INTRO: Headphones range in impedance from 16 ohms to 600 ohms. And some models even come in multiple impedances. So which impedance is best? The answer mostly depends on what you plan to plug them into.
WHAT’S BEST? Are lower impedance headphones better than higher impedance ones? What’s the best output impedance? This is over simplifying, but here are some general guidelines:
- If you’re looking for headphones to use with a portable player or laptop, stick to the range of 16 – 32 ohms with a sensitivity (efficiency) rating of at least 100 dB/mW. There are some higher impedance headphones, up to 80 ohms or so, that are efficient enough to work well with at least some portable gear—especially if you don’t like it very loud. But, in general, the lower the impedance the better the match with battery powered devices.
- If you don’t know the output impedance of your source device, it’s best to avoid balanced armature headphones as they can interact with a higher output impedance in some ugly ways that make them sound substantially worse.
- If your source complies with the European maximum volume regulations (many new phones do even when sold outside of Europe) it is even more important to choose headphones with, ideally, a 16 ohm impedance at at least 100 dB/mW sensitivity. The European compliant devices have even less maximum output than most other portable gear.
- If you’re using a dedicated headphone amp or DAC, check the manufacture’s specifications or guidelines for the recommended headphone impedance range.
- If you’re looking at headphones with less than 100 dB/mW check out at least the first few paragraphs of the More Power article.
DIFFERENT HEADPHONE IMPEDANCES: In the world of consumer speakers, nearly all have an impedance in a relatively narrow range of 4 – 8 ohms. This makes it easier for designers of amplifiers, receivers, etc. as they pretty much know what the speaker impedance will be. With headphones, however, things are a lot different. There are few well defined standards, so impedances vary widely between manufactures and headphones designed for portable, home and studio/professional use. The range from 16 ohms to nearly 40 times higher at 600 ohms. This creates lots of compatibility issues.
WHY IMPEDANCE MATTERS: Headphone sources generally put out very different amounts of power into different headphone impedances. For example the Clip+ portable player can put out 16 mW into 16 ohms but only 0.8 mW into 300 ohms. The FiiO E7 can put out over 100 mW into 16 ohms but only 2.8 mW into 600 ohms. And some sources are not as compatible with low impedances such as the Mini3 and FiiO E9.
LOUD ENOUGH: Most headphone sources can manage at least 5 mW of power into 16 ohms. With headphones rated for 100 dB/mW (the suggested minimum above for portable use) that means 107 dB SPL which is in the range of 105 dB SPL – 115 dB SPL that most consider “loud enough”. See: More Power
WHAT’S CONSIDERED “HIGH IMPEDANCE”? I’m not aware of any hard rule but generally it’s safe to say 100 ohms and higher qualifies as “high impedance”. Such headphones are usually not designed for portable use. And it’s safe to say 32 ohms and lower qualify as “low impedance” and typically work well for all applications. That leaves a gray area between 32 and 100 ohms where other factors determine how suitable the headphones are for a given source.
BALANCED ARMATURE IEMs: Many of the high-end in-ear monitor type headphones use a technology known as balanced armature (also called “micro armature”, etc.). Some examples including popular in-ear models from Shure, Etymotic, Ultimate Ears, etc. These headphones usually have a rated impedance between 16 and 32 ohms but their actual impedance typically varies greatly with frequency. The 21 ohm Ultimate Ears SuperFi 5, for example, ranges from 10 ohms to 90 ohms. These wide variations interact in often unfavorable ways with the Output Impedance of the source.
WHY DO HEADPHONE IMPEDANCES VARY SO MUCH? There are multiple reasons:
- History Favored High Impedance - Before portable nice sounding portable gear came along most high quality headphones were plugged into either home stereo equipment or professional studio equipment. Because it was cheap and easy, those headphone jacks on receivers from the 60’s – 80’s typically had a very high output impedance. They simply used large resistors to drop the power from the speaker outputs.
