Rotary Subwoofer Combines A Speaker Coil W/ A Fan

What happens when you combine a fan with a sub-woofer? Apparently, you get a high-efficiency ultra low hertz (3-5hz) rotary subwoofer!

First thing’s first, believe it or not, these things really do exist. [Chris] got the idea to build his own after seeing the TRW-17, a commercial offering of a rotary subwoofer.

The concept is pretty simple. If you use a giant subwoofer, you can get low frequency response, but it uses an immense amount of power to move a giant speaker coil. So what if you put something on a smaller speaker coil to increase airflow? Like, a fan or something?

The prototype [Chris] has rigged up is only a 300W amplifier (180W RMS), but as you can see in the following video, it literally shakes his house.

After posting the original test video he received a lot of questions about it, so he released another Q&A video:

55 thoughts on “Rotary Subwoofer Combines A Speaker Coil W/ A Fan

  1. I think it would work. On my Chrysler 200, if I leave one back window cracked open a little, the whole car resonates at around 7Hz from wind buffeting. It’s quite uncomfortable.

    1. think it? looks like he’s using one. a complete 5-blade assembly from swash plate and up..

      and his blades aren’t aligned… what a pain that’s gotta be with 5 different one’s to track..

    1. It’s a fan, where the blade angles are controlled by the sound source. At zero amplitude the blades are flat (no air motion). High amplitude deflects blades further, which pushes more air. By varying the degree of blade pulsing you can control amplitude… changing the rate at which you flip the blades can alter frequency.

      You have to be sitting in front of it for it to work. Sit behind it, and the amplitude is reversed.

      1. Or, actually in this case, he’s blowing into a semi-sealed room. So the overall room pressure climbs and falls based on the fan’s overall efficiency. The room leakage acts as a DC blocker : ) For really high efficiency, the blades should be able to swivel in reverse, effectively sucking air back out of the room!

      2. Having encountered the commercial one, it doesn’t actually work like that. The frequencies it can go down to are effectively below what we can truly hear. If you stand in front of it, it feels like any other fan really. It has to push air into a mostly sealed room for you to notice anything, as you’ll then ‘hear’ the sound as it changes the air pressure in the room. It’s below the frequencies we can truly hear, but at the same time it gives some things a much greater depth, as say the sound of a helicopter close by you don’t just hear but also feel as if you were standing right next to a real helicopter.
        They are seriously amazing speakers and can add a lot of depth to movies or music by adding in the sounds you can feel as if you were there.

    2. the subwoofer controls the pitch of the fan blades, so the blades are either pushing or pulling air, which is controlled by the subwoofer. think of the subwoofer acting as a solenoid that controls the fan pitch. Essentially it’s replicating what a speaker does (move air) but by using a much more efficient mechanism for moving air (a fan). Since the pitch of the blades can only be moved so fast, it’s only effective for low frequencies (mechanical mass, inertia, harmonics).

      The fan creates negative or positive air pressure depending on the pitch. Changing air pressure=sound.

  2. Since I was a kid I wanted to find a way of making something that had response this low, I thought of different ways of sealing environments, porting to the outside, everything.. nothing would do it… but this… this is brilliant!

  3. I’ve been a big fan of the rotary subwoofer since I first learned about it. The price however is very prohibitive! I am very interested in this project and may consider pursuing my own build of a rotary subwoofer.

  4. The blades on Eminent’s rotary subwoofer are specifically designed for the job; they are not standard pitch fan blades. It also uses a vector drive to very tightly control the speed of the motor to ensure that the output is as pure as possible. There is a reason that the original product costs $12K.

      1. Ideally you have blades that are extremely stiff, else the transient response just goes down the drain and you introduce massive distorsion. Some esoteric ultra high carbon fiber percentage nailon might approach stiffness levels workable, but really should be carbon fibre sandwich.

  5. Oh noes. Do the neighbors laugh or complain about bangin me juilie? Oh I’m feeling all 7, 5 and 3 hurts. Did not enjoy. Unsure about how Zee Germans feelin bout this.

    1. That is a pretty odd argument. RMS power is a very useful calculation, and is a common standard used in many different industries. The idea that it “isn’t useful” because it isn’t equal to the heating power of a waveform is pretty much a non-sequitur.

  6. Forget dubstep, this is fanstep. A 1 horsepower 220 volt three phase motor can be bought pretty inexpensively, so can a 110 volt single phase to 220 volt three phase variable frequency drive.

    What I’d like to see someone try is a dual voice coil subwoofer using one coil as a shaping control with counter force to restrict over-driving and distortion from high power going through the other coil. Ie, when the sound signal hits maximum amplitude, apply a carefully shaped opposing signal to the other coil so the cone doesn’t overshoot.

    Should be possible with current electronics.

    1. I agree, it is freakin brilliant! I’m an avid DIY nerd, and a materials science consultant by trade, and I couldn’t quite reverse engineer the trw-17 by looking at it. You made it work, and solved several big problems beyond well enough, it works and all you need is a baffle to compensate for the “first draft imperfections”, which IMO are very minor. You have my full respect and I hope you keep it up, solve some other engineering problems, you seem to be put on earth to do exactly that!

      Public school will try to train everyone to hate learning and not trust themselves, which is obviously a load of crap, or some may say “it’s rubbish”. You probably are hooked on the rewards of dreaming something up, visualizing it, then rolling up your sleeves and making a bunch of mistakes, fixing a few, getting stuck, getting stuck some more, finding inspiration and getting past the huge wall roadblock impossibility, then smiling bigger than ever and impressing yourself with your creation, and in your case, making the videos [I still have not done the same, soon though]. Anyway, watching your videos, take it from someone qualified to make the assessment: you have what it takes, 100% sure. I solve the difficult technical problems nobody who works there was willing or able to, so they hire me for. Sometimes I walk into a room full of smart people, engineers or experts usually greet me with these looks like it’s okay to fail, we know you will fail, we just need to prove to the board it’s impossible. I get stuck and overwhelmed every time. But I nearly always find a way! There are 2 types of people in the world, excuse writers and problem solvers. You know which you are. I’m almost never impressed. It’s not the thing you built as much as how you talked about the process of it, how you experienced the project. Meaning the lazy beginning, the inspiration without a plan, the self induced kick in the rear to get going, and then the magic. This is what it’s like for me every time. It’s earned me an international patent, several awards, membership at ASTM because of a project I did, etc.

      Here’s what I would do to improve it to world class levels: 1. the impeller must be nearly weightless and very rigid [aluminum honeycomb & carbon fiber sandwich, or balsa wood and CF, or kevlar honeycomb and CF] 2. separate the voice coil from the mechanical impeller adjusting, use a sensor and servo pair [an even lower powered amp driving a smaller voice coil and convert it to 4 to 20mA or clock ticks for digital then use these values to send commands to a servo that can easily keep up with these low frequencies] 3. use a linear shaft and good quality bearings to stabilize the fan from any off-axis deviation, if it spins dead-steady your baffle can be a single u-turn with hardly any foam and you will hear nothing. 4. brushless motors are the way to go, smaller, quieter, and more precise control can be used [you can fine tune it] 5. for true mastery, make a miniaturized one for gamers, fitting the entire thing in 15x15x15cm or less!

      You inspired me to work on this also, I think pounding techno is begging for it, so I hereby volunteer!

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