**Under normal conditions the impedance itself isn't an issue.**
Hang on... This is going to get a little "technical".

The main issue with impedance is

*related* to frequency response.

The impedance of a headphone (or speaker) isn't constant over the frequency range. Headphones are tested with a low-impedance (constant voltage) source. That is, the voltage out of the headphone amplifier doesn't change when the load impedance changes.

**A good headphone amplifier will behave the same way and impedance doesn't have any effect and the frequency response will be "as specified".**
**If** the headphone amplifier source impedance isn't low relative to the headphone impedance you have a

**voltage divider** where the "top" resistor is the source-resistance the headphone amp and the "bottom" resistor is the headphone impedance.

If these were both pure resistors the only consequence would be a signal loss. If the resistors are equal you'd get a 6dB drop. (Higher headphone impedance or lower amplifier impedance means less voltage drop.)

If the headphone amp has an output capacitor, the voltage divider becomes a high-pass filter.

** If** the capacitor value is too low, or if the headphone impedance is too low, you'll get a loss of bass.

As the headphone impedance varies over the frequency range, the output varies (if the source impedance is not low relative to the headphone impedance).

An impedance-rise in the mid-bass (which is somewhat common) will create a boost in the mid-bass frequency response.

A higher impedance headphone (and/or a low impedance headphone amplifier) minimizes both of these effects.

**Unfortunately, amplifier output-impedance is rarely published.** They usually just publish the recommended load impedance. It's the same with power amplifiers... The source impedance of a power amplifier (designed to drive 4 or 8 Ohm loads) is usually far-less than 1 Ohm, but that's not published in the specs. (Sometimes they publish the damping factor which is the ratio of the load impedance to the source impedance.)

**Impedance is also related to power** (milliwatts in the case of headphones). With a given voltage, half the impedance gives you double the power. If you have a low-voltage headphone amplifier (like what's built-into a cell phone) you may not get enough volume. The volume from laptops or other devices may be limited too.

But although volume/loudness is related to power, some headphones are more efficient than others. Lower impedance headphones

**tend** to be louder, but that's not always the case.