Arash Posted April 28, 2016 Share Posted April 28, 2016 (edited) hey guys I was thinking about starting a project to build a rotary subwoofer capable of delivering frequencies 1hz up to ~30-35hz @ ~120dB levels and I'm submitting this threat like all other my projects to have a timeline archive for futures. I want to hear your ideas about it. oh BTW Eminent Technology has invented this subwoofer before and it works: I'm a big fan of watching movie in big screens and with big and dynamic sound of horns but hearing below 20hz is something more sinister. as a beginning I just ordered this flybarless that is able to change the angle of five blades at the same time. I'm gonna use it to transfer the movement of a voice coil to the blades rotating 800RPM http://www.aliexpress.com/store/product/5-Blades-Flybarless-Quad-Bladed-Main-Rotor-Head-For-T-rex-450-Helicopter/700028_2033589307.html Edited April 28, 2016 by Arash 1 Quote Link to comment Share on other sites More sharing options...
CECAA850 Posted April 28, 2016 Share Posted April 28, 2016 It will certainly be an interesting project to watch. Be sure everything you acquire is heavy enough to withstand the pressures it will be creating. Quote Link to comment Share on other sites More sharing options...
mungkiman Posted April 28, 2016 Share Posted April 28, 2016 Can you get up to 30 Hz at only 800 RPM? Quote Link to comment Share on other sites More sharing options...
CECAA850 Posted April 28, 2016 Share Posted April 28, 2016 Can you get up to 30 Hz at only 800 RPM? The motor is at a constant speed when operating. 1 Quote Link to comment Share on other sites More sharing options...
twk123 Posted April 28, 2016 Share Posted April 28, 2016 It will certainly be an interesting project to watch. Be sure everything you acquire is heavy enough to withstand the pressures it will be creating. I can see the news headlines now: "Our contacts at numerous US Intelligence sources indicate nearby seismographs detected what was initially thought to be an underground nuclear test below an Iranian suburb. Further investigation revealed it was actually a man building a homemade rotary subwoofer. Our sources released this statement, 'Satellite imagery also revealed the guy has a really nice stereo system...' President Obama has been notified of the situation and stated that he has a Bose Wave in the Oval Office and he wants to assure Americans that it probably sounds just as good." 3 Quote Link to comment Share on other sites More sharing options...
mungkiman Posted April 28, 2016 Share Posted April 28, 2016 Can you get up to 30 Hz at only 800 RPM? The motor is at a constant speed when operating. Understood. This is also why rotary subwoofers are 20 Hz and lower. It is my (limited) understanding that distortion will dramatically increase as the input frequency exceeds the rotation rate of the fan, and also that the fan needs to spin at about 1200 RPM for 20 Hz. Quote Link to comment Share on other sites More sharing options...
Arash Posted April 28, 2016 Author Share Posted April 28, 2016 (edited) Can you get up to 30 Hz at only 800 RPM? The motor is at a constant speed when operating. Understood. This is also why rotary subwoofers are 20 Hz and lower. It is my (limited) understanding that distortion will dramatically increase as the input frequency exceeds the rotation rate of the fan, and also that the fan needs to spin at about 1200 RPM for 20 Hz. the rotating speed doesn't have anything to do with frequency. you probably have seen woofers slowly moving playing for example 10hz? but no sound? they are actually making sound but the sound pressure is so low/small you can't hear. you also have probably fanned yourself by your own hand in hot weather. your hand shakes actually ~4-5 times a second so you are making sound in that frequency but still no sound due to weak impact. this subwoofer is also the same as the hand if the rotary motor is off. the blades go back and forth but no sound is audible. as long as everything is rotating at high speed the little impact produced by blades will be much more stronger than before thus the audible sound is produced. Edited April 29, 2016 by Arash Quote Link to comment Share on other sites More sharing options...
CECAA850 Posted April 28, 2016 Share Posted April 28, 2016 My understanding is limited as well. The RPM doesn't determine the frequency rather the pitch of the blade and resistance of the air. The subject is intriguing and something I'd like to learn more about. 2 Quote Link to comment Share on other sites More sharing options...
CECAA850 Posted April 28, 2016 Share Posted April 28, 2016 Seems we were typing at the same time. Quote Link to comment Share on other sites More sharing options...
