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what does the electrical current coming out of an amplifier for a sine wave look like?

post #1 of 16
6/23/11 at 4:38pm
Thread Starter 

so lets say we've got a sine wave that looks like this /\/\/\/\/\/\/\/\/\/\/\/\/\ aside from the fact that that is triangular, what "shape" are the electrical currents coming out of the amplifier? does the amp output only positive current when the pressure increases, so it'd look like (/+)(\-)(/+)(\-)(/+)(\-)(/+)(\-)(/+)(\-)(/+)(\-) or does it look like (^+)(v-)(^+)(v-)(^+)(v-)(^+)(v-)(^+)(v-)?

 

Hope my diagrams make sense.

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post #2 of 16
6/23/11 at 4:51pm

 

It looks lie this: /\/\/\/\/\/\/\/\/\/\/\/\/\

 

se

 

 

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post #3 of 16
6/23/11 at 5:11pm
Thread Starter 

I know it doesnt look like that. When your producing these waves, the driver should move back and forth from its resting point. Otherwise, the bottom of the troughs would need to be located at the drivers resting point. This would mean that to increase the amplitude by 1 cm, the driver's crests would need to be 2 cm further out which sounds inefficient, and generally wrong for another reason. If there is no negative current, e.g. the driver is only pushed outwards by the magnets, this means that you're relying on a constant force to pull the drivers backward to produce the rarefactions that you see in soundwaves. This is critically problematic for a host of reasons. Lets start simple, just killing the power at the crests would result in the drivers snapping back at a constant rate, producing a part triangle, part sine wave. I KNOW this doesn't happen. Now if you modulated the power on the push side of things to provide variable resistance to this constant force, thereby creating a sine wave, you'd have issues at volume. As you increase the amplitude of things, the driver is going to take longer and longer to snap back to its resting position, and will throw off the frequency. Assuming that the driver snaps back at a constant rate (it doesnt actually, but we're saying its linear for the sake of simplicity) when the height of the wave is double, so is the time it takes for the driver to snap back in place. there is simply no practical way to account for this sort of a problem in a pair of headphones. Not to mention that different headphones would have different constants and would likely be incompatible with many output devices. The simpler explanation is that there are, in fact, negative and positive charges working with the headphones. 

 

Now my question is, for a standard sine wave, does the output device output a positive signal after it hits the trough of a sine wave, then a negative signal after it hits the crest? or is it something else entirely.

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post #4 of 16
6/23/11 at 5:18pm
Quote:
Originally Posted by ffdpmaggot View Post

I know it doesnt look like that.


Then I'm afraid I can't help you.

 

Nor will anyone else with the correct answer.

 

se

 

 

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post #5 of 16
6/23/11 at 5:23pm
Thread Starter 

http://electronics.howstuffworks.com/speaker5.htm

does the animation accurately depict the electrical current from an amplifier?

 

If it does, I'm asking when the polarities switch, at the resting point, or the crests and troughs of the sound waves.


Edited by ffdpmaggot - 6/23/11 at 5:24pm
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post #6 of 16
6/23/11 at 5:49pm
Quote:
Originally Posted by ffdpmaggot View Post

http://electronics.howstuffworks.com/speaker5.htm

does the animation accurately depict the electrical current from an amplifier?

 

If it does, I'm asking when the polarities switch, at the resting point, or the crests and troughs of the sound waves.


The animation is depicting voltage, not current.

 

Voltage changes polarity at the 0 volt point, which is the speaker's resting point. The direction of current flow also changes at this point. And the current waveform looks just like the voltage waveform.

 

se

 

 

 

 

 

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post #7 of 16
6/23/11 at 6:03pm
Thread Starter 

Okay, I had a issue with terminology there, thank you for clearing that up. Now I'm not *doubting* you per se, because you have more experience than i do; but, in that animation, assuming that the 0 point is at the speaker cones resting point, the voltage flips to be negative, and lets assume the magnet is positively charged on that end. The voice coil should just stick to that and not move from that point until the voltage flips to be positive, thereby repelling the coil from the magnet. This sounds wrong to me. If you could detail the steps a little more, id be really grateful, in the mean time, im going to log off for the night, so dont expect a response if your one of the impatient types.

