There are a few different electrical measurements that are useful to look at with our range of audio interfaces.
This article will cover what impedance is and what differing impedances can do.
Much like resistance impedance is measured in Ohms (Ω) they also both measure the resistance to electrons moving through a material, for a DC signal the resistance and impedance will be the same.
When you get into more complex signals such as AC (alternating current) impedance is a more useful measurement as it also takes into account frequency information. In more simple terms, impedance can allow you to measure the resistance at a specific frequency.
The two main measurements you will see for an audio interface, will be the input impedance and output impedance.
Input impedance (Z in) measures the resistance to sending a signal into the interface, and output impedance (Z out) measures the resistance to sending a signal out.
At it's most basic, the calculation for impedance is the same as resistance as you can see below;
| Impedance, Z = | V |
| I |
V = Voltage (Volts)
I = Current (Amps)
However using a more advanced formula, you can also input the frequency information as below;
X = Reactance
XL = Inductive Reactance
XC = Capactive Reactance
This adds reactance to the formula, which is a measurement of the opposition of capacitance and induction. This will change based upon frequency information making this a more useful measurement for audio applications.
Reactance
Why is this important?
As we have seen from the previous information, impedance is essentially the resistance to sending a signal from one device to another. This is particularly important for headphones and speakers.
With headphones you will see a wide range of impedances, generally consumer headphones will have low impedances of around 32 Ohms, whereas professional studio headphones can be as high as 300 Ohm.
This will change the effective level you hear from the headphone at the same volume. The 32 Ohm headphones will be louder due to their lower overall resistance, whereas the 300 Ohm headphones will need a larger signal to reach the same volume as they are more resistant.
This is also important with passive speakers being driven by an amplifier, if the impedance of the speakers is higher than the output impedance of the amplifier, you will not be able to get the maximum volume output from the amplifier, as the overall resistance is greater and more power is required to reach the same volume.
You should always avoid connecting passive speakers that have a lower impedance value than the amplifier, as you can potentially damage the speakers and the amplifiers by drawing more current than the amplifier is designed to provide.
Impedance Matching
Where possible, matching impedances is advisable, this makes the resistance appear as very close to 0, which maximises the electrical efficiency of a circuit, and will minimise distortion and EM interference.
You can find more information on this in our variable impedance article here; Variable Impedance Explained