Headphone sensitivity and impedance: what they mean for real-world listening

A customer came into my old shop on Smith Street once — back when I was still selling records out of Fitzroy — carrying a pair of Sennheiser HD 800s and a brand-new iPhone. He wanted to know why everything sounded thin and congested. The headphones retailed for more than his rent. The answer was sitting right there in two numbers on the spec sheet that nobody had thought to explain to him.

Headphone sensitivity and impedance. They look like dry electrical trivia. In practice, they determine whether your headphones sing or sulk, and getting them wrong costs more than the gear itself — it costs you the whole point of buying serious headphones in the first place.

Let me walk through both, clearly and without hand-waving.

What sensitivity actually tells you

Sensitivity is rated in decibels of sound pressure level per milliwatt (dB SPL/mW) or, less commonly, per volt (dB SPL/V). Most headphones fall somewhere between 88 dB/mW and 115 dB/mW. On the surface this sounds like a simple loudness rating. It's a bit more nuanced than that.

A headphone rated at 88 dB/mW needs considerably more power to reach a given listening level than one rated at 100 dB/mW. The relationship is logarithmic. To add 10 dB of loudness, you need ten times the power. So the gap between an 88 dB driver and a 100 dB driver is twelve decibels — meaning the less-sensitive headphone needs roughly sixteen times the electrical power to produce the same volume. That's not a rounding error. That's the difference between being driven properly and being starved.

High-sensitivity headphones — a lot of IEMs fall here, often 100 dB/mW or above — will get loud from almost anything. Your phone, your laptop headphone jack, a cheap dongle DAC. The problem is that those same sources have audible noise floors, and a sensitive transducer will reproduce that hiss faithfully. If you've ever plugged a sensitive IEM into a laptop and heard a faint hiss at low volume, that's the device's noise floor being amplified. Not ideal.

Low-sensitivity full-size headphones — the HD 800 S, the Audeze LCD-series, many planar magnetics — need real current and voltage to open up. Feed them from a phone output and you'll get acceptable volume, but the dynamics compress, the bass loses grip, and the whole thing sounds oddly polite. Which is exactly what my customer in Fitzroy was hearing.

Impedance: the other half of the equation

Impedance is measured in ohms (Ω). It describes the opposition a headphone's driver presents to the electrical signal. Most consumer headphones sit between 16Ω and 600Ω, though IEMs often run even lower — sometimes 8Ω or less.

Low impedance headphones (under about 50Ω) draw more current from the source. High impedance headphones (150Ω and above) draw less current but demand more voltage. This matters because different output stages are better at one than the other. A phone's amplifier stage is built for low-impedance loads — it has relatively high output impedance itself, and it's optimised to push current rather than swing voltage. Put a 300Ω Beyerdynamic on it and you're fighting the design of both devices simultaneously.

There's also the output impedance interaction, which we've covered separately in some depth — but the short version is that a high-impedance source output into a low-impedance headphone will change the frequency response, not just reduce the volume. Bright IEMs can become even brighter. Bassy headphones can lose low-end control. The ratio of source output impedance to headphone impedance should ideally be at least 8:1 in the headphone's favour. Most dedicated headphone amplifiers achieve this comfortably. Many phones and laptops do not.

How these two specs interact in practice

Here's where it gets interesting. Sensitivity and impedance don't tell the same story independently — they interact.

A 32Ω IEM rated at 110 dB/mW is easy to drive and will go loud from anything. Probably too loud, and with audible hiss from noisy sources. You want a low-output-impedance, low-noise amplifier for it — a quality dongle DAC or a dedicated portable amp with a noise floor well below your listening level.

A 300Ω dynamic headphone rated at 100 dB/mW needs voltage swing but not enormous current, and it's forgiving of source output impedance because the ratio stays sensible even if the source isn't perfect. A quality desktop amplifier — or even a decent valve headphone amp — will drive it well.

The genuinely tricky category is the low-impedance, low-sensitivity planar magnetic. Something like a 20Ω planar at 88 dB/mW wants high current and high voltage, demands a low output impedance, and won't get loud without real watts behind it. These headphones are why dedicated headphone amplifiers with proper power supplies exist. They're not marketing fluff for those transducers — they're functional requirements.

What this means for choosing an amplifier

The honest answer is that most people buying mid-tier full-size headphones — something like the Sennheiser HD 660S2 (check price), which sits at 300Ω and 104 dB/mW — will be fine with a reasonable desktop amp or even a quality integrated with a headphone output. The 660S2's relatively high sensitivity offsets its high impedance for most real-world sources.

Where you genuinely need to think carefully is at the extremes: very low impedance planars, very high impedance studio-heritage headphones, or very sensitive IEMs where noise floor matters more than power.

