Push one speaker in, the other goes out

[Markw4]

Yet another way to explain: https://www.electrical4u.com/fleming-left-hand-rule-and-fleming-right-hand-rule/
Also: https://en.wikipedia.org/wiki/Fleming's_left-hand_rule_for_motors#:~:text=Fleming's left-hand rule is,one were to create electricity.
https://en.wikipedia.org/wiki/Fleming's_right-hand_rule

 

[Onemangang]

I was wondering how much voltage/current is actually generated by a typical 3 way speaker system on the input terminals.
Besides the drivers, we also got filters where some of the components are in parallel with the back emf.
Is there any way to determine how much back emf a speaker generates in total for a given power input?
How to measure it?

 

[Markw4]

You could probably measure at the speaker terminals with a scope. If you pulse the speaker with a (voltage limited for safety) current source, at the end of the pulse whatever voltage appears on the speaker terminals should be coming from the speaker. Different pulse widths may produce somewhat different responses. Of course be careful not to damage the speaker in the process.

 

[keantoken]

I tend to think of this in terms of back-EMF (actually MMF). When delivering a positive signal to the speaker, if the cone moves forward then it produces a MMF which opposes the input voltage. The speaker cone is unloaded therefore due to conservation the voltage source will also be unloaded (to the degree allowed by the conversion efficiency). To produce the same MMF without an input voltage therefore you must tap the cone in the same direction, from behind.

So tapping on the front of the cone produces a negative MMF and if another speaker is connected across it in the same polarity, that speaker's cone will move backward instead of forward.

 

[JMFahey]

All the current in a series circuit flows in the same direction (say, CW or CCW). Its only the voltages across the drivers that will be in opposite phases.

But voltages will NOT be in opposite phase, why would they?
"All voltages in a perfect conductor are the same", by definition.
So if + terminals are joined by a conductor, they will always be,at same voltage, no matter what.
Same with the - terminals.

So say I push one cone backwards, it will generate negative voltage between its terminals.
Exact same voltage will appear at matching terminals in the second speaker, so it will also move backwards.
Read my earlier post saying so.

 

[JMFahey]

Okay, I just did an experiment too. I connected the + end of a 1.5v AA cell to one terminal of a speaker (let's call it the red speaker terminal) and the - end of the battery to the other speaker terminal. The cone moved in.

Then that speaker is marked wrong, at least by the usual convention
+ Voltage on the Red must move it forward.

There was also a DVM attached to the speaker terminals with the same polarity as the battery (red meter lead to the + end of battery, and to the red speaker terminal). Then I disconnected the battery negative terminal. Pushing in on the cone produced a + voltage on the meter (again, the red terminal).

Again, wrong marked speaker.

While I let the cone return out to the rest position the meter showed a negative voltage.

Correct.
Opposite voltage because it's moving the opposite way.

Thus what changed was the direction of current through the speaker when it is a generator. The voltage polarity for the cone moving in stayed the same in both cases.

Therefore if another speaker were connected in parallel, while I was pushing in the cone on one speaker (while it was in motion), a + voltage would be applied to the corresponding (red terminal) of the other speaker. IOW, the other speaker should move in too.

Both speakers will have terminals at same relative voltage (+ to - markings) so will move in same direction.

 

[JMFahey]

I posted both above answers as I was rereading this old thread from the beginning.
Now I see all doubts were fully cleared beforehand so these are unnecessary/redundant.

In any case it's nice to see that although starting from opposite ends of the tunnel (current vs. voltage, motor vs. generator) in the end we all met and agreed in the center. 🙂

 

[JMFahey]

I was wondering how much voltage/current is actually generated by a typical 3 way speaker system on the input terminals.
Besides the drivers, we also got filters where some of the components are in parallel with the back emf.
Is there any way to determine how much back emf a speaker generates in total for a given power input?
How to measure it?

Trivial solution: by measuring it.
Already mentioned I commercially make speakers and my go to quick test after magnetizing is to move cone back-forth a lot and hard with my fingers, with a multimeter clipped on.

On 200 mV scale it only registers a few tens of mV .
I actually look at the +/- sign, go figure.

 

[MrKlinky]

Using a 21" speaker to 'drive' a low Fs 6.1/2" one makes the cone movement easy to see.
They both move in the same direction - counterintuitively to what my brain is screaming at me!