PVC tubes :wink:

My overtone flutes are designed with a large length to diameter ratio to promote the playing of overtones. One consequence of this is I cannot play the fundamental of the closed tube - it simply will not resonate that low.

I imagine you already know this, but the open tube can play both odd and even partials of the harmonic series. But a closed tube can only play the odd harmonics. So in the absence of the closed tube fundamental, my closed flute can never play an octave down from the open position.

I can’t tell from your description of the math behind your new module - do you have any input to select which overtone is played from the tube?


I love this overtone flute! That is so cool! Are you articulating your mouth and throat shape to produce some of these formant effects? I ask because it reminds me a little of a didgeridoo.

No, I hadn’t really thought about that, but it totally makes sense now that you say it. It explains some of the stuff I read about trumpets that I didn’t understand at the time. Now it clicks.

I haven’t really thought about that level of control yet, although your question does motivate me to think about that as a design goal. My philosophy of module design (if I can use such a lofty phrase) is more about discovery of emergent properties than planned control. I like starting with physical models because there are so many interesting and complex sounds in the real world. I like to imagine all the trial and error that led from ancient bone and wood flutes to the modern instrument, or how people gradually discovered resonator chambers for violins and guitars. And software allows adding a sprinkle of impossibility to the mix (like complex-valued air pressures moving through a waveguide).

So with your comments about overtones, that makes me think about how to approach further research with the mouthpiece part of my simulated model. Right now it is a very simple (probably too simple) piston-like model that allows air to flow into the tube only at certain displacements. Pressure waves coming back from the open end of the tube interact with the piston to provide a feedback loop. I should study more about how to adjust the frequency response of the piston to resonate at or between higher harmonics.

It’s not clear yet whether it will be practical to have a V/OCT input to this thing. There are plenty of tunable oscillators out there already; I’m far more interested in creating interesting sound textures, with melodic tuning as a much lower priority.

However, I’m also considering the possibility of a separate module that combines a pitch detector with a feedback controller, that could be used in combination with Elastika, Tube Unit, or any other module with CV inputs, to create an effectively pitch-controllable instrument.


A lot of people comment on how the flute sounds similar to a didgeridoo. An important aspect of didge playing is vocalizing while playing a note, and that also works well with the overtone flute, making it even more similar.

But the mechanism of generating sound is quite different - the didge via buzzing lips, and the flute by air turbulence at the fipple. For the flute the turbulence oscillates between being above and below the cutting edge of the fipple.

For didge playing, I believe much of the opening and closing of the mouth cavity has to do with circular breathing - a critical aspect of didge playing. The mouth cavity is opened wide to trap air, and then the throat is closed and while you breath in through your nose, you compress the mouth cavity to continue expressing air so that the sound can continue. (I’m not a didge player, but I have dabbled a bit)

I’ve tried circular breathing with a flute, and it is really hard to maintain a consistent air flow when there is very little resistance (back pressure). I have much better success with a reed instrument like the double reed of a duduk.

For the overtone flute I do vary the mouth and throat cavity to increase control over which overtone is played. The lowest overtones require the smallest air stream under the lowest pressure. Opening the throat and mouth cavity helps buffer air pressure changes, and makes it easier to control and maintain the gentle flow required. The higher overtones require a faster stream under higher pressure. Closing up the throat and mouth cavity makes it easier to quickly vary the rate and pressure so as to reach those high notes. I’ve never read anything about this, but it is how it feels to me.

Beyond that, the mouth cavity and embouchure do seem to subtly effect the tone in a way I don’t really understand. Typically in the higher registers there are multiple overtones that are sounding, each one competing to take over. I suspect that the mouth changes result in different amounts of pressure buffering, and that may be coloring the distribution of the various competing partials.


Oh cool, I learned a new word: fipple. Now I know what the blow-through-it-to-whistle part is called! This makes me think about alternative models of how the mouthpiece could work in Tube Unit. I imagine lower pressures in the tube tend to pull the fipple airstream downward, causing pressure to go back up, and vice versa. I always wondered what exactly was happening in a flute to make it oscillate, and now it’s clearer.

To go off on a bit of a tangent, is there a trick to making the whistle part of the fipple? Are there well-understood principles about the numeric dimensions and cut angles, or is it just a matter of playing around until you get a good feel for it? Right now I have a dedicated workshop area under construction, and when it’s finished, this is the sort of thing I would like to do out there.

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There are definitely construction notes for various fipple instruments out there. There are a lot of variations on the theme, and many opinions as to what works “best”. I think perhaps the most documented variations are for pipe organs, though I don’t know that they translate well to hand held flutes. There are all kinds of crazy things organ builders do to get different timbres.

For the large bore flute I think it is important to make sure the width of the sound hole is wide enough. I’ve heard that the general rule of thumb (at least for Native American style flutes) is 1/2 the diameter of the bore.

I think the few large bore Native American style flutes I have seen others make had comparatively narrow sound hole widths, and the end result was very thin/weak sounding to my ears. For my 2" bore flutes I worked hard to come up with a fairly easily constructed design that gave me a 1" wide sound hole. It requires a lot of shaping to get an even flat cutting edge throughout the 1" width out of a round tube. I looked at the design of Native American flutes and recorders that I own, and came up with a cutting edge bevel shape that was a sort of average of what I saw. My cutting edge has two bevels (asymmetric), one for the top and one for the bottom. My cutting edge is actually carved out of the fusion of an inner tube and an outer sleeve, so the thickness of the wall is doubled. Without that doubling I could never have gotten an even cutting edge across the entire 1" width.

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I’m still working on Tube Unit. It’s not ready for release yet, but it’s getting closer. Here’s what it looks like and sounds like as of tonight.