Hold on to your hats people, the Thigpen Rotary Subwoofer!

[ThomasW]

I was at the RMAF-2005 today, and stopped in to see/hear/feel Bruce Thigpen's astounding rotary woofer.

http://www.eminent-tech.com/RWbrochure.htm

http://www.eminent-tech.com/howitworks.htm

Here's a pic of the demo unit..

Image not available

Bruce ran test tones from 40Hz down to 2Hz. The drywall in the room was flexing as the room pressurized. His intended audience is ultra $$$ HT where it's intended to augment the lower octaves below where your normal sub dies... 8O 8O 8O 8O 8O

Here's a webpage where I uploaded several more pics of the operational unit.

https://web.archive.org/web/20071005220809/http://home.comcast.net/~thomasw_2/RMAF2005.html

[AJINFLA]

Wow.
Just keep it away from pets and children! :E
I almost thought you were joking until I read further. Cost?

Cheers,

AJ

[ThomasW]

They only have handmade prototypes now, ~$12K each I recall. When production begins the cost should plummet, but they still won't be cheap.

[AJINFLA]

Ouch. I'd either have to win the lottery or steal one to get it at that price.
But think of the company motto. It slices, it dices, it plays sub 10hz bass at 150db! :
I recall hearing that Ken's LAT transducers are close to coming out. Those should be pricey at first too. but if successful, mass production should reduce prices. Kens a DIYer at heart too. So there is hope.

Cheers,

AJ

[ThomasW]

Each one of the 'veins' individually pivots like the rotorblades in a chopper. So imagine the engineering involved...... :wink:

[DearS]

does it work as a fan also? I'm not paying 12K if it doesn't. thats what the veins are for right?(j/k)

[ThomasW]

Yes it does, but it's a fairly low speed one. The frequency is set by the pitch of the blades, the motor rotates at a constant speed.

[DearS]

so blades can keep changing frequencies fast?

[ThomasW]

Yep they're actuated by the voice coil, it's the black shape immediately behind the blades. It's a relatively standard VC nothing exotic. If you look closely you can see a fairly normal spider (black) between the blades

[DearS]

I'm not sure how to imagine that.

[GrahamT]

Wow that's brilliant. I'd love to see a video of it.

[ThomasW]

Signal comes into the VC, it moves in and out (back and forth).

The VC motion changes the pitch of the blades which are directly attached to it in a manner almost identical to a helicopter rotator.

VC in = blades flat. VC out = blades at an angle. Angle varies by frequency

[Davey]

Can you make people seasick with it? That'd be a hoot!

Davey.

[ThomasW]

If it were loud enough I think so. His demo room was the only demo room on that entire floor of the hotel (for obvious reasons).

Their demo sort of reinforced the idea that there's no 'brown' note. He stepped down in 1Hz increments from 10Hz to 2Hz, fortunately no one crapped their pants ...... :roflmao:

I put my head deep inside the manifold. In there one could hear the 'flapping' noise made by the blades (just think of "BlackHawk Down"). The labrynth of the manifold and the damping material absorb that aspect of the sound before it reaches the listening room.

[Dennis H]

Their demo sort of reinforced the idea that there's no 'brown' note.

Say it ain't so. I saw it on South Park so it must be true!

[maximumshow]

Brilliant! I'd love to see a video of this, or better yet, experience it in person.

What type of SPL was the driver putting out in that room?

[ThomasW]

I forgot to ask, it wasn't all that loud. One could carry on a conversation during the demo. Bruce said it could play very loud, as loud as many, many 18"s. And depending on the shape of the blades (see the shape difference between the demo vs the operational unit) , the output can go up as the frequency goes down.

Below 10Hz the drywall was visibly flexing, and his assistants were leaning on all the doors, otherwise they were rattling in their frames.

BTW, here's the reason I got access to inner workings and the extra info. Bruce was telling people that "the sub is mounted in a manifold, that's what the IB guy on the internet calls them". I turned to him and said "Hi, I'm that IB guy". We both laughed and the tour began.... :B

[thylantyr]

Each one of the 'veins' individually pivots like the rotorblades in a chopper. So imagine the engineering involved...... :wink:

Maybe we can get surplus these;



and make a bigger monster :lol:

Why pressurize the room when you can do the neighborhood.

[ThomasW]

Why pressurize the room when you can do the neighborhood.

arty: arty: arty:

[cjd]

Does it handle multiple frequencies within its range still?

I'd imagine it does, but have a harder time really comprehending how given my vague understanding of how it actually works. . .

