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Subwoofer-shake cancellation theory...
Subwoofer-shake cancellation theory...
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Old 14th March 2019, 11:37 AM   #1
wolf_teeth is offline wolf_teeth  United States
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Default Subwoofer-shake cancellation theory...

I know a lot of people use dual subwoofers in an opposed assembly to cancel out vibrations. Since some high-Xmax drivers get very pricey very quickly, is it possible that using some active bass shakers as an opposing oscillating mass would be a more cost effective method of cutting down on the subwoofer-shake syndrome?

Most of the time, the cabinets are heavy enough to keep the subwoofer in check, but with the new breed of long-throw high-mass power-hungry mongrels, the boxes really don't have to be very big or stout to accomplish the performance goals- unless you want it to sit still as well.

Here's the theory as to why this would work. Bass-shakers utilize a backwards assembly vs the standard/typical subwoofer driver. The shaker uses a fixed coil and an oscillating mass. Typically a shaker will likely have a higher mass to velocity ratio than the subwoofer. I figure if keeping the motion opposed in subwoofer vs shaker, the active cancellation will actually reduce the vibration of the cabinet in relation to the room and its relative position.

Please- discuss this! I'm looking for any reasonable thoughts on this matter.
Thanks,
Wolf
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Old 14th March 2019, 11:42 AM   #2
wonderfulaudio is offline wonderfulaudio
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Isnt it better to use 2 smaller drivers on opposite sides? The extra coil area comes free. Better dynamics.
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Old 14th March 2019, 11:56 AM   #3
wolf_teeth is offline wolf_teeth  United States
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Not when you already have a larger driver.
Wolf
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Old 14th March 2019, 02:41 PM   #4
kipman725 is offline kipman725  United Kingdom
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Its an interesting idea but I don't think it would ever be cost effective. It would be most effective applied directly to the rear of the speaker magnet. The problem I see if that making the bass shaker move the right amount is very difficult as the drivers excursion varies with frequency and so does the bass shakers. So if its adjusted to cancel one frequency it will not match at another. So for it to work the shaker will require its own amplifier and EQ network. However at extreme excursions (the worst case) there is non linearity in both systems for which the linear EQ won't take into account. In the double driver all this complexity is handled automatically by the symmetry. I would just sell the larger driver and get two smaller if two larger drivers were too big.
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Old 14th March 2019, 03:58 PM   #5
Galu is offline Galu  Scotland
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The force (F) on a voice coil (and hence on the reaction mass) is directly proportional to the current (I), length of wire (L) and the magnetic flux density (B).

Therefore, two opposing identical drivers driven by the same current will produce equal and opposite reaction forces. Whether they can be said to 'cancel' is another debate!

However, a non-identical combination of tactile transducer and woofer will not generate equal and opposite reaction forces since the magnitudes of L and B will not be the same for both transducers.

Discuss!
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Old 14th March 2019, 04:43 PM   #6
TBTL is offline TBTL  Germany
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Agree with kipman725. Something like MFB might be possible to avoid the extra channel and non-linear behaviour, though MFB is not for free either.


A shaker can be useful when a symmetric layout is not possible. One example is the Sunfire in-wall / on-wall subwoofer range SubRosa | Sunfire .
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Old 14th March 2019, 05:17 PM   #7
CharlieLaub is offline CharlieLaub  United States
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Quote:
Originally Posted by wolf_teeth View Post
I know a lot of people use dual subwoofers in an opposed assembly to cancel out vibrations. Since some high-Xmax drivers get very pricey very quickly, is it possible that using some active bass shakers as an opposing oscillating mass would be a more cost effective method of cutting down on the subwoofer-shake syndrome?

Most of the time, the cabinets are heavy enough to keep the subwoofer in check, but with the new breed of long-throw high-mass power-hungry mongrels, the boxes really don't have to be very big or stout to accomplish the performance goals- unless you want it to sit still as well.

Here's the theory as to why this would work. Bass-shakers utilize a backwards assembly vs the standard/typical subwoofer driver. The shaker uses a fixed coil and an oscillating mass. Typically a shaker will likely have a higher mass to velocity ratio than the subwoofer. I figure if keeping the motion opposed in subwoofer vs shaker, the active cancellation will actually reduce the vibration of the cabinet in relation to the room and its relative position.

Please- discuss this! I'm looking for any reasonable thoughts on this matter.
Thanks,
Wolf
The problem you will encounter, I believe, is that the shaker and driver will have different motional behaviors vs frequency and these would have to match to have any hope that the forces will cancel out and leave you vibration free. In the case of two opposing drivers the motional forces will be very similar although even then not necessarily identical (because of TS parameter variations). With the shaker, there will be at some frequencies a large difference in the motion of the moving masses. I think you will be lucky to even get partial cancellation.

It *MIGHT* be possible to create a "servo" circuit and use an accelerometer to sense motion, and then activate the bass shaker as necessary to trim out any motion... ouch, that is complicated. Why do that when there is a much simpler solution at hand: buy another identical driver!

Here are some other approaches if you still want to use only one driver:

(A) If you are stuck (for budgetary reasons, or only one driver exists, etc). then design the cabinet to be "long" along the motional axis of the cone. This will provide a larger bass of support preventing tipping/rocking of the cab.

(B) Vibration is a result of mass reaction between driver and cabinet. Make the cabinet very heavy by adding something like a granite top to it (expensive!) or just a bunch of bricks from Big Lots in between two scraps of plywood. The greater the difference in mass between driver moving mass and cabinet, the less the amplitude of the cabinet's vibration.

(C) Use decoupling. Build/use something like a table that sits just above the sub and hang the sub with rope or chain connecting the table top to the bottom of the sub cab (you want the length of cable to be as long as possible). This won't reduce the reactionary movement that causes "vibration", but it will decouple the sub from the floor and THAT will eliminate vibration. Instead the sub may have a slight swinging movement as driver mass and cabinet move towards/away from each other. You can also add mass as in (B) to reduce the amplitude of this swinging movement. Make sure suspension (rope/chain) is rated for the load...
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Old 14th March 2019, 05:30 PM   #8
Galu is offline Galu  Scotland
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The information on the Sunfire site leaves me wanting to find out more!
Quote:
"As a result of our all-new, patent-pending StillBass technology, the SubRosa combines a powerful subwoofer design with the vibro-tactile to produce prodigious amounts of bass while virtually eliminating destructive wall vibrations."
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Old 14th March 2019, 06:14 PM   #9
Mr Evil is offline Mr Evil  United Kingdom
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How about a driver where the magnet is mounted so that it is free to move axially in the same way as the cone is. With enough compliance, the reaction force would be entirely absorbed by the magnet, with none transmitted through the frame to the enclosure. A little bit of power would be lost in moving it, but not much given the large mass of a magnet compared to a cone. Working out how you would create a surround/spider strong enough to hold a subwoofer magnet is left as an excercise for the reader.
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Old 14th March 2019, 08:06 PM   #10
TNT is offline TNT  Sweden
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I think the two opposing drivers need to be very symmetric in order to cancel. A bass shaker will not mechanically look like your bass driver and therefore not cancel. That would be my best bet.

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