Chris A Posted April 28, 2016 Share Posted April 28, 2016 I was thinking about starting a project to build a rotary subwoofer capable of delivering frequencies 1hz up to ~30-35hz @ ~120dB levels ...I want to hear your ideas about it. There are some interesting considerations: 1) In order to isolate fan noise from the listening room background, the inventors (Bruce Thigpen?) have simply built a separate room enclosure next to the listening room, and ported to the "fan room". The "fan room" is lined with thick absorption to kill as much fan noise as possible. It's basically impossible to attenuate anything below about 70 Hz using absorption pads - you'll have to use structure-borne isolation surfaces - like flexible wallboard--to control those low frequencies. 2) I would think that the mechanism that controls the fan blade pitch will need to be a very low hysteresis mechanism to avoid dead zone errors and to enable the upper frequency band dynamics be possible. also, the design for fatigue life might be an interesting issue in terms of selected materials and allowable stresses. 3) The design of the reciprocating electromagnetic motor to control fan pitch will need to be a very low mass or rotary inertia assembly. 4) The amplifier to drive the assembly may need to be able to drive a higher voltage or current motor, depending on your design specifics. If may require a fair amount of power to drive the assembly. Have you considered using a servo-mechanism to control the fan blade pitch vs.commanded input? 5) Have you considered using more than one assembly to get the requisite amount of SPL to be "tactile" below 20 Hz? 6) Have you considered the source (music, movies) material frequencies? From what I've seen, only frequencies above about 10-12 Hz are usually reproduced using digital disc formats--usually movies. Music is usually limited to 17 Hz (pipe organ) unless you're trying to reproduce extremely low "urban contemporary" music. 7) Rotational speed can be adjusted later to suit the need. But the pitch drive mechanism will be the most interesting part to get right. Chris 1 Quote Link to comment Share on other sites More sharing options...
CECAA850 Posted April 28, 2016 Share Posted April 28, 2016 Arash, would you like this topic moved to the subwoofer section? I believe it would be more appropriate there. 1 Quote Link to comment Share on other sites More sharing options...
Pete H Posted April 28, 2016 Share Posted April 28, 2016 There are so many things that I get exposed to on this forum, that I would have never even contemplated. I would also like to thank Arash, and others like him that build some really odd things that allow me to tell Denise that "I don't have a problem at all, just look at this" You go buddy! This should be interesting. 3 Quote Link to comment Share on other sites More sharing options...
CECAA850 Posted April 28, 2016 Share Posted April 28, 2016 I would think that the mechanism that controls the fan blade pitch will need to be a very low hysteresis mechanism to avoid dead zone errors and to enable the upper frequency band dynamics be possible. I believe subwoofer "motors" have been used. As far as the fan room is concerned, you should be able to build it just like an IB and use your attic. Quote Link to comment Share on other sites More sharing options...
Chris A Posted April 28, 2016 Share Posted April 28, 2016 (edited) I'm referring to the blade pitch-sliding motion mechanism. You're having to build a basic helicopter rotor collective system, and those aren't cheap. ...At least you don't have to build in a cyclic mechanism. I didn't say anything about the fan blades themselves in order to control distortion harmonics, as well as present a low rotary inertia to the pitch mechanism. Chris Edited April 28, 2016 by Chris A Quote Link to comment Share on other sites More sharing options...
stereohermit01 Posted April 28, 2016 Share Posted April 28, 2016 I heard eminent Technologies rotary subwoofer at the CES about 6 years ago and it was phenomenal 1 Quote Link to comment Share on other sites More sharing options...
Taz Posted April 29, 2016 Share Posted April 29, 2016 I know my cousin uses a rotary sub for his band. Overheard one of the band members saying it was a good investment. Me. I know nothing. 1 Quote Link to comment Share on other sites More sharing options...
Frzninvt Posted April 30, 2016 Share Posted April 30, 2016 Phoenix Gold made a 12" cyclone subwoofer for car audio in the 90's I never got to hear one though. I heard they worked well. Quote Link to comment Share on other sites More sharing options...
DizRotus Posted April 30, 2016 Share Posted April 30, 2016 Another very interesting project from the mind and workshop of Arash. I'll follow this closely. Quote Link to comment Share on other sites More sharing options...
CECAA850 Posted April 30, 2016 Share Posted April 30, 2016 Neat Fritz. I had never seen them before. Quote Link to comment Share on other sites More sharing options...
Chris A Posted April 30, 2016 Share Posted April 30, 2016 (edited) Taking a TRIZ-like approach, if you think about the problem a little differently, it is possible to come up with alternative solutions that use the same principles... Just like propulsion and lift are separated in an airplane vs. a bird, separating functions of the TRW-17 air-flow subwoofer could make things much easier--for instance: 1) build a constant flow air supply fan box that uses a fan to push air into a box, then 2) use variable-opening ports using something like a throttling valve to let the air flow escape into behind the ports into the room--like valves in an internal combustion cylinder head. See picture below for a throttle valve example: This is more like a supercharger or turbocharger setup. That way, you don't need a rotating/sliding contact of a helicopter collective system--and the problems of eliminating dead zone - the functions are separated into two mechanical functions--and much easier to manufacture. Chris Edited April 30, 2016 by Chris A Quote Link to comment Share on other sites More sharing options...
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