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post #8 of 16
6/23/11 at 7:00pm

 

Ok, here's what you need to understand how this works.

 

With no signal present, and therefore no current flowing through the voice coil, the speaker's cone is at its point of equilibrium.

 

Now let's apply a sinewave, starting at 0 volts, and let's assume we're starting out with the voltage polarity being positive.

 

As the voltage increases, so does the current through the voice coil. The current through the voice coil creates a magnetic field of its own, which interacts with the static magnetic field of the speaker's magnet assembly. Since the voltage is positive, the direction of the current through the voice coil creates a magnetic field of a polarity which pushes forward from the point of equilibrium and subsequently pushes the cone outward.

 

This continues until the voltage has reached its peak, after which the voltage diminishes, as does the current and therefore the force of the voice coil's magnetic field, allowing the cone to return to its point of equilibrium.

 

Once the voltage drops down to 0, the polarity changes and the voltage increases with a negative polarity. Because the polarity has switched, so does the direction of the current through the voice coil as well as the polarity of the magnetic field produced by the voice coil. So now instead of the cone moving forward from the point of equilibrium, it now moves backward, and simply repeats the same process as before.

 

Hope this helps.

 

se

 

 

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post #9 of 16
6/23/11 at 11:36pm
Good explanation, Steve Eddy.
Although I'd add the fact that the diaphragm of the speaker/headphone moves due to the change in the B-field of the voice coil.

The force on the diaphragm is equal to:
Code:
F = B I L
Force = B-Field * Current * length of voice coil
Where the B-field is created by the use of permanent magnets.

This is true because moving charges in a B-fields (the current in the voice coil), will always experience a force. And since the voice coil is mechanically attached to the diaphragm, the diaphragm will also experience a force.

It should be noted that the diaphragm experiences a FORCE. Which is acceleration, and not an actual change in position. And furthermore the sound is created by the change in position of the diaphragm. Hence:
Code:
Sound produced = ds/dt = velocity of diaphragm
Since this velocity is caused by the force on the wire:
Code:
dv/dt =  Integral of Force
d^2v/dr^2 = F = proportional to current.
dF/dt = proportional to dI/dt

Giving: Sound produced = d^3 I/dt^3
Meaning that the sound produced is proportional to the third derivative of the current with respect to time (rate of change of rate of change of rate of change of current)

But since current is changing like a sine wave, the third derivative will be a cosine function. Which is of the same shape, but just a little bit out of phase.
And I could add that Current is proportional to voltage, so it doesn't matter whether current or voltage is changed, as long as you understand that current is the actual determining factor for the movement of the diaphragm.

I hope my mathematical jargon isn't too confusing. Never done this so mathematically, just applying my limited knowledge of electro magnetics
Edited by Tilpo - 6/23/11 at 11:39pm
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post #10 of 16
6/24/11 at 8:22am
Thread Starter 

I see how it works now, that should be enough information for the time being, thanks a lot.

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post #11 of 16
6/25/11 at 5:10pm

except for the fact that with headphone loads the relation can be complex (a little engineering humor there - complex numbers are used to describe the headphone impedance)

 

the mass of the headphone diaphragm gains momentum from the force caused by the current in the voice coil interacting with the magnetic field so it keeps moving in the same direction after the the current reverses, the voltage across the voice coil is partially determined by the velocity of the coil/diaphragm so the relation between the current and the voltage isn't simple proportionality - the voltage and current are both sinusoidal at the same frequency but not necessarily "in phase"

 

there is a similar issue with the spring force of the headphone surround that keeps the voice coil/diaphragm centered in the magnetic gap -

at some low frequency the mass and spring "complex impedance" effects just balance and you get the "mass-spring resonance" which can be seen as a bump in the impedance vs frequency curve

 

for a simple linear dynamic system there is a relation between the slope of the impedance vs frequency curve and the phase shift between the "across" and 'through" variables - in the headphone driver example, voltage and current

