The following article goes into the complexities of how impedance affects your headphone volume and frequency response. For general advice on which headphones to use, please see this article: What impedance headphones should I use with my audio interface?
Electrical impedance is the sum of inductance, capacitance, and resistance to an electrical current. Simply put, higher-impedance headphones require more power to deliver the same volume as lower-impedance headphones. You can calculate the power transferred between two loads — two sources of resistance or impedance — by using the equations below.
Inductance and capacitance are mainly relevant to alternating current (AC) circuits. To simplify things, let's treat the output of the interface as a direct current (DC) circuit, leaving only the resistance of the output and the resistance of the headphones to consider:
= the output voltage from the headphone output
= the output resistance of the headphone output
= the resistance of the headphones
Using a few simple equations derived from Ohm’s Law, we can calculate the power transferred between the two resistors in this simplified circuit with varying values of .
The circuit is in series, so the current remains constant throughout. According to Ohm's law, we can calculate the current, , with the following equation:
Once we know , we can calculate the voltage transferred to the headphones,
, with this equation:
And we can calculate the power transferred to the headphones, with the following:
If we assume nominal values of = 100V and
= 25Ω, then we can calculate the voltage and power transferred to the headphones for varying values of
, as in the table below:
Maximum power transfer to occurs when
=
. This characteristic would be the same when considering a more complex AC circuit, like your interface’s headphone output.
It is worth noting that, whilst attaining higher power transfer to your headphones will ensure a higher audio output level, higher voltage transfer will provide the flattest frequency response — or at least a frequency response most faithful to your headphone’s performance specifications. You can achieve higher voltage transfer with a headphone impedance that is higher than the output impedance of your interface: the greater this difference is, the greater the voltage transfer will be.
However, if high output levels are your ultimate goal, we'd advise you to try to use headphones with an impedance as close as possible to that of the headphone output. It isn't necessary to match the headphone output impedance of your interface exactly, but the closer they are, the higher the output levels will be.
You can find the headphone impedance and other specifications for all our current audio interfaces in the User Guides in Focusrite Downloads.