C

[cdwitmer]

Ditto here, concerning the inability to imagine how it works. A cone transducer is actually also a mechanical crossover, and different sections of the moving system will be producing different frequencies simultaneously, but I can't imagine how this would do something like that. At the same time, we do perceive sound differently at really low frequencies and if it is only reproducing really low frequencies then it may still be performing some real "magic" even if it has to "average" frequencies in its range (which it what it seems to me that it must be doing). Fascinating.

[ThomasW]

Does it handle multiple frequencies within its range still?

Yes supposedly it plays complex passages up to 40Hz. So it's not just a tactile transducer.

The prep before and setup at the show was so complicated, they didn't have time for the HT demo they planned on presenting. Things will be fully operational for the CES

[Feyz]

I think this is how it works, at the positive cycle the pitch of the blades are such that they compress air forward, with negative cycle they compress air backwards IOW decompress the room. The rate of change of the pitch of the blades follow the input signal from amplifier. At 1Hz, they change at a 1Hz rate; for 0.5 seconds they push air into the room, for the next 0.5 secs they pull air out of the room and so on. At 10Hz they do this ten times more quickly. If multitone is played, the pitch angle follows the signal, so both frequencies are played. Though I am guessing there must be something else at play that causes higher frequency tones to cause less pitch change than low frequency ones, because sound pressure is proportional to air accelaration.

It would become a fan with only dc applied, and the direction of air flow will be dictated by the dc being negative or positive.

[Dennis H]

It would become a fan with only dc applied, and the direction of air flow will be dictated by the dc being negative or positive.

Yeah, this thing should be able to play all the way down to DC, i.e. pressurize the room and hold the pressure as long as the room is reasonably well sealed. Make it part of a system with linear phase crossovers and you should be able to play a perfect step response or a 10 Hz square wave with a flat top.

[ThomasW]

Yes it operates to DC. That's really 'subsonic' ...... :B

Here are some posts by Bruce from an AVS thread about this sub.

The rotary woofer is a solution to the impedance mismatch between the air and a moving plate or cone at very low frequencies.
The blades of the rotary woofer move at constant speed. In theory the rotary woofer does not have any x-max limitation. Roughly its effective area doubles for each halving of frequency. With this design a single 4 inch driver can play 30Hz in a car without difficulty. This represents an elegant way to reproduce very low bass with a small device.
The problems with the rotary woofer are cost, random noise generated by blade motion (signal to noise ratio), modulation noise, a high frequency limit and mechanical integrity or reliability. The acoustic problems are mostly aerodynamic and not easy to overcome.
The maximum SPL of the rotary woofer design is above 120dB. Achieving high acoustic pressure is no problem. There is a delicate balance between designing a transducer that can play very low and causing structural or hearing damage. It most cases this will not be a problem. This is something we have to be concerned with.
On the first prototypes the signal to noise ratio was poor. The model 17 version has very good signal to noise ratio and more output than any other commercial woofer below 20Hz.
The Phoenix Gold cyclone (a very clever design by Tom Danley) is still a moving or oscillating plate and it must increase its displacement significantly for each halving of frequency. Therefore it has impedance mismatch with the air at low frequencies with a low frequency limit, as with a cone it will always run out of volume velocity and must get bigger to play lower frequencies. On the flip side, in theory the cyclone could play mid range or even high frequencys, the rotary woofer cannot. These are not really comparable devices.
There are psychoacoustic studies which suggest perception down to as low as 2Hz. My experience is that sound ceases to become a "tone" for people somewhere between 10 and 20Hz. But the individual cycles are still audible and clearly perceivable. If you play 5 or 10Hz with a conventional woofer there just isn't enough output to make a judgment. Everyone who has listened to the rotary woofer hears something. They are not hearing harmonics from the transducer.
The material cost is many times the guesses. The retail price of the model 17 is way too high in my opinion. However it reflects several years of R&D and the inevitable continuous R&D that will occur in the future if the product enjoys even a small amount of success. I have to include the R&D cost in the product to have a going concern. If the past is an accurate predictor we can now manufacture planar drivers for several hundred times less cost than the first product.
It is difficult to compete even in a limited way with a technology that has been around 60 years that enjoys an established market and engineering input from hundreds of companies. You need a solid real advantage to do so. My hope is to be able to compete with a cone woofer at very low frequencies on price and build lower cost versions as soon as practical. It will always be more expensive. If the problems can be overcome, with four times the output for the same area at 30Hz, and many times more as you go lower in frequency it has a chance to be price competitive in a few markets.
A legitimate question would be is it really necessary? If you get to experience sound at a high enough level below 20Hz, I believe at a minimum under certain circumstances the answer will be a definite yes.
I posted some 1Hz-30Hz distortion measurements on the web site. These are completely honest a repeatable and there was headroom at all frequencies tested. These will improve over time. I will be glad for anyone to make their own measurements.
I will also be glad to answer any questions I can, just send me an email. Because of patent applications there is a limit to how much I can disclose. I hope this helps.

