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Shorting speaker terminals

S

Superampman

AK Member
Okay, for the life of me I can not find a single entry addressing this topic on google. No matter how I word it, the same results come up. I didn't want to pester you people about something as mundane as this but it looks like I have to. We all know if you place a piece of wire across the terminals of a driver (completing the circuit), it serves as a brake, locking the diaphragm in place which is useful in protecting it during transportation. My question is does this set up a DC current 'flow'? What does the "brake" consist in?
 
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It's a brake because the wire across the terminals acts as a zero ohm resistor. When a dynamic driver's diaphragm isn't in its rest position, it acts as a generator (coil of wire in a magnetic field; this is how a dynamic microphone works). It's essentially trying to generate a high amount of current due to the extremely low resistance path (i.e. a short circuit). Just like a standard generator with a high load, that causes the coil to resist the movement in the magnetic field, because it's trying to do more work than it's capable of. I'm sure there's a more scientific way of describing it, but that's my general understanding.
 
I played around with this once. I shorted the binding posts on some speakers because they were in a listening room, yet unused. I didn't want them radiating while the other speakers played.

Well, it didn't work worth a crap as they still radiated all the same.
 
Superampman said:
Exactly, so is there current flow through that zero ohm resistor? Perpetually? I suppose by induction?
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No, there's no current flow, or hardly any at all.
swechsler said:
When a dynamic driver's diaphragm isn't in its rest position, it acts as a generator (coil of wire in a magnetic field; this is how a dynamic microphone works).
Click to expand...
No, that isn't correct. Being offset from the rest position isn't what generates the voltage, it's motion through the magnetic field which generates the voltage.

This is how the brake works:
When the speaker's cone is caused to move by soundwaves impinging on it, the motion of the voice coil through the magnetic field produces a voltage at the speaker's terminals. No current can flow if the terminals are open circuit, but there's a voltage there.
If you place a short circuit across the terminals a current can flow when the cone is caused to move, but the flow of current produces a magnetic field around the voice coil which interacts with the permanent magnetic field and opposes the motion of the diaphrgm. The effect is called "back EMF".

If you google "back EMF" you should get plenty of results. An amplifier with a very low output impedance has the same effect as a short circuit across the speaker terminals.

Superampman said:
Exactly, so is there current flow through that zero ohm resistor? Perpetually? I suppose by induction?
Click to expand...
Actually, yes current does flow, but only when something causes the speaker cone to move.
 
Superampman said:
Okay, for the life of me I can not find a single entry addressing this topic on google. No matter how I word it, the same results come up. I didn't want to pester you people about something as mundane as this but it looks like I have to. We all know if you place a piece of wire across the terminals of a driver (completing the circuit), it serves as a brake, locking the diaphragm in place which is useful in protecting it during transportation. My question is does this set up a DC current 'flow'? What does the "brake" consist in?
Click to expand...

Speakers work by AC current, not DC. Strapping a wire across the speaker terminal will not act as a brake (or short). as Rothwellaudio points out, AC current will still continue to be made (back-current) --- as long as the speaker cones are allowed to move.

Practically speaking, I would not worry about this small back current --- as the speaker excursions will be very small, even in the roughest of transport (at least compared to when they are in connected to an amp and cranked).

What maybe important in tube amps, is to short out the secondary leads from the OPT, if the speakers are disconnected. Just in case the amp get turned on, without seeing a speaker load.
 
Mr Boochie said:
Strapping a wire across the speaker terminal will not act as a brake...
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Yes, it will. The short circuit allows current to flow, and the flow of current creates the back EMF which opposes the motion which caused the current in the first place.
Anyone can test this. It's probably easier with a drive unit in free space but it can also be done with complete speaker/enclosure. Just tap the cone lightly with your ear close to it. You should hear a bit of a "bong" sound, something like a drum. Then put a short across the terminals and tap the cone again. You should hear that the "bong" has become more of a dull thud.
 
Loss factor has not yet been mentioned. Drivers are inefficient at converting Electrical to Acoustic energy, many being only 1 to 2 percent efficient. The reverse is true.

If they were 100% efficient a dead short would cause a woofer to feel nearly glued into place. Consider Thevenin's Theorem; connected to a dead short, any current produced by an 8 Ohm source will be consumed by it's own internal loses. This is why shorting is effective in theory but rarely realized in practice.
 
I'm not sure what speaker efficiency or Thevenin's theory have to do with it. The OP's question was...
Superampman said:
My question is does this set up a DC current 'flow'? What does the "brake" consist in?
Click to expand...
Yes, DC flows as long as the cone moves in one direction. When the cone moves in the other direction the DC will flow in the opposite direction, ie it's really AC.
The "brake" is produced by the back EMF.
 
z-adamson said:
I played around with this once. I shorted the binding posts on some speakers because they were in a listening room, yet unused. I didn't want them radiating while the other speakers played.

Well, it didn't work worth a crap as they still radiated all the same.
Click to expand...
Lol ... myth debunked.
 
I would think the "braking" would be negligible after running through the crossover components. Directly shorting the individual driver elements would almost certainly be more effective- but even then I doubt it would make much difference in a room or in transport.
 
62caddy said:
I would think the "braking" would be negligible after running through the crossover components. Directly shorting the individual driver elements would almost certainly be more effective- but even then I doubt it would make much difference in a room or in transport.
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If you have a loose woofer available, try it.

Just try pushing on the cone, with, and without the terminals shorted.

I think the results will surprise you, even with a woofer installed in a speaker system. The ESR of the LF crossover section will have a small, but mostly negligible effect.
 
It seemed to me that this is way too easy to test, rather than speculating about what might happen, so I did just that. I shorted the speaker terminals on my almarro M1A speakers, and observed a noticeable difference in both the effort required to deflect the cone, and the range of travel. Without wires, the cone floated back quite easily, to what felt like a mechanical stop. Shorted, the cone presented more resistance, and traveled only a very short distance before it was asking for more push, in order to move, than I was comfortable with using.

QED
 
A brush type permanant magnet DC motor will practically lock the rotor if the wires to the brushes are shorted together and the rotor is rotated.
It was common on the early CNC machines to short all the motors when E-Stop was engaged in case of an axis runaway. As was stated, it's back EMF in action.

BillWojo
 
Interesting observations. Remember that there is a major difference between shorting driver terminals and shorting loudspeaker terminals. If passive crossovers are present, the inductors will absorb generated EMF current and convert it into heat. Shorting the terminals beyond the crossover will surely have little effect.
 
This thread is getting a lot deeper than I thought it would. Interesting though.

I usually have unused speakers in the same room as my main system. I’ve heard about shorting the terminals on the unused speakers, but haven’t really played around with it.

So then, what happens if you have speakers connected to an amp, but not powered on? And another pair of speakers and amp that are playing. The amp must provide some kind of braking for the unused speakers. Or at least a place for the generated voltage to go.
 
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