Impedance and Sensitivity, Without the Math
Two numbers on every spec sheet, routinely misread, and one rule about your source that genuinely changes how things sound.
Impedance
Impedance is how much the headphone resists the electrical signal, measured in ohms. Higher impedance means the source has to supply more voltage to reach the same loudness. It is not a quality rating. A 32 ohm headphone is not worse than a 300 ohm one; it is easier to drive.
Sensitivity
Sensitivity is how loud the headphone gets for a given input. This is where most confusion lives, because it is quoted two different ways and they are not interchangeable.
- dB per milliwatt describes loudness per unit of power.
- dB per volt describes loudness per unit of voltage.
A figure quoted in dB/V will look dramatically higher than the same headphone quoted in dB/mW. Comparing a dB/V number against a dB/mW number tells you nothing at all. Check the unit before you compare two products.
The practical rule
- Under about 50 ohms and above roughly 100 dB/mW: essentially anything will drive it, including a phone.
- Above about 250 ohms, or below about 90 dB/mW: expect to need real amplification to reach satisfying volume with headroom intact.
- In between: probably fine off a laptop, better off a dongle.
The rule that actually changes the sound
Your source has its own output impedance. The long-standing guideline is that it should be no more than about one eighth of the headphone's impedance.
Below that ratio, nothing interesting happens. Above it, the frequency response of the headphone genuinely shifts, and it shifts most on multi-driver balanced armature IEMs whose impedance varies a lot across the frequency range. This is the real, measurable reason some IEMs sound bass-heavy or thin depending on what you plug them into. It is not mysticism and it is not the DAC chip. It is an impedance mismatch.