Bruce T
Eminent Technology Inc.

I will try and answer some of the questions. In view of some of the feedback the thick skin comment seems appropriate.
For low frequency content in movies look at Keith Yates subwoofer reviews titled "Way down Deep", Be sure to look at the woofer test spectrograms. An employee of DLC also compiled a list of CD's with low frequency content, try SMR home theater. I could not post with the URL's.
This transducer has low frequency response more like an amplifier and I think it will be straightforward to establish new generally accepted low frequency hearing limits which can be useful for movie special effects and other applications.
The first model is not a traditional sub replacement. It will fill in below 40Hz. We might be able to get useable response up to 80Hz in time.
Anyone who appreciates bass reproduction has to respect the work of Mark and Tom Danley.
Mark-
Your description of operation is correct. A conventional audio amplifier drives the mechanism which pitches the blades, based on a traditional voice coil motor structure in this version. A motor controller maintains constant blade speed. The limits: At low blade velocities, (low rpm) the pressure wave will stall the blade at high angles of attack and sound will stop being produced, the system unloads. As the blades go faster (higher RPM) essentially the driver gets effectively larger and larger, like a giant cone and the efficiency improves until you overload the pitching mechanism with the air load. The air load becomes too stiff for the force of the voice coil. For pro use we can probably use higher blade velocities than in the home.
Our steel building side wall has a resonance at seven hertz. Homes seem to be a little better with lower Q. I appreciate very much your comments about potential structural damage.
I have constructed the sonic boom simulator on a smaller scale as well as similar designs in acoustics textbooks (Tom had the best design) and I do not view this as the same. Besides being an entirely different way to produce acoustic pressure,(from an airfoil, a complex subject) this transducer design represents a practical way to get very low frequency high efficiency sound out of a small unit. It is a cone that effectively gets bigger as frequency goes down, exactly what we need to reproduce very low bass. The impedance curve is flat. Air foil parameters enable you to expand or compress the signal. It is somewhat like a giant low frequency horn without the horn but with its own set of problems which are not easy to overcome.
You make good points about pressure limits. We can define the back wave volume to enhance the low frequency output, with blade stall below cutoff. The effective air displacement is proportional to swept area. Half the frequency doubles the swept area, thus the box volume is larger when compared to a cone. A two cubic foot box volume with an 6 inch rotary woofer makes a fairly nice subwoofer with a very rapid rolloff below 25Hz. But the blade stall is ugly, a bad form of modulation distortion.
The theoretical half space maximum SPL is fairly high, we have measured 126dB or more with 100 watts amplifier input at very low frequencies. I do not know what the real limit is.
Noise and box volume represent the biggest engineering hurdles toward a practical product. The Model 17 needs a manifold with some length to make it acceptably quiet. In operation the noise level is much less than the computer fan next to my desk and much less the fan in a video projector. Most would not even notice it. I do not think it will be a problem.
I wanted the first product to break all the rules and boundaries. We specify an infinite baffle so it can go to DC. Use the model 17 with about 1000 cubic foot room behind it and it will maintain 110dB or more down to about 5Hz. Below this the blades will stall.
CME - the noise spectrum is much like very low level white noise being played through speakers. Modulation noise is audible near blade stall, this won't happen with typical use. We try to design around these forms of distortion.
Rugerfan - good comment about pricing. The price will come down over time. Just give me a chance. Our test room ceiling begins to shake almost violently at 110dB at 10Hz. At any other frequency it is fine. The doors in the test house begin to rattle aggressively at a few hertz. Because of this and the speakers bandwidth I choose 110dB as a design goal rather than 120dB which the speaker can do.
This is not a joke product, on the other hand, those who make a living with cones and horns don't need to be too worried just yet. Thanks very much for the comments

BruceT

Here's a LINK to the entire thread

[aud19]

:drool: Ok, that's just cool! :yesnod:

I'm so happy there's wacky people thinking up crazy stuff like this for the rest of us schmoes to enjoy! :banana:

Off to win the lotto! :later:

[David Meek]

Damn, that's a clever way of doing things! 2 Hz, huh? 8O I would liked to have experienced that.

[Nick M]

I read about this and posted on AVS a few months back. Real interesting stuff, but I'm afraid this is where I have to draw the line due to living in an apartment.