 

http://hyperphysics.phy-astr.gsu.edu/hbase/oscdr2.html#c2 driven damped oscillator example may help - or not


Edited by jcx - 6/25/11 at 5:16pm
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post #12 of 16
6/26/11 at 2:46am
Quote:
Originally Posted by jcx View Post

the mass of the headphone diaphragm gains momentum from the force caused by the current in the voice coil interacting with the magnetic field so it keeps moving in the same direction after the the current reverses, the voltage across the voice coil is partially determined by the velocity of the coil/diaphragm so the relation between the current and the voltage isn't simple proportionality - the voltage and current are both sinusoidal at the same frequency but not necessarily "in phase">
So are you saying that the movement of the diaphragm induces a current in the voice coil, causing weird behavior?
Because otherwise I don't see how the momentum of a mass can induce a voltage.
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post #13 of 16
6/26/11 at 6:52am

the velocity of the voice coil relative to the stationary magnetic field of the magnet/pole piece structure gives a motional EMF - a voltage across the voice coil terminals that adds (or subtracts) from the I*R term, the I*R term is 0 when I=0 but there will still be a motional emf V when the voice coil mass is still moving due to its accumulated momentum (you also have to consider that the spring of the suspension also stores and releases energy as it is compressed or extended by the diaphragm motion)

 

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/farlaw.html#c1

http://en.wikipedia.org/wiki/Faraday's_law_of_induction

 

putting the electrical and mechanical parts together is an entertaining exercise of undergraduate engineering system dynamics principles

 

the mass-spring mechanical resonator of the diaphragm+voice coil mass and the suspension spring is coupled to the electrical terminals by the voice coil and magnetic field motor/generator, for dynamic headphone drivers the resistance of the wire of the voice coil is the biggest part of the electrical impedance at most audio frequencies

 

http://en.wikipedia.org/wiki/Thiele/Small

 

it turns out that the radiated sound is a negligible part of the energy budget for the system - typically single digit percent, even less for orthodynamic drivers

 

for some headphones the mass spring impedance hump is clealy visible, for others, like orthos they look almost like a pure resistor to the amp:

 

graphCompare.php?graphType=7&graphID[]=573&graphID[]=2621&graphID[]=2871


Edited by jcx - 6/26/11 at 7:19am
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post #14 of 16
6/26/11 at 7:28am
Thread Starter 

Okay, I don't understand anything technical that I'm looking at. Does anybody have a good link to start given than I have practically no knowledge of how electromagnets and such work? My initial plan was to create a functional driver first then tweak it over time, but I'd love to be able to understand whatever it is that I'm looking at.

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post #15 of 16
6/26/11 at 2:33pm
Quote:
Originally Posted by jcx View Post

the velocity of the voice coil relative to the stationary magnetic field of the magnet/pole piece structure gives a motional EMF - a voltage across the voice coil terminals that adds (or subtracts) from the I*R term, the I*R term is 0 when I=0 but there will still be a motional emf V when the voice coil mass is still moving due to its accumulated momentum (you also have to consider that the spring of the suspension also stores and releases energy as it is compressed or extended by the diaphragm motion)

 

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/farlaw.html#c1

http://en.wikipedia.org/wiki/Faraday's_law_of_induction

 

putting the electrical and mechanical parts together is an entertaining exercise of undergraduate engineering system dynamics principles

 

the mass-spring mechanical resonator of the diaphragm+voice coil mass and the suspension spring is coupled to the electrical terminals by the voice coil and magnetic field motor/generator, for dynamic headphone drivers the resistance of the wire of the voice coil is the biggest part of the electrical impedance at most audio frequencies

 

http://en.wikipedia.org/wiki/Thiele/Small

 

it turns out that the radiated sound is a negligible part of the energy budget for the system - typically single digit percent, even less for orthodynamic drivers

 

for some headphones the mass spring impedance hump is clealy visible, for others, like orthos they look almost like a pure resistor to the amp:

Interesting, I think I see what you mean.

I'm by no means an engineer, but it does seem like something interesting to consider.
I hope I will get to do something similar when I study physics. biggrin.gif. But that's gonna take another year before I begin my bachelor.
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