What Do Headphone Impedance and Sensitivity Specs Mean?
The three numbers on the box
Headphone spec sheets lead with three figures: impedance (in ohms), sensitivity (in dB per milliwatt or dB per volt), and frequency response (a range in hertz). Each describes one property of the electrical-to-acoustic conversion. None of them, alone, describes how the headphones sound.
Impedance (ohms)
Impedance is the headphone's opposition to the electrical signal driving it. Low-impedance models (commonly tens of ohms) are designed to reach useful levels from battery-powered devices; high-impedance models (hundreds of ohms) draw less current and are traditionally matched with dedicated headphone amplifiers, a convention with roots in studio equipment design. Two practical notes:
- Impedance interacts with the source: a source with high output impedance can alter both level and tonal balance into a low-impedance headphone.
- More ohms is neither better nor worse; it is a matching parameter, not a quality score.
Sensitivity (dB/mW or dB/V)
Sensitivity states how loud the headphone plays — in dB SPL, the sound-pressure scale explained in what is a decibel — for a given input, either one milliwatt of power or one volt of signal. The two conventions are not directly comparable, because power delivered at a fixed voltage depends on impedance; when comparing models, make sure the units match.
A higher-sensitivity headphone reaches a given loudness with less input. That cuts both ways: it pairs easily with weak sources, and it also reaches high sound-pressure levels readily. Since listening level is what connects headphone use to hearing health, the background references worth knowing are the NIDCD's explainer on noise-induced hearing loss and the WHO's fact sheet on deafness and hearing loss, which discusses safe listening in the context of personal audio devices. Questions about your own listening levels or hearing are for an audiologist, not a spec sheet.
Frequency response (Hz)
A range like "20 Hz – 20 kHz" states the band the driver reproduces, nominally matching the span of typical human hearing (see the NPS overview of frequency and amplitude). Read this figure skeptically: a bare range without tolerance bounds (such as ±3 dB) says nothing about how evenly the band is reproduced, and evenness is what shapes the sound. Two headphones with identical printed ranges can sound entirely different.
What the specs cannot tell you
Distortion behavior, fit and seal, isolation, and the actual shape of the frequency response drive perceived quality far more than the headline numbers. Specs are best used negatively — to rule out mismatches, such as a high-impedance, low-sensitivity headphone on an underpowered source — rather than to rank sound quality. For the dB values quoted in environmental-noise contexts, which use the same unit differently, see how to read a decibel chart.