Headphone impedance and sensitivity, explained with actual numbers

Every headphone spec sheet leads with the same two numbers, and every month someone asks me why their new headphone sounds limp from the source that drove the old one perfectly. The answer is nearly always sitting in those two numbers - they just needed to be read together instead of separately.

Impedance: what the ohms mean#

ImpedanceThe total opposition (resistance + reactance) a speaker or headphone presents to the driving current, measured in ohms and varying with frequency. is the opposition a headphone presents to the electrical signal, measured in ohms (Ω). It isn't a quality metric - it's a description of what kind of electrical load the source sees, and it largely determines whether a headphone needs voltage or current to play loud.

The practical brackets: low-impedance designs (16-32 Ω, most IEMs and consumer over-ears) draw current easily and reach loudness from small voltage swings - phone-friendly, at least on this axis. High-impedance designs (250-600 Ω, the classic studio dynamics) need several times the voltage for the same power, which is exactly what battery-powered outputs don't have. In between sits most of the modern audiophile catalogue.

One wrinkle matters more than the textbook version admits: for dynamic drivers, impedance isn't one number. It rises sharply at the driver's resonant frequencyThe frequency at which a mechanical or electrical system oscillates with minimum energy input; for a speaker driver, this is where cone excursion peaks. - the impedance humpA peak in headphone impedance at the driver's mechanical resonance frequency; with a high-output-impedance amplifier, this produces a bass emphasis. - and this is where your source's output impedanceThe impedance looking back into an amplifier's output terminals. A high output impedance interacts with headphone impedance curves to alter frequency response. starts editing the sound. A high-output-impedance amp delivers extra voltage right at that hump, audibly inflating the bassSay: BAYSS /beɪs/The low-frequency foundation of audio, roughly 20-250 Hz - felt as much as heard, carrying a track's weight, warmth, and impact. (Said "BAYSS", like the guitar, not the fish.) around resonance. The defense is the rule of eighths: keep the headphone's impedance at least eight times the source's output impedance and the response stays essentially flat. Planars opt out of this drama entirely - their flat, resistive impedance looks the same at every frequency, one of the quiet reasons they pair so predictably.

Sensitivity: what the decibels mean#

SensitivityThe output sound pressure level for a standardized input, typically dBSPL at 1W/1m for speakers, or dBSPL at 1mW or 1V for headphones. tells you how loud a headphone plays for a given input - quoted either as dB per milliwatt of power, or dB per volt. The two conventions are not interchangeable, and mixing them up is the single most common spec-sheet error: a 32 Ω headphone rated 100 dB/mW and a 300 Ω one rated 100 dB/V are wildly different loads in practice. Check which unit the manufacturer used before comparing anything.

The decibelA logarithmic ratio unit; 3dB represents a doubling of power, 6dB a doubling of voltage or pressure, and 10dB a perceived doubling of loudness. scale compresses huge differences into small-looking numbers. Ten dB of sensitivity difference means ten times the power for the same loudness; the gap between an easy 104 dB/mW IEM and an 83 dB/mW planar is over a hundred-fold in power terms. That's the difference between a fraction of a milliwatt and hundreds of milliwatts for the same peak - and why "it has a headphone jack" tells you nothing about whether something can drive your headphone.

Reading them together: three real loads#

Spec pairs only mean something in combination, so take three characters from this catalogue's shelves. The HIFIMAN HE400se is the classic "gateway planar" case - moderate impedance, modest sensitivity. It makes sound from a phone, which fools people; it makes music from a desktop source that can feed it real current, which is why its review keeps insisting on amplification despite the budget price.

The HIFIMAN Arya Organic, the catalogue's open-backHeadphones with perforated or meshed ear cups allowing free air exchange; produces a more natural, spacious presentation with no isolation from ambient sound. reference, sits in the same valley but deeper: its planar load is utterly benign in character - flat impedance, no reactive surprises - yet insensitive enough that proper crest-factor headroomThe decibel margin between the loudest expected signal and an amplifier's clipping point. 10-20dB of headroom is generally needed for unclipped reproduction of dynamic recordings at realistic listening levels. demands hundreds of clean milliwatts. Gear like the HIFIMAN EF500 exists for precisely this class of load: 4.5 W balancedA signal transmission method using two opposite-polarity signal lines plus a ground; noise induced on both lines is cancelled at the differential input. means the amp idles where lesser sources strain.

And at the far end, the glossary's favorite cautionary tale: the HIFIMAN SusvaraHiFiMAN's flagship planar magnetic headphone, famous for requiring enormous amplifier power (at least 1-2W) due to its extremely low sensitivity of ~60dBSPL/mW., famous for needing speaker-amp levels of power. You don't need to own one to learn its lesson - sensitivity floors exist below which no portable source, however well-reviewed, is a serious proposal.

Doing the math without doing the math#

The formula chain is short - sensitivity plus headroom gives required power, power and impedance give required voltage and current - but there's no reason to push the logarithms by hand. The headphone power calculator takes the two spec-sheet numbers and a target level, and returns exactly what your source must deliver, with the crest-factor headroom already included. Aim it at any headphone you're considering and any source you own, and the "do I need more amp?" question collapses into a yes or no.

Two closing habits worth keeping. First, treat published sensitivity with mild suspicion - manufacturers measure at different frequencies with different conventions, and a couple of dB of optimism is common; headroom in your source covers the fudge. Second, remember what these numbers can't tell you: nothing about tonality, stageShort for soundstage; the perceived three-dimensional acoustic space of a stereo recording. Often used to describe headphone presentation specifically ("the Arya has a deep stage")., or comfort lives in ohms and decibels. They answer one question only - what does this headphone need to perform as designed - but they answer it completely, which is more than most specs in audio can claim.