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Two magnet speakers

A stiff membrane witha coil and magnet is a speaker; placing the stiff membrane between two magnets could up efficiency
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One way of thinking of an audio speaker is like this

M] 3||

The M] is a magnet, and then next to it is a coil (3) near a stiff membrane (||)

What if the stiff membrane was placed between two magnets and two coils

M] 3 || 3 [M

that way the stiff membrane could be driven on both sides, creating greater volume, and possibly enhancing energy efficiency. Greater efficiency at headphones would cause things like iPhones to function longer between electricity refills.

beanangel, Feb 10 2017

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       I don't see where the greater efficiency is coming from.
MaxwellBuchanan, Feb 10 2017
  

       Instead of two magnets, one each side, you could use a single double-acting magnet, capable of both attraction and repulsion
pocmloc, Feb 11 2017
  

       If you had two magnets, they'd interact. How would you arrange them to overcome this? You'd position them inline + pole to - pole along the same axis. In that configuration, they'll try and attract. The other way round, they'd try and repel, so doing that is not a brilliant idea. If you've got two similarly configured + -   + - axially aligned magnets, you might as well just have a long single one that takes the same room. The two voice coils could operate in antiphase around either end of the single longer magnet.   

       If you've got two antiphase voice coils, you'd require twice the energy to operate them, and you'd also get differential bias errors unless you used quite accurately selected and matched output transistors to drive them. If you drove them with a differential pair, in push-pull configuration, this might work well, if you could overcome the crossover distortion through accurate biasing. However, it might prove more efficient to simply connect the two voice coils together in series. In which case, you might as well just have one longer voice coil.   

       Now you have a longer single magnet, and a longer single voice coil. This will require more energy to operate, not less (or fewer).
Ian Tindale, Feb 11 2017
  

       Dual-voice-coil drivers are real, but a loudspeaker needs to couple the generated mechanical energy into the air somehow, preferably without a magnet getting in the way.
Wrongfellow, Feb 11 2017
  

       [+] Magnetic strength decreases with distance. With two magnets, when the push from one drops off, the pull from the other increases. Same effect as two coils bracketing one magnet, but the cone assembly is lighter.   

       //getting in the way// not an issue; put a cap in the cone, as is done currently.
FlyingToaster, Feb 11 2017
  

       Has anyone considered moving the listener back and forth instead of the speaker diaphragm?   

       //Magnetic strength decreases with distance.// I'm not sure that applies, with the right geometry and over the distances involved.
MaxwellBuchanan, Feb 11 2017
  

       I remember back in the late 70s there was a newish type of active speaker called Motional Feedback, from Philips. It used a sensor on the woofer as part of a feedback loop to the amplifier (being active, the amplifier was integral to the speaker). It could have used a second coil but the main voice coil would cause induction effects distinct from the action of the sensor coil. In the end they used piezo sensing, directly on the cone.
Ian Tindale, Feb 11 2017
  

       The eficience would be the same as a double sized coil inmersed in a larger magnet gap in a standard loudspeaker.. Ohh, Ian already note that. With one coil and one magnet at the front of the device, how do you install a cone or something to convert movement to air pressure ?
piluso, Feb 12 2017
  
      
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