If you're considering buying high quality headphones, you're going to come across the term "high impedance" very quickly.
That's because headphones with high impedance typically deliver far superior sound – and unlike their low-impedance peers they don't distort or self-destruct if you connect them to audiophile-level or DJ equipment.
High-impedance headphones: what you need to know
Impedance is a measure of resistance to electrical current, and it's measured in ohms: the higher the figure, the more resistance there is.
In the world of headphones, low impedance is generally considered to be a figure lower than 50 ohms. High impedance headphones may be 250 or even 600 ohms.
There are some crucial differences between low impedance and high impedance headphones. The first and most important difference is that low impedance headphones don't need much power to deliver sound, and as a result that makes them ideal for applications where battery life is more important than sound quality.
For example, the amplifier in a typical smartphone is designed with 32-ohm headphones in mind. It may be able to drive much more resistant ones, but it won't get the best from them or from its battery.
The other really important point is that high impedance headphones really do sound better.
Why high impedance headphones simply sound better
When you listen to high impedance headphones on a compatible hi-fi system or studio equipment, you'll notice that there's a stark difference in sound quality.
The sound stage is bigger, the bass is more defined, and the individual instruments and voices are clearer and better positioned.
That's because the voice coil in high impedance headphones is engineered differently to the one you'll find in low impedance models. Its wiring is much, much thinner, and that enables the manufacturers to wind more wire more times and more tightly than they can on lesser models.
That does two things: it creates more magnetic force to move the coil's diaphragm, which is the bit that actually produces the sound; and it makes the coil much lighter and more responsive. That means much lower distortion, better dynamics and more accurate sound representation.
Other specs to study
There are some other figures to consider when you're looking at headphones. The first is the frequency range, and the second is the sensitivity.
Frequency range tells you how much of the sonic spectrum the headphones can deliver, and it's common to see 20Hz to 20kHz, which is the typical range of human hearing.
But some headphones are capable of much more, so for example you might see the lower limit as 8 or even 5Hz and the upper at 37 or even 44kHz.
The second figure is sensitivity, which is measured in decibels of sound pressure level – dB SPL for short. dB SPL tells you how many decibels of sound will be created from one unit of power, either 1 milliwatt or 1 volt, at a frequency of 1kHz.
The figure effectively tells you how loud the headphones are, with the higher numbers indicating higher volume.
Just make sure you're comparing like with like: you need to know if you’re looking at dB SPL/V, which is based on voltage, or dB SPL/mW, which is based on milliwatts.
Unhelpfully, some manufacturers only list dB without clarifying which measurement they're using. That can make it difficult to compare different models from different manufacturers.
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Together in perfect harmony
It's important to match your headphones to the amplifier or other hardware you'll be listening to. Headphones with too low impedance may deliver harsh, clipping distortion or may simply fail when connected to a high impedance output.
On the other hand, headphones with too high impedance will struggle to produce the full dynamics and volume or may have inconsistent volume levels.
As a rule of thumb, if you're buying for use with a smartphone or similar device then look for low impedance headphones around the 32 ohm mark; if you're buying headphones for your existing or planned audiophile hi-fi hardware, or for recording studio hardware or professional DJ kit, high impedance is the way to go.
That's why some manufacturers offer the same headphones with different impedance levels: they'll make a 32 ohm version for maximum compatibility with everyday audio devices alongside higher impedance versions for audiophile or studio use.