Headphone Impedance and Why It Matters

A headphone box prints its impedance the way it prints frequency response: as one tidy number, like 32 Ω or 250 Ω, that looks like a fixed fact about what you bought. Neither is fixed. Frequency response changes across the range by design, and so, less obviously, does impedance. The "32 Ω" on the box is one reference point on a curve, not what the headphone actually presents at every pitch. The more useful thing behind that number is electrical, not about sound: it's why the same phone can make one pair of headphones uncomfortably loud and barely wake up another. That comes down to voltage and current in a circuit, not "power" in the loose way 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 lowest point or a typical mid-range point, standing in for something that actually changes with pitch. 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 dynamic headphone, to "more than 1.5 times its nominal value at the self-resonance frequency." A headphone labelled 32 Ω can present well over 40 Ω at the pitch where its diaphragm naturally resonates, and something different again elsewhere. The label is a reference point the maker chose to publish, not a constant.

Most makers' spec sheets 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 range, with no pitch given for either and no note of whether the 96 dB is measured against one milliwatt or one volt. That's not sloppiness so much as the normal level of detail for a consumer spec sheet. The nominal number is what most buyers compare, and the curve behind it only starts to matter once you wonder why two headphones with similar-looking numbers sound or behave differently on the same device.

Same device, two headphones: why 32 Ω and 250 Ω behave differently

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 split exists because the headphone jack on a phone or laptop can only put out a small, roughly fixed voltage before it starts to distort, whatever you plug into it. How loud that voltage plays depends on how much current the headphone lets through, and a higher-impedance headphone lets less current through from the same voltage. That means less sound at the same volume setting. A 250 Ω headphone isn't "harder to power" in some vague way; it needs more volts to get to the same volume, and a phone only has so many volts to give.

This is also where sensitivity figures get mixed up. Sound On Sound's "Phones & Ohms" separates the two units makers 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 in the voltage unit. That's handy precisely because a phone or laptop works by voltage, so a dB/V figure tells you directly how loud a given device will play it. A dB/mW figure has to be converted through the headphone's impedance to get the same answer, and treating the two as the same thing, or not checking which one a spec sheet uses, is the usual mistake in casual headphone comparisons.

Both DT 770 PRO versions show the same 96 dB figure, even though one has five times the impedance of the other. That only makes sense if you read it as "96 dB at whatever reference beyerdynamic used", most likely 1 mW rather than 1 V, because a higher-impedance headphone needs more voltage to use that same milliwatt. The HD 600's 97 dB per volt, on the other hand, tells you directly what to expect from a given device, with no conversion needed.

Output impedance and the "rule of eighths"

The device you plug into has an impedance too: its amplifier's own internal resistance, as seen from the headphone plug. Because a headphone's impedance changes with pitch, a device with a noticeable output impedance ends up boosting some pitches and cutting others a little. That adds a small colouring on top of the headphone's own sound, 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 because of how the device's output impedance and the earphone's impedance curve interact.

A popular guideline says a device's output impedance should be about an eighth of the headphone's impedance or lower: the "rule of eighths". It isn't an official standard. The clearest public explanation of it comes from a 2011 blog post by the audio-engineering writer NwAvGuy, who works the figure out from how we hear rather than from any regulation: "the smallest audible difference most can hear is about 1 dB," and an output impedance of one-eighth of the headphone's keeps the resulting change under about that 1 dB across typical headphones. Treat it as a practical rule of thumb for keeping the effect inaudible, not a certification. Most dedicated headphone amps and DACs beat it easily, but plenty of budget devices and dongles don't publish an output-impedance figure at all.

When an amp really helps

The real case is the voltage shortfall described above, and you can see it clearly in amplifier specs. 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". The power drops as impedance rises, but it starts from enough voltage that 300 Ω headphones still get a healthy share of a watt. FiiO's own specification page for the K3 shows a similar pattern on a portable scale: "220mW(16Ω) 120mW(32Ω)" from its standard output. A phone or laptop's built-in jack, designed for the low-impedance, sensitive headphones beyerdynamic itself markets for "Mobile Devices," has nowhere near that much voltage in reserve. That's exactly why a 300 Ω studio headphone like the HD 600 plays so much quieter on a phone than a 32 Ω pair at the same volume setting.

That's also where the real case ends and audiophile folklore tends to begin. Low-impedance, sensitive headphones that already get loud enough straight from a phone or laptop, like the 32 Ω DT 770 PRO, which beyerdynamic makes for exactly that, aren't short of voltage. An external amp can't give them volume headroom the phone never lacked in the first place. What an amp reliably adds in that case is a lower, steadier output impedance and a cleaner supply, not more volume or a different sound. The spec figures above are how to tell which case you're in with a given pair of headphones and device, rather than guessing by ear.

We can't measure impedance on this site. There's no tool here for reading a headphone's impedance curve or an amp's output impedance, just the makers' figures cited above. And if your headphones sound quiet, how loud is too loud explains why it's better to find a better-matched device than to keep turning the volume up. This guide only covers volume and frequency response. What a demanding pair of headphones does to distortion is a separate question, covered in total harmonic distortion.