I get a solid 11Hz>105dB with my current subs, but it dies off to almost nothing at 10Hz. Damn I WANNIT! :drool:

[SteveCallas]

Let's say it's on but the passage calls for no low bass, is it making noise because it is still rotating? People complain about fan noise from their amps, I would imagine this would be quite noticable when just running with no signal, no?

[ThomasW]

All the acoustic noise from the blades is filtered out by the manifold.

Real interesting stuff, but I'm afraid this is where I have to draw the line due to living in an apartment.

Having one in an apartment would certainly draw some lines........ :rofl: :rofl:

[mikec]

Bruce has been working on this thing for years. Did he say how close it is to being "production ready"? Or, maybe a better question would be, does he even plan to bring it to market? If this thing can produce adequate output at 20Hz and it's priced within range of Rel's & Velodyne's he will not be able to build them fast enough.

[maximumshow]

I love how, in that avs thread, people automatically disbelieved it. People do not like to think outside the box. Anything that defies convention is automatically dismissed hogwash.

If there weren't people out there like this Bruce T, we wouldn't even have ANY stereo equipment today, let alone rotary subwoofers. A person 100 years ago would have been locked up if the talked about magical spinning metallic disks, lasers, and solid state amplification.

I personally get excited when I see truely new ways of doing stuff. Thats real innovation!

[Feyz]

As mentioned in the AVS thread, I think one very interesting usage of this will be a dipole subwoofer. Dipole woofers are velocity sources, this thing is also, but this thing doesn't seem to have Xmax problems as the conventional drivers suffer so much in dipole woofers. Put this thing on something like an H Frame open baffle, with added padding to suppress fan and motor noise, add some dipole equalization, then have the best dipole subwoofer in the world, with not so much volume used. Really cool!

[Dennis H]

Yep, you could build an Orion-size speaker with awesome dipole bass. And it shouldn't have the mechanical vibration of conventional sub drivers either. Of course at $12K per side......

[ThomasW]

Put this thing on something like an H Frame open baffle, with added padding to suppress fan and motor noise, add some dipole equalization, then have the best dipole subwoofer in the world, with not so much volume used. Really cool!

It can't work in a dipole since it must have the long/large damped manifold to supress the acoustic noises of the blades.

[Dennis H]

How long does the manifold need to be? Couldn't it serve as the "H-frame?"

[Bing Fung]

Very Nice :drool:

[ThomasW]

Did you look at the link in my first post that shows the operational unit?

This is the manifold. They were using the door frame between the two rooms as the actual plane of the baffle.

Image not available

[Bent]

I get it - it took me a minute, but the little picture sealed it for me,
for a 5Hz tone, the blades change direction 5 times each way, allowing the "woofer?" to suck-blow-suck-blow-suck-blow-suck-blow-suck-blow, all in one second. I can see how it becomes more efficient at low frequencies, the air moving through it can change direction easier when called upon to do it less often (lower frequency).
I think this subwoofer could really blow my hair back. :lol:

[mikec]

Yep, you could build an Orion-size speaker with awesome dipole bass......

The Orion is already an Orion-sized speaker with awesome dipole bass. :W

[ThomasW]

Right, the same way a cone moves in and out 5 times/sec for a 5Hz note, the blades flutter back and forth for a similar manner. If one puts their head inside the manifold one can hear the blades fluttering. In the listening room the flutter is inaudible.

[Dennis H]

If one puts their head inside the manifold one can hear the blades fluttering. In the listening room the flutter is inaudible.

I suspect, with time and a lot of work, the fluttering can be reduced with changes in the planform and airfoil of the blades. There is a considerable body of research into ducted fans for aircraft propulsion and this is sort of the same thing.

[Brandon B]

This would be a great way to get lower bass in movie theaters right now. They don't go much below 40 or 30 right now.

BB

[ThomasW]

There is a considerable body of research into ducted fans for aircraft propulsion and this is sort of the same thing

Yep and then the per unit price would be $200K ....... :wink:

Brandon,

My thinking exactly, unfortunately I don't know if they'd be interested in the investment.

[dwk]

Managed to get in on the last demo of this on Sunday. Very interesting stuff, particularly how perception of 'sound' changes noticably as you get down into infrasonic territory. From talking to one of the Eminent guys as I was waiting to get into the demo this was actually a part of the point of their show presence - to see how people hear/percieve the output of the system and determine whether there is really a point to it.

It made me realize that I probably have a couple of engineering acid tests that I tend to apply to things. To wit, it makes me
a) smile/laugh at the insight and clever-ness
and
b) want to start copying/reproducing to understand how it works.

My only slight disappointment is that I think the technology is really perfect for an A/B demo of HT material with/without the infrasonic augmentaiton. I suspect that they will get there eventually, but since this is so new they had limited demo capability.