# Push one speaker in, the other goes out

#### presscot

I have been told that if you have two cone drivers. Put them in their individual “closed” boxes. Then, wire them together in parallel. If you press cone of one driver inward, cone of the other driver, despite locating in different box, will move outward. Is this true?

#### Galu

Well, let's see!

Pushing the cone of one speaker will generate a voltage which will then generate a movement of the second speaker cone.

If the voltage induced current comes out of the positive terminal of the pushed speaker, it will enter the second speaker by its positive terminal.

Does that mean the second cone will move in the opposite direction to the pushed one? I think we need a diagram!

#### jan.didden

Paid Member
They way you describe it (which I think is correct), the direction of current in the 2nd driver is opposite to the direction of current in the pushed one, so the 2nd should move in opposite direction.

Jan

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#### DonVK

You may have an air leak between the chambers. I use a 2S+2P woofer arrangement with 4 separate chambers. Pushing in one (slowly by hand) does not move the others if the chambers are sealed.

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#### tmuikku

If circuit is closed, input terminals shorted or low impedance amplifier connected, then there ought to be some current in the circuit when pushing a cone. Voice coil in magnetic field should induce voltage (back-EMF), which makes into current as long as circuit impedance is low. The current would be such that opposes the movement of the driver that is being pushed, so the other driver in same circuit in series with the woofer ought to move opposite direction as long as enough current happens.

If push is slow nothing should happen, inductance depends on frequency. If one pushes it 1kHz fast then why not, perhaps less is fine, as long as the push is fast Also, nothing happens if the woofers are connected in parallel and the (speaker input) is shorted with a very low impedance like an amplifier or just cable short, it would short the other driver, all current runs through there and not through the other woofer. If woofers are parallel, leave the speaker input open.

Test rig, connect two low impedance drivers together and nothing else, they are in series now and best case scenario to make it happen.
What if you talk to the other, can you hear it coming out from the other?

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#### Davey

I have been told that if you have two cone drivers. Put them in their individual “closed” boxes. Then, wire them together in parallel. If you press cone of one driver inward, cone of the other driver, despite locating in different box, will move outward. Is this true?
If wired in parallel (assuming plus-to-plus and minus-to-minus) you'll find the cones move in the same direction.
However, I suggest a quick practical demonstration to prove it to yourself.

Dave.

2 users

#### Galu

Ah! There is dispute amongst the ranks!

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#### Galu

I just tried it, but only with two small radio type speakers I had handy.

There was no perceptible movement of the second cone.

We may need a bigger pair of drivers!

#### jan.didden

Paid Member
If wired in parallel (assuming plus-to-plus and minus-to-minus) you'll find the cones move in the same direction.
However, I suggest a quick practical demonstration to prove it to yourself.

Dave.
Why is that Dave? Since one is the generator and the other the load, the current in them runs in opposite directions.
That should move them in opposite directions.

Jan

#### Davor D

If wired in parallel (assuming plus-to-plus and minus-to-minus) you'll find the cones move in the same direction.
However, I suggest a quick practical demonstration to prove it to yourself.

Dave.

Yup.
Electrodynamic speaker is basically a linear electric motor.
If you apply positive voltage to plus terminal, cone will move forward. At the same time, back-EMF is generated wich has same polarity as applied voltage.

Now, two drivers are connected in parallel, plus to plus, minus to minus.
If you push the cone of one driver in, thats backward movement and induced EMF will be in opposite polarity.
So, second driver will be fed with opposite polarity on its terminals and it will also move backwards.

But, efficiency of electrodynamic speaker is very low, so I doubt that movement of second driver will be visible.

1 users

#### Ixnay

This is a speaker driver interaction that I have considered before. FWIW, I doubt that the back EMF has much if any real effect for the playback of music, correct? And this back EMF is something that the amplifier sees as well. Probably of little consequence though.

#### planet10

Paid Member
Pushing the cone of one speaker will generate a voltage which will then generate a movement of the second speaker cone.

I use the same a little different. When i ship finished loudspeakers i short the terminals to add a brake to any cone movement.

dave

1 users

#### Galu

If you apply positive voltage to plus terminal, cone will move forward. At the same time, back-EMF is generated wich has same polarity as applied voltage.

Won't the back EMF oppose the change causing it, and therefore be of the opposite polarity to the applied voltage?

#### nigelwright7557

I use the same a little different. When i ship finished loudspeakers i short the terminals to add a brake to any cone movement.

dave
They do the same on car windscreen wiper motors.
They short the motor out otherwise wiper slowly moves up the screen due to wind.

#### Nico Ras

Not if you have a Ferrari with little spoilers on them

1 user

#### GM

Well, I've got dual 15" super high Vas, low Qt drivers in acoustically large vented boxes, but not connected to an amp and any perceptual inward movement of the top one makes the lower one move outwards and just as quick settles to rest, presumably because it's vented.

1 user

#### PetruV

When I was a kid,I took a long piece of telephone wire and hooked up a pair of small speakers together,then I cupped one speaker with my hands and spoke into it,at the other end,the speaker reproduced my voice quite clearly,all the kids thought I was a wizard or something,Later in life,when we took apart our pa speakers,we did the same with the hf horns,my mate,was on the other end,awaiting transmission with the horn pressed to his ear,I decided it would be funny to dig my face into the horn and scream as loud as I can.You can guess how that went

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#### JMFahey

If you push a speaker backwards (cone towards magnet system), the "+" marked terminal will become negative (relative to the other).
A similar speaker, connected in parallel, will also get that negative voltage and also move backwards.

So both will move in phase, whether forwards or backwards.

.

1 user

#### Davey

Well, I've got dual 15" super high Vas, low Qt drivers in acoustically large vented boxes, but not connected to an amp and any perceptual inward movement of the top one makes the lower one move outwards and just as quick settles to rest, presumably because it's vented.
The OP specified the woofers were in separate boxes.
In your case the woofers are in the same box.....so, a different configuration....even though the box is vented. In that case the momentary air pressure differential is somewhat greater than the electrical drive, and thus you see the woofers moving in opposite directions.

The OP's query turned out to be more thought-provoking than it might seem, at first glance.

Dave.

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#### Davey

This is a speaker driver interaction that I have considered before. FWIW, I doubt that the back EMF has much if any real effect for the playback of music, correct? And this back EMF is something that the amplifier sees as well. Probably of little consequence though.
Yes, pretty much inconsequential. Most amplifiers nowadays are pretty close to voltage sources, so the drivers "look back" into something that appears very much like a short circuit.
In certain cases though the back EMF can find its way into the feedback loop of an amplifier and cause some weird happenings.

Dave.