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Rotary Subwoofers


ElliottDesigns
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I've seen some interesting posts about rotary Subwoofers and very few videos on them. I was wondering if anyone has seen, heard/felt one in person, or even made one? If so, what are your impressions, are there any difficulties in getting one to run properly, or issues with high noise levels created from it effectively being a fan? I'm interested to hear!

Elliott Dyson - Mechanical Engineering Student and 3D printing & Design Freelancer


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123Toid
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I have not personally heard them. But one of the issues with rotary subwoofers is the fact that they only go so high in frequency. I can't remember off hand, but I'm thinking something like 30 to 40 HZ. So they're good from like 1 Hertz to 30 to 40 HZ. It's pretty amazing really. But you'll definitely need a secondary subwoofer to cover up higher. Also I know a lot of PA amplifiers have built-in High passes around 20 Hertz. So you need to find an amplifier that is true full range.

https://www.youtube.com/123toid


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ElliottDesigns
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@123toid

Thanks Nick, I heard that too. I think it has to do with the fact that the blades have to be rotate to create the positive and low pressure for the needed frequencies, and that the wider the blades sweep outwards (to a certain point) the louder the sound, since the spin of the blade system would be more difficult to control I assume. If the blades sweep out further (for more volume) then it means there is further for the blade to turn for that shift in pressure. I think this means that the quieter it operates the higher in frequency it can go (apart from over limiting factors such as accuracy of course). This is just what I believe to be true from 1st year fluid dynamics in engineering so I might be wrong, but I think that's all I know about rotary subs.

The amplifier part is an interesting factor, maybe a bass shaker amplifier would be well suited?

Another thought that occured to me is, would it need some sort of enclosure/separation between the front wave and back wave? I would have though so.

Elliott Dyson - Mechanical Engineering Student and 3D printing & Design Freelancer


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chedwin
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If you are looking for something that can produce infrasonics down to 1HZ but want to keep normal subwoofer ability to play up to 120hz in a single unit a better option than rotary subwoofers would be the powersoft M-Force linear motor transducer, they are very interesting and unlike any other driver on the market. In fact they arent really a driver at all by the standard definition.

They move a magnet inside a static voice coil, have mindblowing cone acceleration and the t/s parameters are largely configurable by you to suit the enclosure its housed in

https://www.powersoft.com/wp-content/uploads/2019/01/powersoft_M-Force_data_en_v3.0.pdf

although powersoft spec it down to around 16hz they absolutly can go down to 1hz as demonrated in this video by dave rat, FOH engineer for bands like red hot chilli pepers

 

On the topic of high pass filter in PA amplifiers. alot of PA amps will be specced 20-20k -1db. that doesnt neccesarily mean it cant work below 20 just that they arent measured below 20 so will roll off at an unknown ammount, they could go down to 5 or they could stop at 18 and its pot luck which youll get.

Higher end DSP amps can be specced down as low as 7HZ -2.5db but these cost alot more, likely in the area of £3500+ for only 2 channels

Josh Evans - Professional Live Sound Engineer


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123Toid
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@elliottdesigns  You are correct. They are typically housed in a closet with an opening into the main room, just big enough for the fan blades. 

https://www.youtube.com/123toid


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123Toid
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@chedwin

That thing is awesome!  What a beast!  That would be fun to play around with for sure.

As far as the PA amplifiers, I was referring to the main ones used in Home Theater (the crowns and Berhinger).  Both of those have a 20hz high pass that is built-in.  There was a Chinese off-brand that offered no high pass, but I can't recall the name.  Having said that, I haven't kept up with the new Berhinger amplifiers, so it is possible they do not have one. 

I can't even imagine buying a $3500 amplifier for my subwoofers. That is insane!  It sure would be cool though.

https://www.youtube.com/123toid


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chedwin
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@123toid

I hadn’t realised the amps were high passing at 20hz, although to be honest I’m normally using hired much higher end amps made in house by the same company the line arrays I use comes from.

 

looking into it it seems they are often using -12/octave filters probably to cheap out on the size of capacitor bank given that more and more power becomes required the lower you go. Although as previously mentioned you still get some output below 20hz. That said if you are using one of those amps enough below it’s rated power e.g. 2000W amp on a 500W sub I don’t see a reason electrically you couldn’t very carefully eq around the high pass on the way into the amp. DISCLAIMER-DO THIS AT YOUR OWN RISK

Josh Evans - Professional Live Sound Engineer


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