Headphone impedance is measured in ohms and appears in the specifications of almost every pair of headphones. It determines how hard a source device has to work to drive the headphones to a useful volume, and matching it correctly makes a practical difference to what you hear.
Low-impedance headphones (16–32 Ω)
Most consumer headphones — earbuds, on-ear, and many over-ear models — have an impedance in the 16–32 Ω range. These are designed to work directly from smartphones, tablets, and portable players, which have relatively weak headphone amplifiers. Low impedance means the driver is easy to push to adequate volume from a low-power source.
High-impedance headphones (150–600 Ω)
Studio and audiophile headphones — particularly open-back designs — are often rated at 150 Ω, 250 Ω, or higher. These require a dedicated headphone amplifier or an amplifier with a capable headphone output. Connected directly to a smartphone, they will play at low volume and may sound thin, because the phone's output cannot deliver enough voltage swing to drive a high-impedance load to its full dynamic range.
Output impedance of the source
The source device also has an output impedance — typically very low on a well-designed dedicated amplifier (under 1 Ω) and higher on a smartphone headphone output (sometimes 10–30 Ω). When the source output impedance is a significant fraction of the headphone impedance, frequency response can be affected because impedance varies with frequency (as with speakers). The general recommendation is that the source output impedance should be less than one-eighth of the headphone impedance.
Sensitivity
Impedance is not the only factor in how loud headphones will play. Sensitivity — typically expressed in dB/mW or dB/V — tells you how much sound the headphones produce for a given amount of power or voltage. A high-impedance headphone with high sensitivity may actually be easier to drive than a low-impedance one with low sensitivity. When assessing whether a source can drive a headphone, look at both figures.
Frequently asked questions
Can I damage headphones by connecting them to an amplifier output rather than a headphone output?
Yes. A speaker output on a power amplifier provides far more voltage than a headphone can handle. Even a few watts into a headphone at its rated impedance would be thousands of times its maximum safe power. Always use a dedicated headphone output or a headphone amplifier with appropriate output power.
My headphones work fine from my phone but sound poor from my amplifier's headphone socket. Why?
Headphone outputs on integrated amplifiers vary considerably in quality and output impedance. Some are afterthoughts connected directly to the speaker output through a resistor — in which case the output impedance is very high relative to headphone impedance. This produces coloured frequency response and is why dedicated headphone amplifiers exist. A dedicated amplifier with a known low output impedance is the reliable solution.