All headphones have an impedance rating, measured in ohms. But what do headphone ohm ratings mean? In practical terms, impedance tells us how much power your headphones require to deliver a reasonable listening volume. Put simply, the higher your headphones’ impedance, the higher source voltage they need. Knowing your headphones’ impedance helps you determine which audio sources will best suit your headphones.
Let’s compare low-, mid-, and high-impedance headphones, and look at the use cases they fit best.
Low-impedance headphones: Up to 32 ohms
Low-impedance headphones are a good match with the headphone outputs on battery-powered consumer devices like smartphones, tablets, and laptops. These headphones don’t require much voltage to drive them, so they’re likely to be plenty loud with all your small, portable devices. But be careful — if you plug an 8-ohm pair of earphones into a pro DJ mixer and turn it up to the max, you’re likely to blow them out. Even if you don’t damage your earphones permanently, you’ll get bad-sounding clipping distortion.
Low-impedance does not mean low-quality. In fact, Audio-Technica’s high-end ATH-IEX1 earphones have impedance of only 5 ohms! Most in-ear headphones and professional IEM earphones are relatively low-impedance. For example, Sennheiser IE 400 PRO earphones are 16 ohms, and Shure SE215 earphones are 20 ohms.
Mid-impedance headphones: Between 32 and 100 ohms
You generally can’t go wrong with headphones in this middle ground — they can work with a wide range of equipment. Pair them with the headphone output on your DJ mixer, audio interface, or standalone headphone amplifier, and they’ll really shine. You can also plug them into portable consumer devices like your smartphone, and they’ll sound good, although they’ll likely be noticeably quieter than low-impedance headphones. If you want your mid-impedance headphones to be loud enough when driven by battery-powered devices, look for a sensitivity rating of at least 100 dB SPL per milliwatt. (See “What about headphone sensitivity?” below for more on sensitivity.)
Most general-use over-ear headphones, popular DJ headphones, and studio-workhorse headphones fall into this middle range. For example, Pioneer DJ HDJ-X10 headphones are 32 ohms, Audio-Technica ATH-M50x headphones are 38 ohms, AKG K240 Studio headphones are 55 ohms, Sony MDR-7506 headphones are 63 ohms, and Sennheiser HD 280 PRO headphones are 64 ohms.
High-impedance headphones: 100 ohms and up
These relatively high-impedance headphones require more source voltage than a consumer device can typically provide. If you plug a pair of 250-ohm headphones into your mobile phone, you won’t hear much, even at max volume. They’ll usually require a standalone headphone amplifier, or a quality audio interface with a dedicated headphone amp built in, to drive them properly and produce enough voltage to unleash their great potential.
High-end studio headphones for critical listening, as well as headphones intended for audiophiles, often fall into this category. For example, Neumann NDH 20 headphones are 150 ohms, Beyerdynamic DT 990 PRO headphones are 250 ohms, and Sennheiser HD 650 headphones are 300 ohms.
Do high-impedance headphones sound better?
Back in the old days, way before laptops and iPhones were common, traditional studio and broadcast gear was relatively high-voltage. If you were an engineer plugging your headphones into the board, you wouldn’t want to overload your headphones and risk damaging your ears. High-impedance headphones offered protection against that risk.
But if high-impedance headphones require so much voltage to drive properly, and generally don’t play nice with mobile devices that are the norm nowadays, why do they still exist?
Well, manufacturers don’t exactly set out to make headphones with high impedance. It’s more of a “side effect” of choosing components that sound good. For example, Beyerdynamic is known to use delicate, super-thin wire to wind their moving-coil headphone drivers. This results in densely packed voice coils with very little air between each layer of wire, so you end up with a driver that’s light in weight, yet has a strong electromagnetic field, which reduces distortion. And of course, that super-thin wire has fewer electrons to carry current, so it impedes the flow — resulting in a high-impedance headphone.
So, do high-impedance headphones sound better than low-impedance headphones? If you’re an audiophile or pro engineer, you may be able to tell the difference. But it’s far more important to choose headphones that are the best suited for your use case.
What is “Nominal Impedance”?
Manufacturers report their headphones’ impedance as a single number — the “average” or nominal impedance. But that single snapshot doesn’t tell the whole story. In the real world, impedance varies — sometimes drastically — across the headphones’ frequency range. This is true of headphones with conductive moving-coil drivers (most full-size cans), and headphones with multiple balanced armature drivers (most earphones). A notable exception: headphones such as the Avantone Planar Open-Back Headphones, which use planar magnetic drivers, resulting in a relatively flat impedance range.
Your so-called “64-ohm” moving-coil headphones may have impedance that peaks at 160 ohms at 80 Hz, and dips down to 60 ohms at 3 kHz. The nominal impedance of 64 ohms is measured at 1 kHz. It doesn’t tell the whole story, but it’s a useful shorthand spec.
What about headphone sensitivity?
Low-impedance headphones are generally louder than high-impedance headphones, for a given source voltage. But impedance isn’t the only spec that determines how loud your headphones can get — sensitivity also plays a big role. Just as in loudspeakers, sensitivity measures how efficiently your headphones convert power into sound. Sensitivity ratings are measured in decibels SPL (Sound Pressure Level) per milliwatt. They vary from low sensitivity (around 85 dB SPL/mW), to very high sensitivity (around 118 dB SPL/mW.) If you want your headphones to get very loud, look for sensitivity of 100 dB SPL/mW and up.
Impedance Matching: The 8:1 Rule
Have you ever noticed that the same pair of headphones can sound quite different when plugged into different devices? That’s because each device’s output has an impedance rating, too.
Most headphones work best when the source impedance (the output impedance of the device you’re plugging into) is much lower than the load impedance (the nominal input impedance of your headphones). For proper impedance matching, or impedance balancing, audiophiles recommend following the 8:1 rule of thumb: plug your headphones into a source with output impedance no higher than 1/8 your headphones’ impedance. For example, plug your 80-ohm headphones into a source with an output impedance of 10 ohms or lower.
Why does the 8:1 rule matter? If the source output impedance is more than 1/8 of the headphone impedance, you’ll get distortion in the form of audible variation in the frequency response. Highs, lows, or mids may get extra emphasis. These variations in frequency response can range from subtle to easily audible, and often unpredictable.
There’s just one problem with this 8:1 rule: Audio gear manufacturers rarely publish their devices’ output impedance — and there’s really no standard!
Fortunately, some high-end headphone amplifier makers actually do publish their output impedance specs. And high-end headphone amps are likely to have very, very low (near-zero) output impedance, so they’ll definitely be less than 1/8 the input impedance of any headphones you plug into them. For example, the Rupert Neve Designs RNHP headphone amplifier has an output impedance of only 0.08 ohms, and the headphone jack of the Apogee Boom audio interface has an output impedance of just 0.5 ohms. That means no matter which headphones you plug into those jacks., you’ll never break the 8:1 rule. Those low-impedance headphone jacks play nice with all sorts of headphones, and that’s why you’ll hear gear manufacturers touting their “zero ohm” headphone outputs.
But what if you want to listen to a device with a high output impedance — or an unknown output impedance? You could follow the 8:1 rule and bust out your fancy high-impedance headphones — or simply choose to accept that your low-impedance headphones may sound a bit different when plugged into different sources.