[noona1227]

'Headphone Impedance is the reaction when the voice coil found in the driver is affected by the magnet inside the headphone.'
 
I know there's more to it but is this sentence correct?
 
Edit: maybe reaction isn't the right word?

 

[liamstrain]

I might use the word "resistance" - I suppose it is more or less correct, with that change (reactance does play a role, but resistance is the "real" part of impedance). I would probably phrase it:
 
"It is the resistance caused by the interaction of voice coil and magnet inside the headphone driver under a given voltage."

 

[noona1227]

Thanks I understand now.
 
I have another question.
so high impedance headphones need more voltage but less current. And low impedance headphones need less voltage but more current. Can you explain to me why? Is there a formula to represent this?

 

[Mauricio]

I would define impedance in general as: the frequency-dependent opposition to the flow of current.  The opposition has three components:  resistance, inductance and capacitance.
 
The basic formula is Ohm's Law:  V=IR (Voltage equals the product of Current and Resistance).

 

[bellsprout]

Impedance is a property of the headphone itself. It is not dependent on the voltage you pass across it.
 
Impedance (Z) has 2 components, resistance (R) and reactance (X) such that
 
Z=R+X *
 
R is the resistance of the voice coil, caused by it being a long thin wire. Higher impedance headphones** use thinner wire for the voice coil and/or more coils of it. You can check this with a multimeter.
 
X is the reactive part due to the wire being in a coil, so it acts as an inductor. It's not strictly correct to say it's affected by the magnet, as it would have the same reactance even if it isn't in the magnet - the magnet is just the reason it moves. X is frequency dependent.
 
* Normal addition doesn't work here since Z, R and X are vectors (have direction). R and X are perpendicular to each other, so 3 ohms of resistance + 4 ohms of reactance = 5 ohms of impedance, like if you drove 3km west and 4km north you'd be 5km from where you started.
 
** I think most manufacturers (incorrectly) quote the impedance as the resistance. Impedance is frequency dependent so it would be a graph rather than a single ohmic value. Some reviews with measurements show how impedance changes with frequency. Dynamic drivers don't fluctuate too much. IEMs (balanced armatures) fluctuate wildly. Also, in these real tests output impedance from the amp also comes into effect.

 

[Head Injury]

Quote:

Thanks I understand now.
 
I have another question.
so high impedance headphones need more voltage but less current. And low impedance headphones need less voltage but more current. Can you explain to me why? Is there a formula to represent this?

Quote:

I would define impedance in general as: the frequency-dependent opposition to the flow of current.  The opposition has three components:  resistance, inductance and capacitance.
 
The basic formula is Ohm's Law:  V=IR (Voltage equals the product of Current and Resistance).

 
Expanding on that:
 
Headphones require an amount of power to reach a certain volume depending on their sensitivity (usually given as dB/mW, sometimes dB/V). The amount of power a headphone will receive for a given voltage is P = V^2 / R, in Watts, Volts, and ohms. The amount of power a headphone will receive for a given current is P = I^2 * R, I being amps. This is just a special application of Ohm's Law, since P = I * V (so if V = I * R then I * V = I * I * R, and if I = V / R then I * V = V * V / R, etc.)
 
Because different headphones need different amounts of power for a given volume, it's not so rare to find a low impedance headphone that needs more voltage (and much more power) than a high impedance headphone to reach a certain volume. As far as loudness goes, sensitivity is actually just as important.
 
You know what's funny? I knew all those equations before, but I didn't know the original Ohm's Law equation until today, or at least I don't remember knowing it. This is the subject we're on this week in an energy/physics course I'm taking.

 

[Mauricio]

Quote:

Impedance is a property of the headphone itself. It is not dependent on the voltage you pass across it.
 
Impedance (Z) has 2 components, resistance (R) and reactance (X) such that
 
 
 
 

Impedance is dependent on the voltage.  Ever see a Frequency vs. Impedance graph for a headphone or speaker.  It is anything but a straight horizontal line.  Impedance is not dependent on the magnitude of the voltage (within a certain thermal range), but it is--by definition--dependent on the frequency of the voltage.

 

[RazorJack]

I think he meant nominal impedance. Given at a fixed frequency, usually 1000 Hz.

 

[tranhieu]

Quote:

I think he meant nominal impedance. Given at a fixed frequency, usually 1000 Hz.

 
Yeah, it's often written on the package that the nominal impedance is measured with 1khz signal, though I heard from somewhere some manufacturers might use different values for their own measuring but not sure how valid that claim is.