Has anybody heard about or build a Roto-sub. It would be nice to have a speaker system,
with a freq, range from approximately 5Hz

//Henke

Isnt that the one that uses a spinning propeller that changes angle to produce different tones?

See this thread. It starts off with reg. sub, but someone posted the "roto" sub with it's ridiculous claims to respond to DC.

http://www.lumenlab.com/forums/index.php?showtopic=16709

Actually DC isn’t as far fetched as it might seem. When AC current is converted to acoustic energy it is simply alternating positive and negative pressures. The problem with cone type speakers is there inability to produce low frequencies because the cone needs to displace large amounts of air.
DC would be just positive or negative pressure alone. In theory a fan could pressurize a room to produce a representation of DC acoustically. How efficient it is would be another story.

DJ

Nah, not buying today, Dazz. If that's the case, you just need a regular $9.99 fan from Mal-Wart, not some fancied up one for an astronomical price.

AC is alternating, DC, by definition, isn't. All DC will do to audio equipment is make it thump once, then it's just heating the driver as well as limiting travel in one direction. Therefore, an audio amplifier that puts out DC is poorly designed.

If anyone really wants to feel the thump, get/make some tactile transducers instead. Lots cheaper, and I bet it's just as good a thump @ 5Hz as with the fan. Maybe better. Assuming your equipment will feed it the signal, because even if you have one of these beasts, you still need the source, pre-amp, and amp to drive it. I haven't gone out and priced these items looking specifically for flat response below 10Hz, but I'm willing to bet that when you get done, the $25k fan will probably be half or less of the system price. You're probably looking at custom equipment. And that doesn't include your L&R mains. If you're spending $25k on your sub, you should spend at least $50k on your mains.

At some point, it all just gets really silly. I think the fan is considerably over the line, and their claims are at least partly (maybe mostly) bogus.

I'll have to ask the resident audiophile at work if he's ever experienced one. He has a pair of multi-thousand $ (ea) mono tube amps, plus all the other doo-dads and gee-gaws to go with them. Who knows, maybe he has one at home. Maybe it's better than sex. I just have to laugh about the "true response to DC" thing. I used to have a Rat Shack amp that put out DC every time I turned it on. THUMP!

My point was that DC can be represented acoustically. A cheap fan blowing into a sealed room is pressurizing it so in theory you have 0Hz. Take it one step further and alternate the pressure from positive to negative in 1 second and you have 1Hz. It’s generally believed that human hearing is not capable of detecting sounds under 20Hz. I believe that we are capable, it’s just that there isn’t a good way to reproduce those frequencies loud enough for us to hear. There are certain model cars when driving with the window partially open that can produce very low frequencies. Here in OZ this noise is called putting and can make you feel ill. The thing is that you can hear it despite what the general expectance is. Where am I going with all this? I have no idea, just babbling.

DJ

I just went and re-read your post in the other thread It sounds like I work for them, I said nearly the exact same thing they did.

I don't work for them,really

I’ll put my augment forward another way.

Can you represent DC using a fan? Yes.
Can you represent DC using a piston? NO
Can they say that their speaker is capable of DC? Yes.
Is there allot of equipment designed for DC audio? NO
Can you hear DC? NO
Can you hear close to DC? Yes
Do these particular fan speakers sound any good? Don’t know

DJ

I won't disagree that very low frequency "sound" is capable of affecting your body/ears, I just think that the whole thing is silly, particularly because of the way they wrote their copy. DC cannot carry any audio content. They, and by extension their product, are silly. Batteries are a power source, not an audio source.

DC, if it could be said to carry any audio content, (which I'll disagree with) would be a single thump. Period. End of story. For the whole "song".

And the really silly thing is that even if there is a DC signal that the AC rides on, all it does is limit the movement of the driver, (in this case fan blades), which means the driver wont be capable of the output it should in one direction. And there aren't any recording/reproduction media that support a DC signal, in fact, great pains are taken care to eliminate any DC offset in audio recording/reproduction equipment, because it's "a bad thing" for the reason given above.

It's just my visceral reaction to a laughable claim.

I have start to build one anyway from an old Zachry BX15 woofer, but some things still missing...fan blades motor etc....

Some pictures....

Good luck. What kind of motor are you going to use to drive it? It will have to be pretty stout, or it's going to lose speed quickly when you start deflecting the blades, and your output will drop like a rock.

I think a 3-phase engine is to prefer, but maybe 1-phase motor with gear reduction could work

I don't know the trade secrets, but it looked like the one in the video I saw was spinning at a pretty fair clip.

A 5Hz roto sub seems a little unessesary to me Just to put pressure in the air. A really good regular sub should do that anyway. Plus, not a lot of theatrical encodings offer 5Hz in a digital sub output.

