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
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
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.
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!
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!
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 .
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 .
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...
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.
My other thought was to use a 120/12 transformer from the original signal, and step a 500W amp to 50W for a perfect power and arrangement without needing another amplifier. Then it was just if the drivers would oscillate in tandem where it would be feasible.
Still- I hadn't thought of this before,
Wolf
Still- I hadn't thought of this before,
Wolf
It's possible.
What, I suspect, they're doing is this:
- Pick a driver/cabinet combination that produces constant excursion with decreasing frequency.
- Add a tactile transducer which, when driven, also produces constant excursion with decreasing frequency.
- Set the mass of the tactile transducer so that the vibrational forces become equivalent. NB - not necessarily equal to the Mms of the speaker
- EQ the feed to the driver AND tactile transducer so that the LF response is flat.
In theory, that'd work until things go non-linear.
Doesn't work for any kind of resonant enclosure, either.
Chris
Thanks Chris!
I'm still leery of this 'force cancellation' idea.
Look at how a loudspeaker works. The force exerted on the voice coil/cone is accompanied by an equal and opposite reaction force on the magnet/basket of the speaker.
Regardless of the relative masses of the cone assembly and magnet assembly, each experiences the same force.
Now, since force equals rate of change of momentum, both the cone and the magnet undergo the same change of momentum in the same time. Since momentum is mass x velocity, this simply means that the initial recoil velocity of the massive magnet is smaller than the initial forward velocity of the less massive cone.
Therefore, regardless of the relative masses, the forces will always be equal provided the magnetic parameters I, L and B are equal.
That's why I'm pondering over your statement, 'Set the mass of the tactile transducer so that the vibrational forces become equivalent.'
I'm still leery of this 'force cancellation' idea.
Look at how a loudspeaker works. The force exerted on the voice coil/cone is accompanied by an equal and opposite reaction force on the magnet/basket of the speaker.
Regardless of the relative masses of the cone assembly and magnet assembly, each experiences the same force.
Now, since force equals rate of change of momentum, both the cone and the magnet undergo the same change of momentum in the same time. Since momentum is mass x velocity, this simply means that the initial recoil velocity of the massive magnet is smaller than the initial forward velocity of the less massive cone.
Therefore, regardless of the relative masses, the forces will always be equal provided the magnetic parameters I, L and B are equal.
That's why I'm pondering over your statement, 'Set the mass of the tactile transducer so that the vibrational forces become equivalent.'
So that would translate to higher velocity when the mass is lower, to compensate and be equal in force.
Hard to combine this with audio signals.
Edit:
But maybe if you had 2 individual amplifier channels with DSP, one for the driver, and one for the bass shaker. Then it would be no problem using eq and phase shaping to get what you want. One would probably run out of "oomph" before the other, though.
Hard to combine this with audio signals.
Edit:
But maybe if you had 2 individual amplifier channels with DSP, one for the driver, and one for the bass shaker. Then it would be no problem using eq and phase shaping to get what you want. One would probably run out of "oomph" before the other, though.
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I'm quite familiar with how a loudspeaker works.
Lets go back to basics.
F=ma.
Let's say capital letters are for the loudspeaker, and lower case are for the tactile transducer.
All we want is sum(F) = 0.
Substitute and we get, MA+ma=0
Or, MA= -ma
Thus, we can see that a tactile transducer undergoing smaller accelerations (ie, smaller displacements for a given frequency) will need more mass in order to cancel vibrations.
F=BIL tells us the forces acting between the voice coil and the magnet. We don't particularly care about those forces.
Consider this - if we had a tactile transducer with the same motor as the main driver (ie, BIL is the same), but a very small moving mass, would it still be able to cancel out the forces being applied to the cabinet?
Chris
Lets go back to basics.
F=ma.
Let's say capital letters are for the loudspeaker, and lower case are for the tactile transducer.
All we want is sum(F) = 0.
Substitute and we get, MA+ma=0
Or, MA= -ma
Thus, we can see that a tactile transducer undergoing smaller accelerations (ie, smaller displacements for a given frequency) will need more mass in order to cancel vibrations.
F=BIL tells us the forces acting between the voice coil and the magnet. We don't particularly care about those forces.
Consider this - if we had a tactile transducer with the same motor as the main driver (ie, BIL is the same), but a very small moving mass, would it still be able to cancel out the forces being applied to the cabinet?
Chris
I thought this was what you meant, and that was what I replied to.
But now under the context of what you're saying: *higher* mass and less motion on the tactile transducer, then I completely agree.
I wonder where it would be best to have the transducer though, the opposite boundary does not seem like a smart choice, but in sum, it might be better than the rear of the magnet.
It does not matter that much as you are only controlling the rigid body motion of the enclosure. Modes for which the enclosure is flexible are at frequencies above the pass band of a subwoofer.
Forces acting on the basket of a driver (which are 'transmitted' to the enclosure) are not only caused by the motor, but also by the suspension.
Forces acting on the basket of a driver (which are 'transmitted' to the enclosure) are not only caused by the motor, but also by the suspension.
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