Headphone Impedance and Why It Matters

A headphone box prints its impedance the way it prints frequency response: as one tidy figure, 32 Ω or 250 Ω, that looks like a fixed property of the thing you just bought. Neither figure is fixed. The frequency response varies across the audible range by design, and so, less obviously, does the impedance — the "32 Ω" on the box is a single nominal reference point on a curve, not the number the driver actually presents at every frequency. The more useful fact hiding behind that nominal figure is electrical, not acoustic: it is the reason the same phone can drive one headphone to an uncomfortable level and barely wake another one up, and that difference is a story about voltage and current in a circuit, not about "power" in the loose sense most buying guides use the word.

Impedance is a curve, not a number

The single ohm figure on a spec sheet is a nominal value, usually the minimum or a representative mid-band point, standing in for a quantity that actually moves with frequency. Sound On Sound's technical article "Phones & Ohms" measured impedance curves across 797 headphone models and found that "57 percent have some variance of impedance with frequency," rising, for a typical moving-coil dynamic driver, to "more than 1.5 times its nominal value at the self-resonance frequency." A driver that ships labelled 32 Ω can present well over 40 Ω at the frequency where its diaphragm and suspension resonate, and a different value again elsewhere in the band — the label is a reference point a manufacturer chose to publish, not a constant.

Manufacturer spec sheets mostly don't show this. beyerdynamic's own product page for the DT 770 PRO lists "Nominal impedance" and a single "Nominal sound pressure level" of 96 dB for the line, with no frequency attached to either figure and no note of whether the 96 dB is referenced to one milliwatt or one volt. That's not sloppiness so much as the industry's normal level of detail for a consumer spec sheet — the nominal number is what most buyers compare, and the curve underneath it only starts to matter once you ask why two headphones with headline numbers that look similar end up sounding, or behaving, differently on the same source.

Same source, two headphones: why 32 Ω and 250 Ω don't behave the same

beyerdynamic sells the DT 770 PRO in three impedance versions built on the same driver family, and its own product page names what each one is for: 32 Ω for "Mobile Devices," 80 Ω for "Universal Studio" use, and 250 Ω for "Professional Devices." That three-way split exists because a phone or laptop headphone jack is a voltage-limited source — its internal amplifier can only swing a small, roughly fixed peak voltage before it clips, regardless of what's plugged into it. How loud that fixed voltage plays depends on how much current the headphone's impedance lets through, and a higher-impedance driver pulls less current from the same voltage, which means less acoustic output for the same volume-wheel setting. A 250 Ω headphone isn't "harder to power" in the abstract; it needs more volts to reach the same level, and a phone has a limited supply of volts to give it.

This is also where sensitivity specs get conflated. Sound On Sound's "Phones & Ohms" distinguishes the two units manufacturers actually use: a figure in dB/mW is efficiency, "how much of this power is delivered as sound," while a figure in dB/V "measures the output level for a given voltage swing" directly. Sennheiser's own specification page for the HD 600 gives that headphone's 300 Ω, 97 dB (1 V) sensitivity figure in the voltage-referenced unit — useful precisely because a phone or laptop is a voltage source, so a dB/V figure tells you directly how loud a given source voltage will play. A dB/mW figure has to be converted through the headphone's impedance to get the same answer, and treating the two units as interchangeable, or not checking which one a spec sheet is quoting, is the routine error in casual headphone comparisons.

Both DT 770 PRO versions above carry the same published 96 dB figure despite the fivefold difference in impedance between them, which only makes sense once you read it as "96 dB at whatever reference beyerdynamic used," most plausibly 1 mW rather than 1 V; a higher-impedance driver needs more voltage to dissipate that same milliwatt. The HD 600's voltage-referenced 97 dB figure, by contrast, tells you directly what to expect from a given source voltage without that conversion.

Output impedance and the "rule of eighths"

The source has an impedance too — the amplifier's own internal series resistance, seen from the headphone's plug — and because headphone impedance genuinely varies with frequency, a source with non-negligible output impedance forms a voltage divider that also varies with frequency. The result is a small, frequency-shaped coloration layered on top of the headphone's own response, and it can be audible: Sound On Sound's own measurements found a Shure SE535 in-ear monitor's frequency response shifting by "a very audible 7.5dB" between a near-zero-ohm source and a 50 Ω one, purely from the interaction between the source's output impedance and the earphone's own impedance curve.

A widely repeated guideline holds that a source's output impedance should sit at roughly an eighth of the headphone's impedance or lower — the "rule of eighths." It is not a standard; the clearest public derivation of it comes from a 2011 blog post by the audio-engineering writer NwAvGuy, who builds the figure from a psychoacoustic assumption rather than citing a regulation: "the smallest audible difference most can hear is about 1 dB," and an output impedance at one-eighth of the load keeps the resulting frequency-response error under roughly that 1 dB threshold across typical headphone impedance curves. Treat the ratio as a practical rule of thumb aimed at inaudibility, not a certification requirement — most dedicated headphone amplifiers and DACs comfortably beat it, but plenty of budget devices and dongles do not publish an output-impedance figure at all.

When an amplifier genuinely helps

The genuine case is the voltage shortfall described above, and it shows up clearly in published amplifier specifications. Schiit's own specification page for the Magni Unity lists maximum output power of "3.0W RMS per channel" into 16 Ω, "2.5W RMS" into 32 Ω, and "415mW RMS" into 300 Ω, with an output impedance under "0.1 ohms" — power that falls as impedance rises, but from a voltage rail high enough that 300 Ω still gets a substantial fraction of a watt. FiiO's own specification page for the K3 gives a similar shape at portable scale: "220mW(16Ω) 120mW(32Ω)" from its single-ended output. A phone or laptop's built-in jack, sized for the low-impedance, high-sensitivity headphones beyerdynamic itself markets for "Mobile Devices," has nowhere near that voltage headroom in reserve, which is exactly why a 300 Ω studio headphone like the HD 600 plays back so much quieter on a phone than a 32 Ω pair does at the same volume-wheel position.

That is also where the genuine case ends and audiophile folklore usually begins. A low-impedance, high-sensitivity headphone that already reaches a comfortable level straight out of a phone or laptop — the DT 770 PRO's 32 Ω version is beyerdynamic's own example, built for exactly that use — is not voltage-starved, and an outboard amplifier cannot deliver more loudness headroom a phone genuinely lacks when the phone was never short of headroom for that headphone in the first place. What an amplifier reliably adds in that case is a lower, more consistent output impedance and a cleaner voltage rail, not additional volume or a different sound signature. The specification numbers above are the way to tell which case applies to a given headphone and source rather than guessing from listening impressions alone.

This site cannot measure impedance directly — there is no tool here for reading a driver's impedance curve or an amplifier's output impedance, only the manufacturer figures cited above. A headphone that sounds quiet is exactly the kind of result how loud is too loud warns against chasing with the volume wheel rather than a better-matched source. The electrical story in this guide only covers level and frequency response; what a difficult load does to distortion under load is a separate question, covered in total harmonic distortion.