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Vibrating a piston at low frequencies isn't hard, but getting it to move enough air is very hard. You're right that "big enough" with "enough power" could solve that problem, but the mechanics of a conventional driver result in unavoidable output roll-off as you go lower, whereas the fan/rotary solution won't roll off because it has essentially infinite excursion - it just keeps blowing in the required direction at the required pressure level for any low-enough frequency, even fractions of Hz.

Re: custom electronics - my understanding is that the electronics just need to handle very low frequencies (in part by being able to sustain enough current without fading), which would also be true for a conventional subwoofer system trying to operate at those frequencies.

I remember reading about this thing a long time ago and wondering how close I could get with a DIY solution. It would probably sound terrible but it would be fascinating as an transducer learning platform.

The Thigpen Roatary sub used an entire hotel room as it's enclosure.

I'm willing to bet you could outperform it using about $1000 worth of 18" subwoofers and 10,000 watts or so.

And where you you put those subwoofers? Probably in a room of some sort. Thus considering the room as part of the BOM for one but not the other is a bit disingenuous.

> It's very interesting to use a fan to create such waves when an electromagnetic piston might fail. But I wonder how useful it is since making a piston vibrate at low frequencies isn't that hard, it's enclosing it that is the problem. I suspect a large enough enclosure with a big enough woofer and enough power in an amplifier (eg, a commercial subwoofer) can do just as well without being so exotic as to require custom electronics.

See my comment above. Tom Danley came to the same conclusion that you did, and he's literally a rocket scientist.

I feel like anyone with a basic understanding of loudspeaker design would come to the same conclusion. Ultimately, it's well understood that the "infinite baffle" design will give you the best low frequency performance for a given woofer but it's impractical given the amount of volume behind it to actually act as a practically infinite baffle.

The goal of loudspeaker design is how to choose a port geometry and enclosure volume that makes the best tradeoff of low end rolloff and resonance for a given woofer at a price point for consumers. The cleverness of a waveguide is that it can manipulate the combined acoustics of small woofer and enclosure volume to provide good sounding low end that is extremely surprising compared to the infinite baffle design for the same woofer.

That's why big speakers sound good but are not surprising. There's an inherent tradeoff between low end response and acoustic compliance which is proportional to the volume of the enclosure. A large enough enclosure gives you effectively infinite volume behind it, hence the "infinite baffle" design. That volume tends to be prohibitively large for commercial products. If you choose a smaller volume you get a noticeable rolloff in the bass, but you can trick the listener by turning the enclosure into a Helmhotz resonator by adding a port that creates a resonator with critical frequency below the lower rolloff of the enclosure that extends the bass response. Waveguides further allow better extension by creating a natural acoustic amplifier for a range of frequencies that behave much better than the Helmhotz resonator.

It's not rocket science, although it is control systems and signal processing. In short I don't think an exotic loudspeaker is better, because under ideal conditions it still has to deal with the physical properties of whatever encloses it. It's interesting to consider how they interact in a different way that affects the model but I don't think there's anything fundamental that makes it better than a traditional piston model. It's still just moving air and working against the compliance of the air behind it in the enclosure.

Essentially the problem of low frequency reproduction is not moving air in front of the piston that is the problem, it's isolating the pressure wave behind it and the effects of the compression of that volume that prohibit its movement at low frequencies (and how they diffuse around the front) that contribute to the issue of reproduction. I don't see how this design fundamentally alters such problems.

I agree, I don't think it's better or different.

What sticks in my mind is that I think its a rate problem. Meaning audible bass volume is relative to volume of air displaced.

It's like when we design pumps. There are positive displacement pumps, flow equals stroke volume times stroke rate. Stroke rate is frequency, stroke volume is stroke times surface area. So bigger subwoofer diameter equals bigger surface area. You can also increase the stroke. Anyway, that all integrates to flow.

We also have rotary pumps like centrifugal and axial flow pumps. With those flow is a function of rpm and pressure gradient across the pump. (Pressure gradient across the pump is not a factor in positive displacement pumps). To me this is why they needed such a large chamber. Below a certain size they were sucking down the Pressure too much. Also it's much flow that the pressure gradients ar such that it limits its frequency response to 10 hz or so.

All this to say, I think the mistake was simple. They said existing subwoofers are 12" so let's make a 12" rotary sub.

I think they should have said a 12" sub has a volumetric displacement rate (diameter times stroke times frequency) of X mm^3/s and if we design a rotary sub to have that same volumetric displacement what are it's dimensions.

It would be far less than 12", likely 2" or so, and very compact yet powerful. That form would also dramatically change its frequency response and performance.

This is what I've been wanting to build for years, a 2" rotary sub with a very powerful fan.

I’d be very interested to hear a recording of large waves crashing at the ocean with and without this woofer. That comes to mind as the ultimate low-frequency soundscape I know.