I've tried measuring audio signal out of my DMM one time, and thought occurred to me: Wait a minute, I'm measuring a audio signal(it was fluxuating), maybe I should use a O-Scope!!! One thing I don't understand is, how is offset of 10mV(thats 100th of a volt) dangerous if the signal swings way beyond that if you raise the gain?

All these concepts may help with understanding headphone impedance.

From wiki:

## Complex impedance

Impedance is represented as a complex quantity and the term *complex impedance* may be used interchangeably; the polar form conveniently captures both magnitude and phase characteristics,

where the magnitude represents the ratio of the voltage difference amplitude to the current amplitude, while the argument gives the phase difference between voltage and current and is the imaginary unit. In Cartesian form,

where the real part of impedance is the resistance and the imaginary part is the reactance .

Where it is required to add or subtract impedances the cartesian form is more convenient, but when quantities are multiplied or divided the calculation becomes simpler if the polar form is used. A circuit calculation, such as finding the total impedance of two impedances in parallel, may require conversion between forms several times during the calculation. Conversion between the forms follows the normal conversion rules of complex numbers.

#### Inductor

For the inductor, we have the relation:

This time, considering the current signal to be

it follows that

And thus

This tells us that the ratio of AC voltage amplitude to AC current amplitude across an inductor is , and that the AC voltage leads the AC current across an inductor by 90 degrees.

This result is commonly expressed in polar form, as

Or, more simply, using Euler's formula, as

Edited by High_Q - 4/30/11 at 2:30pm