Rotary Woofer (2006)
rotarywoofer.com
rotarywoofer.com
* I heard the Thigpen Rotary Subwoofer when Thigpen premiered it at the Rocky Mountain Audio Fest in 2005. Something that's not obvious from most of the published pics is that the rotary subwoofer was using an entire hotel room to serve as an enclosure. IE, you're not going to easily get loud bass at 10Hz without an enclosure, and the Thigpen sub used an entire room.
* Tom Danley is a national treasure and designed something similar at Sound Physics Labs, about 22 years ago. Tom also licensed the technology to Phoenix Gold for use in their "Cyclone" car subwoofer. I've only met Tom once, but as I understand it, he abandoned the technology because subwoofers from companies like Faital, 18 Sound, MTX and Eminence offered comparable performance with improved reliability. Basically the subs he made for Sound Physics Labs were crazy, but reliability was an issue, especially since they used a lot of custom parts.
If anyone is curious about these designs, let me know, I love this stuff.
He had a hand the design of the BassTech 7 servo drive subs as well. Their not being made anymore as I think they went out of business 20 years ago or so. All the reports I've read about those indicated that those subs were a pinnacle of rock concert audio technology and were devastatingly loud, lol.
Once BassTech folded, he worked with an online forum known as the Live Audio Board (LAB) to open source the design and replace the servo based driver with a conventional voice coil and cone driver.
The resultant design became known as the LAB Sub. A friend of mine built 12 of these beasts ~ 45x45x22 inches and let me tell you, they were amazing performers, that you could build yourself for around $500 (at the time).
Mr. Danley truly is a treasure to audio engineering and loudspeaker design.
Never thought I'd see these words together.
Danley is awesome and I would kill to have synergy hornes for my apartment (if they'd fit)
Seems like they do the same in their subs what Backes & Müller and Silbersand do for every speaker. Cinema for the ears, if you ask me. Even at volumes below speech, you can hear EVERYTHING! Even low bass, from a closed enclosure!
This is called "infinite baffle." Basically just a wall that provides isolation from the other side of it. It is merely a type of speaker enclosure.
https://en.m.wikipedia.org/wiki/Loudspeaker_enclosure
Normal subwoofers can also be used with infinite baffle. You can use an attic, basement, or crawlspace to create it.
(Which has me wondering if you could do some fun directional things with the sound? Of course low bass is not exactly known for its directionality)
https://en.wikipedia.org/wiki/Leslie_speaker
Although its rotation was for tremolo, not bass.
I thought it was more accurate to say that the lowest frequency a human can perceive as sound is also the threshold between determining if a sequence of pulses is a sequence of distinct events and one continuous sound, whose period is around 5ms (or 20Hz) and this is pretty easy to verify experimentally, even with woofers that can't produce a pure sine below 20Hz.
That said there is another factor which is that we don't just experience pressure waves in air with our ears, but with our full body of senses. And if you can definitely be impacted by infrasound even if it isn't just vibrating your ears, since it will absolutely vibrate your chest cavity and the rest of your body and you can feel that in your bones (or so to speak).
There's also a long history of scientific/pseudo scientific and almost apocryphal stories about infrasound. Off the top of my head I've heard of fans in a building causing subsonic vibrations that disoriented people, subsequent research into weaponizing it, whistles that were designed to vibrate below the threshold of hearing, the legendary "brown note" (a sound low enough and powerful enough to cause evacuation, of a building or a person) and even Havana syndrome was allegedly attributed to infrasound.
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.
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.
I'm willing to bet you could outperform it using about $1000 worth of 18" subwoofers and 10,000 watts or so.
See my comment above. Tom Danley came to the same conclusion that you did, and he's literally a rocket scientist.
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.
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.
> Now take your hand while driving down the road and stick it outside the window of a car. The faster you go the thicker the air feels.
So could you have a normal fan blowing air into a normal subwoofer to increase the pressure, and get more bass?
Also very expensive.. which makes sense, it's mechanically complex product (variable-pitch propeller) with strict requirements (don't want things to rattle or while) which is marketed to rich people (hi-fi fans).
I wonder if modern power electronics and brushless motors can achieve the same effect with a fixed-pitch propellers. This could be much cheaper and more accessible to hobbyist.
It's the tall cabinet behind Money Mark in the Beastie Boys video for "Gratitude" at around 2:19.
I mean, if we could find something that knocks off a zero at least this might make an interesting project but at this price point, it's for die-hards only.
Looks like they have a demo DVD. Was hoping for more.
I wouldn't be surprised to discover that the original "Iron Man" movie reproduced effects at that level, but I know the "War of the Worlds" film does.
Audacity can generate sine waves of arbitrary frequency, but take care not to set the volume too high and break things if you want to experiment.