For IEMs specifically, the single most important amplifier spec isn't wattage — it's output impedance and noise floor. A dongle DAC with a 1Ω output impedance and a -110 dBu noise floor will outperform a powerful desktop amp with a 10Ω output impedance for a sensitive 16Ω IEM, every single time. Power means almost nothing if the fundamental electrical relationship is wrong.

For demanding full-size headphones, power matters more. But I'd argue — and this will make some solid-state enthusiasts uncomfortable — that a well-designed valve headphone amplifier, particularly one with an output transformer that can present a consistently low impedance across the audio band, can drive high-impedance dynamic headphones as well as almost anything solid-state at a similar price point. The measurements will look less tidy. The listening experience often doesn't suffer for it. That's a subjective call, yes, but one I'll defend.

IEMs: the sensitivity trap

IEMs deserve their own moment here because the sensitivity numbers can get extreme. Some custom and universal IEMs break 115 dB/mW. At those levels, the amplifier's noise floor becomes the primary sound-quality variable. You're no longer shopping for power; you're shopping for quiet.

The Chord Electronics Mojo 2 — which we've reviewed at length here (check price) — has become something of a standard reference for this. Its measured noise floor is low enough to stay below audibility with even very sensitive IEMs, and its output impedance is comfortably low. That's what you're paying for when you buy a quality portable DAC/amp for IEM use. Not watts. Silence.

I'll also say this: if you're building a more complete desktop setup for both IEMs and full-size headphones, a single amplifier that compromises both — low enough output impedance for IEMs, enough voltage swing for planars — is a genuinely difficult brief. Lots of desktop amps with a gain switch handle it reasonably. Dedicated separate amplifiers handle it better. Something to consider before you buy one device and expect it to serve two very different transducer types equally well. Our guide to building a serious desktop rig goes into this in more detail.

Reading the spec sheet without getting fooled

A few things to watch for when you're comparing specs on paper.

First, sensitivity ratings are not always measured at the same reference point. Some manufacturers quote dB/mW, others dB/V. These are not directly comparable without knowing the impedance. At 32Ω, 1mW equals about 179mV. At 300Ω, 1mW equals about 548mV. A headphone rated at 100 dB/V might look less sensitive than one rated 102 dB/mW, but depending on impedance, the real-world difference could go either way.

Second, impedance is not flat across the frequency range. It's a nominal figure, usually measured at 1 kHz. High-impedance dynamic drivers typically have a significant impedance peak at resonance — often 2x to 4x the nominal value at the bass resonant frequency. This is why output impedance ratios matter even for high-impedance headphones: a high-output-impedance source will have a varying frequency response as the headphone's impedance varies across the spectrum. It won't be dramatic, but it's not neutral either.

Third, efficiency figures from manufacturers are sometimes optimistic. Measured data from independent sources — the Headphone Database maintained by volunteers at the major headphone forums is one reasonable reference, as are published measurements from publications that actually measure — will occasionally disagree with box specs by a couple of dB. Not enough to make a high-sensitivity headphone sound like a low-sensitivity one, but enough to matter in borderline matching situations. The published IEC 60268-7 standard covers headphone testing methodology, for anyone who wants the full technical picture direct from the source.

A simple matching framework

If you want something you can apply quickly when comparing headphones and sources, here's how I think about it.

Sensitivity above 100 dB/mW: prioritise low noise floor and low output impedance over raw power. Works from almost any source volume-wise, but sounds best from a clean one.

Sensitivity 94–100 dB/mW: the middle ground. Most decent dedicated headphone amplifiers will drive these well. Quality portable sources are adequate; a desktop amp lifts the ceiling noticeably for full-size headphones in this range.

Sensitivity below 94 dB/mW: plan for a dedicated amplifier. For low-impedance planars in this bracket, check the amplifier's current capability and output impedance, not just its wattage into 32Ω. For high-impedance dynamics below this sensitivity threshold, you want voltage swing and a quiet background.

Impedance below 32Ω: output impedance of your source matters a lot. Aim for 4Ω or less from the amplifier. Many quality portable and desktop amps achieve under 1Ω.

Impedance above 150Ω: output impedance becomes less critical, but you need a source that can swing voltage without clipping at low gain. Desktop or stationary use is usually the right context for headphones in this range anyway.

None of this replaces listening. But it does mean you'll know going in whether the pairing you're considering has any structural problems — before you spend money on gear that'll never perform as it should. My old customer on Smith Street would have thanked me for the five-minute explanation before he walked out the door with the HD 800s and no amplifier. He came back for one eventually, anyway.