But it's fun to try to build one

I think the point has been missed. If you look at the short comings of a regular piston driver you can see the advantage of a speaker that can go to 5 Hz or DC, not that you’re going to listen to anything that low. Same deal as having an amplifier that has way more watts than needed, headroom.
Here’s the problem with cone drivers. As frequency goes down either the cone needs to get larger or the distance it travels gets larger and this size/travel limitation is far from linear. Most cone drivers work reasonably well down to their system resonant frequency, under this their output drops very fast. Different box designs can change the slope that they roll off, some have shallow slopes and some sharp so their bass content varies but they still all decrease their sound output. The slope can be extended down further but there are compromises involved, large amounts of extra power and speakers that have extra travel. While cone drivers do a good job their roll off means that they can’t reproduce a frequency at the correct level if that frequency is in the roll off regain. An Ideal speaker would be one that has no roll off at all
The way in which a piston driver creates positive and negative pressure prohibits it from working at very low frequencies; the piston simply travels out or in with no apparent pressurization to the air. This is where I believe the fan driver is much more efficient. Its blade’s pitch can be oscillated at very low frequencies but the pitch oscillation is not what is producing the pressure differences. If the fan was not spinning, not much would happen. But when spinning, the pitch of the blades convert the power of the motor to air pressurization. Just as an example think about a piston driver operating at 5 Hz, it moves in and out 5 times a second. It would do very little to the air pressure near it unless it was the whole size of a wall or it was able to travel the length of the room. Now look at a fan that varies its blade pitch at 5 times a second while spinning. It would be able to affect the pressure nearby without its size being huge.
Again I’m in no way saying that this manufacturer’s fan speaker sounds good but I do believe the theory behind it. I have to admit that I’m a bit biased on the idea though. I thought about a fan speaker many years ago but never got motivated enough to build one, maybe if I new that I could charge that amount of money for one I might have. But I’m glad someone did it just to know that I didn’t waste all that time thinking about it for nothing.

DJ

the vision of a wall sized speaker moving in and out several feet is quite scary .

If you're talking to me, Dazz, then I did not miss your point. Like I said, it's just my visceral reaction to a laughable claim made on the manufacturer's website:
Complete Crap. That one claim just completely blows their credibility with me.

As for reproducing very low frequency pressure differentials in the room, I'm certain that it does a much better job than a cone, but again, I think a Buttkicker or Aura or a DIY tactile transducer will give you basically the same effect, and probably better, as the medium used to conduct the pressure wave is much more efficient than air in doing so.

And you still need equipment that plays with authority that low. That's going to be very expensive, and that's why they charge what they do for their equipment.

Actually, here's the pricing from their web site.


$700 seems an awfully low price for an amp that's going to drive this piece of equipment. I've seen speaker cables that were a lot more than that. And you still need the source equipment and preamp that will give you response to those low frequencies. $$$

I'm just going to stick with my 38Hz -3dB response from my mains until I can get a decent powered (cone) sub that will take me into the upper teens.

tenzip...maybe you could go with a car sub..they have a tighter suspension and most have a lower frequency response than HT subs....the 450w 12" i have is rated 20-20,000Hz. The only thing i have to worry about is the magnetic field. (wich isnt that bad)

OK.... the amp cost 700$...and a controller for 350$ etc. But lets say if You build the
Rotary-sub Your self, the 12. 900$ cost for the TRW-17 Transducer is gone, so the total cost
overal will not be to high and best of all......You have buildt it Your self isn't that what
DIY stands for ...and why not even build a pair of "plasma tweeters" in the same time
...and maybe the amps?

If I wanted to DIY it, I probably would. But looking at the calculations and fiddling that folks do to build their boxes, I'll probably just spend the extra cash and buy one. (Or two? )
In that case, I think the time saved by taking it out of the box and connecting rather than selecting components, designing the box, building, and testing is worth the money spent.

I've never said it was a BAD product, just silly, because of what the manufacturer's web site claims, and I think that tactile transducers will give you essentially the same sensation without all the hullabaloo. If money grew on the tree in my back yard, I'd probably already have one of these, just to say I had it. I'd have to do something with all that money, or it would pile up and kill the grass.

I really do hope you can DIY it, Henk, I'll be watching your progress. If I can help, I will. It really is kind of cool, but I just have this reaction to statements like "the first audio product with true response to DC.", that are obviously false.

So, what kind of equipment will you be driving it with?

Have you considered using the rotor pitch mechanism out of an R/C helicopter? It should work pretty well for this type of thing, even though there are a lot more blades.

I just have mine in a normal, all wood, single ported box(for a car). Its louder and deeper than my friends 15" JBL home sub, in a bandpass box, with the same amount of wattage.

Altho i think mine has a lower Ohm rating.

What ya think about this driver?
Got it from someone cause the voice coil was completely wrecked. It's a dual 6-ohm coil speaker.