Okay, let's start with a basic definition ofimpedance. We should first think about electrical resistance (represented byR), measured in Ohms (symbol Ω). Imagine a simple circuit consisting of abattery and a resistor. The battery generates a voltage which tries to force acurrent around the circuit connected between the battery's two terminals. Theresistor resists that current — the higher the value of the resistor, the lowerthe current will be, and vice versa. In resisting the current, a voltagedifference is developed across the resistor. This important phenomenon isdefined mathematically in Ohm's Law, where the battery voltage (represented byV and measured in Volts) equals the current (represented by I and measured inAmps) multiplied by the resistor's resistance value. Expressing this lawalgebraically, V=IR, a simple bit of algebraic rearrangement gives I=V/R. So ifthe battery is 12V and the resistor is 120Ω, the current flowing around thecircuit will be 12V/120Ω, which is 0.1A, or 100mA.

This simple example is of a Direct Current (DC)circuit — the battery voltage is steady and unchanging (ignoring the effect ofthe battery losing energy over time). However, when we are dealing with audioelectronics, the signal voltage changes amplitude continuously to represent thechanging amplitude of the audio signal, and it alternates between positive andnegative cycles. The currents that flow therefore have varying amplitudes andalternate in direction as well, and we have what is known generically as anAlternating Current (AC) circuit.

This is where things become slightly morecomplex, because, in addition to the resistance, there are two otherfundamental components which affect the current flowing around an AC circuit.In addition to the simple resistance we have already discussed, there is alsocapacitance and inductance to consider. In simplistic terms these also act likeresistors, except that their resistance to current changes in proportion to thefrequency of the signal voltage fluctuations — the rate at which the currentflowing through the circuit is made to change direction by the audio signalvoltage, in this case.

All audio electronics have combinations ofresistors, capacitors and inductors connected in circuits, along with 'active'components like transistors or valves which provide amplification or act asswitches. To make life slightly easier for ourselves, we often consider thetotal 'resistance' of a complex circuit involving resistors, capacitors andinductors as a composite lump, and that's what we call the impedance.

Impedance has the symbol Z — hence referencesto high-Z inputs, for example — and is still measured in Ohms. However, theactual value depends to some degree on the frequency of the signal voltagesinvolved. In audio input and output circuits the impedances are principallyresistive to make interconnection easier — the impedance won't change too muchover the range of audio frequencies. However, the impedance to radio frequency(RF) signals will often be very different to that at audio frequencies in orderto keep RF interference out.

Because the unit of impedance is ohm, manyfriends think that the impedance of the earphone is the resistance of the coil,which is actually wrong. Impedance refers to the obstruction of alternatingcurrent in a circuit with resistance, inductance, and capacitance, and the unitis ohm. There is no particular absolute correlation between the impedance andthe sound quality of the headphones. It is not that the sound quality of acertain earphone is good, and it is expensive because of the high impedance. 

Since a low impedance is enough, why do youhave so many headphones with large impedance? Although the impedance does notdirectly reflect the sound quality, high-impedance headphones do have someadvantages in terms of sound field and level. Of course, high-impedanceheadphones also need more power amplifier support, generally desktop systemscan be driven by multi-stage amplification.

Since a low impedance is enough, why do youhave so many headphones with large impedance? Although the impedance does notdirectly reflect the sound quality, high-impedance headphones do have someadvantages in terms of sound field and level. Of course, high-impedanceheadphones also need more power amplifier support, generally desktop systemscan be driven by multi-stage amplification.

If you listen to songs on your mobile phone orportable player, try to choose headphones with low impedance and highsensitivity when choosing headphones. Usually the impedance of the earphones inthe 16 or 32 ohms is very good.