- Outdated 1996 Standard – A standard was set in 1996 for headphone output impedance to be 120 ohms—apparently more as a convenience for makers of gear with with headphone jacks. Stereophile has since summed up their opinion of the standard with: “Whoever wrote that must live in a fantasy world.” A much lower output impedance has significant advantages but the poorly conceived standard still likely had an influence towards higher impedance headphones.
- High-End Headphones Favor High Impedance - There can be some significant advantages to higher impedance headphones. A higher impedance allows for more turns of wire to be used in the voice coil of the driver. This can result in a better “motor system” with fewer compromises resulting in better overall sound. Higher impedance headphones also require less current to drive and that typically reduces distortion in multiple ways. It makes them more immune to output impedance differences between sources and also less sensitive to long cables and sharing a common wire in 3 wire cables and 3 wire connectors (i.e. headphone plugs and jacks). The amplifiers driving headphone nearly always produce lower distortion into higher impedances.
- The iPod Revolution Favors Low Impedance – As of 2009 over 220 million iPods have been sold. And before iPods there were other portable devices. It’s probably fair to say, if you include the headphone output on music-friendly cell phones, there are now over a billion portable audio devices in use. This is important because battery powered devices don’t work well with high impedance headphones. So they forced development of lower impedance headphones that would work well. But this was in direct conflict with the above three paragraphs. So you have high-end goals, and lots of history competing with the massive market potential of a billion portable devices.
WHAT HEADPHONES CAN I USE WITH MY SOURCE? This is main question most people have. It comes down to just three things:
- Power Requirements – Will a given source have a enough power to drive a given pair of headphones to a loud enough volume level? The 100 dB/mW guideline mentioned earlier should get someone close. For headphones with a lower sensitivity (or no sensitivity) specification, see the More Power article.
- Device Output Impedance – This is tricky because the output impedance of most sources is unknown. But the idea is to follow the “1/8th Rule” described in Output Impedance Explained. If you multiply the output impedance of your source by eight, that’s the lowest impedance headphones you should use with that source. The FiiO E9 amp, for example, has a 10 ohm output impedance. So it should only be used with headphones of 80 ohms or higher if you want to be assured of the best sound quality.
- Source Distortion – Some sources have a hard time with lower impedance headphones. Tube amps with no output transformers (like the Bottlehead Crack), for example, have much higher distortion into low impedance headphones.
CAN A MISMATCH DAMAGE ANYTHING? Using the wrong headphones for a given device generally won’t harm anything besides the sound. But some sources are capable of very high output and might damage more sensitive headphones. But this is only an issue if you turn up the volume well past sane levels—which could happen accidentally. Some headphone amps have a gain switch to help prevent this problem.
WHEN IS A HEADPHONE AMP OR DAC REQUIRED? If a given pair of headphones will not play loudly enough, or has other obvious problems with a given source, then a headphone amplifier or DAC may help. But these devices can also create their own problems. And if a given source already has a low output impedance and enough output power, than adding an amplifier can often make things worse instead of better. See: Headphone Amps/Dacs Explained.
BOTTOM LINE: It’s best to follow the guidelines above unless you otherwise can be fairly certain a particular pair of headphones will work well with a particular source.
VOLTAGE AND CURRENT: It’s important to have at least some understanding of voltage and current to understand impedance and what follows. Voltage is analogous to water pressure (i.e. PSI) while current is analogous to the volume of water (i.e. gallons per minute). If you let water run out of the end of your garden hose with nothing attached you get a lot of flow (current) and can fill a bucket quickly but the pressure at the end of the hose is near zero. If you put a small nozzle on the hose the pressure (voltage) is much higher but volume of water is reduced (it takes longer to fill the same bucket). The two are typically inversely related. High pressure usually means low flow and visa versa. The same is true of voltage and current.
HOSE NOZZLES: Impedance is roughly analogous to the size of a hose nozzle. A high impedance headphone is like a tiny nozzle. To get much water out you need a lot of pressure (voltage). A low impedance headphone is more like filling a bucket and requires more flow but not much pressure. A lot of headphone outputs on devices are good at one, OR the other, but not both. So it’s important to know which you have and match the headphone accordingly.
HEADPHONE LOAD IMPEDANCE: This article talks about two different kinds of impedance—the impedance of the headphones (usually easy to find) and the impedance of the source (usually difficult to find). A “perfect” source has an output impedance of zero ohms. This means it will always deliver the same output into any load. In practice, any output impedance below about 1 ohm approximates a “zero ohm” source. The blue circle on the left above represents a “perfect source”, the blue resistor (zig zag line) in the middle represents the output impedance. And the resistor on the right represents the load impedance (the headphones). If the output impedance is not zero, the voltage produced by the source will be reduced when a load is connected. The higher the output impedance, the greater the drop in voltage at the load. This drop is given by the formula: Load Voltage = Source Voltage * ( Load Resistance / ( Load Impedance + Output Impedance) ). For more information see Wikipedia Voltage Divider:
R vs Z: Resistance is measured in ohms and represented in electrical engineering by the letter R. It’s typically measured with a DC current and it’s what a Digital Multi-Meter measures. Impedance, however, is more complex and measured with an AC current and the letter Z is the correct representation (but some still use R). Because music is an alternating signal, not DC, impedance is how headphones are rated. Impedance is made up of resistance, capacitance and inductance. Few headphones “look” like perfect resistors—most have significant inductance and at least small amounts of capacitance. They’re what’s known as a reactive load and the math is considerably more complex than just the most basic form of Ohm’s Law.
MEASURING HEADPHONE IMPEDANCE: You can’t measure headphone impedance with a DMM (although many incorrectly try). The more reactive the headphones are, the less accurate the reading will be as the DMM is only measuring the DC resistance “R” not the AC impedance “Z”. The DC resistance will nearly always be lower than the AC impedance. And an AC impedance measurement is only valid at one particular frequency so it’s best to plot if versus frequency in a graph. To do this right requires specialized equipment that can monitor the current and voltage independently over the entire audio band. I use my Prism Sound dScope.YOUR IMPEDANCE MAY VARY: Virtually all passive headphones (without their own powered electronics) have a varying impedance that changes with frequency. As described above, they don't behave like a simple resistance when driven with typical audio signals. The gold colored graph below shows the impedance, in ohms, of the Ultimate Ears SuperFi 5 Pro in ear monitors. You can see they’re only at their rated 21 ohm impedance below about 200 hz. The impedance rises to nearly 90 ohms at about 1200 hz and drops to below 10 ohms at 11,000 hz:
PHASE: When impedance varies with frequency so will the phase. In this case “phase” is the time difference between the peak voltage and peak current. The greater the phase variation the more “reactive” the load. Generally, the greater the phase shift, the more difficult the load is to properly drive. The white curve above is the phase shift shown in degrees. A simple resistor will have essentially zero phase shift with frequency over the audio band.
HEADPHONE DESIGN COMPROMISES: Drivers, in speakers or headphones, have moving mass. The diaphragm of the driver is suspended in a way that allows it to move. The stiffness of this suspension, the weight of the diaphragm, and sometimes the enclosure it’s in, work together to form a natural resonant frequency. The main resonance in full size headphones is usually in the bass frequencies and most high quality drivers require some sort of damping to control this resonance. If it’s not controlled, it tends to seriously degrade the bass performance of the driver. The impedance peaks at this resonance. Here’s the popular Sennheiser HD280 headphones showing an obvious bass resonance at 75 hz:
WHAT ABOUT TYPICAL EARBUDS? The vast majority of reasonably priced dynamic (not balanced armature) earbuds headphones have a 16 or 32 ohm nominal impedance that might vary by just 1 or 2 ohms. Here's the popular Sony MDR-EX51 which is mostly 17 ohms and rises to 18 ohms at its 5 khz resonance:
HIGH END HEADPHONES: Here’s the 300 ohm Sennheiser HD 650 which vary from about 305 ohms to 530 ohms:
OTHER RESOURCES: If you missed the links earlier, you might want to check out: