The Complete Guide To Understanding Headphone Impedance

Headphones work with AC electrical voltages (audio signals) and, therefore, have impedance. The impedance of a headphone model is critical to its performance yet is often overlooked by users.

What is headphone impedance? Headphone impedance refers to the inherent opposition the headphone circuitry has to the flow of electrical current. As the headphones create a circuit with an audio source, the impedance will dictate the voltage (audio signal) transfer as well as the quality and amplitude of the headphone's sound.

A quick answer like that doesn't really cut it. There's a lot more to know about headphone impedance, and in this article, we'll discuss everything there is to know about headphone impedance in detail.

For the simplified description of headphone impedance that will cover the basics of what you need to know, jump ahead to the section Headphone Impedance Simplified.

Related article: Microphone Impedance: What Is It And Why Is It Important?

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Table Of Contents

• What Is Impedance?
• Source And Load Impedance
• Impedance Matching/Bridging
• The Relationship Between Headphone Impedance And Sensitivity
• Headphone Impedance Simplified
• Headphone Driver Types And Their Impedances
• Low-Impedance Headphones
• High-Impedance Headphones
• Mid-Impedance Headphones
• Headphone Amplifiers
• Electrostatic Headphone Impedances
• Wireless Headphones And Impedance
• Active Headphone Impedances
• Low Output & Blow-Out
• Related Questions

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What Is Impedance?

Electrical impedance is defined as the measurement of opposition that a circuit presents to a current when a voltage is applied.

Impedance essentially extends the concept of electrical resistance to AC circuits. Unlike resistance, which applies to DC circuits and only has magnitude (measured in ohms), impedance has both magnitude (in ohms) and phase (in degrees) components.

Every electrical device that has AC circuitry has an electrical impedance. Impedance, then, is inherent in all analog audio signal paths and equipment include headphones.

Microphones also have impedance. To learn more about mic impedance, check out my articles Microphone Impedance: What Is It And Why Is It Important? and What Is A Good Microphone Output Impedance Rating?

Headphone impedance is one of the most important specifications we'll find on a headphone's datasheet. Knowing the impedance of a pair of headphones will help us determine which audio sources will best suit the headphones for optimal performance.

Note that the impedance value given by the manufacturer is only the nominal (average) impedance of the headphones. The actual impedance of headphones will vary across the headphones' frequency response.

As an example to help us better understand, let's have a look at the impedance graph of the popular Sennheiser HD 280 Pro headphones:

In the above graph, we see that the impedance of the HD 280s peaks at about 155 ohms at roughly 80 Hz before dropping back down as low as 60 ohms by 2,500 Hz. The impedance then increases slightly in the upper-frequency range.

Why do headphones have these peaks in impedance? It may seem counter-intuitive, but it's due to the natural resonant frequency of the headphone driver. More specifically, it refers to the resonance of the conductive coil of the headphone driver that completes the circuit with the audio source.

The Sennheiser HD 280 Pro is featured in the following My New Microphone articles:
• Top 5 Best Moving-Coil/Dynamic Headphones Under $100
• Top 5 Best Closed-Back Headphones Under $100
• Top 5 Best Circumaural (Over-Ear) Headphones Under $100

In our example, the HD 280 Pro's moving-coil has a natural resonance at about 80 Hz that coincides with the impedance peak.

The driving analog audio signal (AC voltage) has the easiest time moving the coil at the resonant frequency. The driving signal (from an audio device or headphone amplifier jack) only has to apply as much current as is necessary to prevent the decay of the coil oscillation.

As we approach the resonant frequency of the headphone driver coil from above or below, it essentially fights harder against the audio source (playback device/amplifier). The driver generates its own reactive electromagnetic force (voltage) in an attempt to oscillate at its resonant frequency. The source/amplifier will have to damp in order to maintain the desired signal.

It is at the resonant frequency that the audio signal has the easiest time driving the coil. It is also the most difficult frequency to drive the coil away from and so it has the highest impedance.

I like to think of it this way: if impedance opposes electrical current and the coil requires the least amount of current at its resonant frequency in order to move, then it would make sense that the impedance peak would coincide with the resonant frequency.

This is largely due to the way that moving-coil dynamic drivers work on the principle of electromagnetic induction. As electrical current passes through the coil, a coinciding magnetic field is produced within the coil, which causes the coil to move within an external permanent magnetic field.

As the resonant frequency, the coil requires less current in order for it to continue oscillating.

Note that moving-coil headphone drivers are by far the most common driver type and so we've begun our discussion with this type of driver. Skip ahead to the section Headphone Driver Types And Their Impedances to read about the impedances of the other driver types.

For more information on moving-coil dynamic headphones, check out my articles What Are Dynamic Headphones And How Do They Work? and Why & How Do Headphones Use Magnets?

The phase of the headphones, put simply, shows us a positive value when the driver resonance is “pulling” the electrical audio signal up towards resonance and a negative value when the driver resonance is “pulling” the electrical audio signal down to the resonance. At the resonance, the phase is midway through a flip and is effectively 0°.

This is a bit complicated. I'll do my best to provide more simplified information in the rest of this article to better educate about headphone impedance.

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Source And Load Impedance

To best understand how impedance works in the context of audio signal transfer and headphones, we need to know the terms source impedance and load impedance.

The source impedance is the inherent impedance of the audio output device. Output devices include the headphone jacks on headphone amplifiers, audio interfaces, recording devices, mixing boards, smartphones, laptops, etc.

Nominal headphone jack impedances are typically around 0.1 Ω to 24 Ω in regular device jacks and can get up to 120 Ω or more in dedicated headphone amplifiers.

The load impedance is effectively the “input impedance” of the headphones. When headphones are connected to a headphone jack, the headphone jack's device acts as the source and drives the headphone drivers, being the load.

Nominal headphone impedances typically range from about 8 Ω to 600 Ω though this range is loose.

Electrostatic headphones work a bit differently and can have nominal impedances in the hundreds of kiloohms with dedicated high-impedance amplifiers to drive their high-voltage low-current signal requirements.

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Impedance Matching/Bridging

In order to ensure optimal headphone performance, the headphones and the source impedances must pair well together. This is not to say that they are the same (they shouldn't be). Rather, it means that the impedance values should be complimentary. What does this mean?

Equal source and load impedances maximize the power transfer but not the voltage transfer. Having the impedances match also reduces frequency bandwidth and so it is not what we want when pairing headphones to an audio output source.

What we the load impedance (the impedance of the headphones) to be significantly higher than the source impedance. This is known as impedance bridging and will typically yield the best results for our headphones.

Impedance bridging is commonly used in audio signal transfer including the transfer between microphones and their connected preamplifiers.

For more information on microphone impedance bridging, check out my article Microphone Impedance: What Is It And Why Is It Important?

There are three reasons for impedance matching between an audio source and a pair of headphones that are worth mentioning:

• Damping factor
• Bass roll-off
• Distortion

Damping Factor

The damping factor between a source and headphones refers to the ability of the source/amplifier to control the motion of the driver once the audio signal has stopped. Kinetic friction plays a role in bringing the oscillating driver to a halt after a signal is removed. However, the electrical circuit (between the source and driver) also plays a major role and is described by the damping factor.

More broadly speaking, the damping factor is the amount of control the source/amplifier has over the driver.

The damping factor is a simple ratio of the load (headphone) impedance to the source impedance.

Ideally, we want the damping factor to be between 2.5:1 to 8:1 or even greater in some instances to ensure the driver is well-controlled within the audio circuit.

The loose rule of thumb is the “rule of eights” which suggests an 8:1 damping factor for optimal headphone results.

There are two reasons for this.

The first is that low-end clarity will suffer greatly with low damping factors. Low-end frequencies require relatively slow but large oscillations in the driver diaphragm. If the audio signal has poor control over the movement of the diaphragm, it will affect the driver's ability to accurately perform the oscillations required of low frequencies (more so than higher frequencies).

The result is a boomy and undefined low-end with a poor transient response, which is unwanted in any headphones.

The second reason is that impedance spikes in moving-coil headphone drivers response. A lower damping factor may seem sufficient relative to the nominal impedance but may cause issues if the headphones have a spike in impedance.

At the frequency of the impedance peak, the impedance matching may not be sufficient to provide ample damping and may lead to distortion and/or alteration to the headphones' frequency response.

To learn more about headphone frequency response, check out my article What Is Headphone Frequency Response & What Is A Good Range?

Bass Roll-Off

In addition to poor low-end clarity, a low damping factor can actually roll-off bass frequencies in the headphone frequency response.

This is most noticeable with a damping factor less than 2:1 and affects the perceived roll-off below the resonant frequency.

Distortion

A properly matched source and pair of headphones, as discussed, has a load (headphone) impedance that is 8x that of the source.

Compared to lower damping factors, the 8:1 ratio circuit will require less current to drive the headphone driver. This ultimately reduces distortion by not overloading the drivers' circuits and by mitigating crosstalk between drivers in unbalanced stereo headphones and

It's always best to test headphones before buying with the intended sound source rather than a random sound source at a retailer. A change in the source impedance has the potential to alter the character and quality of the headphones.

The rise in popularity of mobile audio devices (i.e., mp3 players and smartphones) has caused a shift in headphone standards.

It used to be that high-end headphones were designed with high-impedance for use with amplifiers. Today, with the current technology and market, headphones are often designed around 32 Ω for versatility and use with the aforementioned mobile audio devices.

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The Relationship Between Headphone Impedance And Sensitivity

Headphone sensitivity relates the loudness of the headphones to a given power level. It is typically measured at a specific frequency (1 kHz) at 1 mW of power.

Headphone sensitivity specifications generally fall between 90 dB SPL and 105 dB SPL though there are outliers beyond this range.

Though there is no correlation between impedance and sensitivity in headphone design, there is certainly a correlation between the impedance matching values and the volume of the headphones.

Remember that sensitivity is simply the sound pressure level produced by the headphone when a certain electrical power is applied to headphone drivers.

Impedance variations at the source may certainly alter the potential power transfer between the source and the headphones and, therefore, affect the volume capabilities of the headphones. However, the sensitivity value is fixed.

Altering the source impedance will not change the headphone sensitivity rating.

Let's have a look at some headphone examples to see that impedance and sensitivity are not causally linked:

Headphones Model	Impedance	Sensitivity
Sennheiser HD 280 Pro	64 Ω	113 dB
Bose QuietComfort 35 Series II	40 Ω (passive mode) 480 Ω (active mode)	99 dB (passive mode) 96 dB (active mode)
Sony MDR–7506	63 Ω	104 dB
AKG K 240	55 Ω	91 dB
Bang & Olufsen Beoplay H4	20 Ω	91 dB
Beyerdynamic DT 770 Pro	32 Ω 80 Ω 250 Ω	96 dB 96 dB 96 dB
Audio-Technica ATH-M50x	38 Ω	99 dB
Focal Utopia	80 Ω	104 dB
Shure SE215	17 Ω	107 dB
Grado Labs PS2000e	32 Ω	100 dB
Bowers & Wilkins PX7	20,000 Ω	111 dB
STAX SR-007A MK2	170,000 Ω	100 dB/100 V RMS*
Audeze LCD-X	20 Ω	95 dB

The headphones in the table above are taken from My New Microphone's article The Top 13 Best Headphone Brands In The World.

There's some pretty wild variation in the above chart. Here are a few things to consider about the headphones in the list:

• The Bose QuietComfort 35 Series II; Bang & Olufsen Beoplay H4, and Bowers & Wilkins PX7 are wireless headphones. More on wireless headphone impedance here.
• The STAX SR-007A MK2 is a pair of electrostatic headphones. More on electrostatic headphone impedance here.
• The Shure SE215s are in-ear monitors.

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Headphone Impedance Simplified

Headphone impedance is perhaps the most important specification to understand with any given pair of headphones.

There's a lot to know when it comes to impedance and for the non-electrically inclined, it can be difficult to wrap our heads around what impedance means in complete detail.

However, there is a simple way to understand headphone impedance specifications for the practical use of different headphones with different headphone jacks and amplifiers. Let's get into this simplified definition.

As we've discussed, impedance is measured in ohms (Ω) and is a specification that all headphones have. It basically tells us much power the headphones will need to get to a reasonable listening volume.

The higher the impedance, the more power the headphones need.

Headphone specifications can be interpreted in the following ways:

• 32 Ω and below: the headphones will work well with consumer devices (relatively low-level) such as smartphones, laptops, etc. without issue.
• 32 Ω to about 100 Ω: the headphones may suffer slightly when connected to consumer devices or low-level headphone output jacks though not necessarily. Headphones in this range would likely benefit from a headphone amplifier, though a headphone amp is not vital to getting good results.
• 100 Ω and above: the headphones will require a headphone amplifier to properly drive their high-impedance drivers and produce their optimal result.

So then, why would we want high-impedance headphones? They're often more expensive; require extra gear (amplifier), and are less portable.

Well, since high-impedance headphones are able to handle stronger electrical signals, reproduce sound more accurately in theory. For example, many professional mixing/mastering headphones are high-impedance.

That being said, many professional-grade headphones are now being designed with lower impedances to better meet the demands of the market. These low-impedance headphones allow us to connect to our favourite consumer devices with clean, clear and professional-sounding results.

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Headphone Driver Types And Their Impedances

So far we've been mostly discussing moving-coil dynamic headphones. This is appropriate since the vast majority of headphones and earphones on the market are designed with these types of drivers.

However, there are 5 headphone driver types on the market today and it's worth discussing their general impedances.

These driver types are:

• Moving-coil dynamic
• Planar magnetic
• Balanced armature
• Electrostatic
• Bone conduction (piezoelectric)

Moving-Coil Dynamic Driver Impedance

The impedance of the typical moving-coil dynamic driver has a peak that coincides with the natural resonance of the conductive coil. In most dynamic drivers, this resonant peak is somewhere in the lower midrange or bass range of frequencies.

Let's have a look at the 3 versions of the moving-coil dynamic Beyerdynamic DT 880 headphones (32 Ω, 250 Ω and 600 Ω):

Beyerdynamic DT 880:

These 3 headphones effectively sum up the typical range of moving-coil dynamic impedance values.

We notice that the impedances peak in the lower frequencies (around 80 Hz) and rise again in the high-end. This is typical of moving-coil dynamic headphone impedance graphs.

To learn more about moving-coil dynamic headphones, check out my Complete Illustrated Guide To Moving-Coil Dynamic Headphones.

Planar Magnetic Driver Impedance

Planar magnetic headphones are dynamic (they work on the principle of electromagnetic induction). However, they do not have a conductive moving coil. Rather, they have a layout of conductive traces on the diaphragm surface.

For more information on dynamic headphone drivers, check out my article What Are Dynamic Headphones And How Do They Work?

Planar magnetic drivers generally have very low impedances. The planar conductor in the planar magnetic circuit exhibits a mostly resistive load to the source/amplifier. The design of planar magnetic headphone drivers yields a very flat impedance over the entire frequency ranges and makes them very easy to drive.

Let's have a look at a couple of examples of planar magnetic headphone impedance curves:

This is the curve of the previously-mentioned Audeze LCD-X. It has a published nominal impedance rating of 20 Ω though, in the graph shown below, we can see that the actual impedance is closer to 15 Ω across the entire frequency response.

Audeze LCD-X:

The Audeze LCD-X is featured in My New Microphone's Top 5 Best Planar Magnetic Headphones.

Next up is the impedance graph of the HiFiMan HE1000 planar magnetic headphones. These headphones have a nominal impedance of 35 Ω. As we'd expect from a pair of planar magnetic headphones, the impedance line is practically flat.

HiFiMan HE1000:

The major takeaway here is that planar magnetic headphones are generally low-impedance and have very little variation in their impedances over the range of their frequency responses.

To learn more about planar magnetic headphones, check out my article The Complete Guide To Planar Magnetic Headphones (With Examples).

Balanced Armature Driver Impedance

Balanced armature drivers are notorious for their small bandwidths when it comes to frequency response. They are also only used in earphones due to their size limitations.

Many “balanced armature” earphones are, therefore, designed with multiple balanced armature drivers to fill out a wide frequency response. This design calls for cross-over components, which add complexity to the overall impedance of the earphones due to the added reactive components (inductors and capacitors).

This often results in seemingly odd impedance graphs without many similarities between different balanced armature driver designs.

Below is the impedance graph of the Shure SE535 triple-driver in-ear monitors (nominal impedance is 36 Ω).

Shure SE535:

The Shure SE535 is featured in My New Microphone's Top 5 Best Balanced Armature In-Ear Monitors Under $500.

Here is the impedance graph of the Klipsch X20i dual-driver earphones (nominal impedance is 50 Ω):

Klipsch X20i:

Balanced armature earphones require low-impedance amplifiers to drive them properly. Other amps/sources may very well lead to an overly coloured listening experience.

To learn more about balanced armature earphones, check out my article The Complete Guide To Balanced Armature IEMs/Earphones.

Electrostatic Driver Impedance

Electrostatic headphone drivers are quite different than the aforementioned dynamic drivers. These drivers work on the principles of electrostatic electricity.

Essentially, a biased diaphragm sits between two stator plates that act as a sort of capacitor.

In order to properly charge the stator plates, the audio signal must have its voltage cranked way up while its current is dropped way down. The headphones must have incredibly high impedances to ensure a negligible stray charge from the system.

Though impedance graphs for electrostatic headphones are rarely, if ever, produced, some manufacturers offer nominal impedance values for their electrostatic headphones. Let's have a look at a few examples:

• Koss ESP-950: 100,000 Ω
• STAX SR-007A MK2: 170,000 Ω

The Stax SR-007A MK2 is featured in My New Microphone's Top Best Electrostatic Headphones.

Needless to say, electrostatic headphones require specialized headphone amplifiers design to properly boost the voltage and impedance of the audio signals while also often providing the biasing voltage for the diaphragm.

To learn more about electrostatic headphones, check out my article The Complete Guide To Electrostatic Headphones (With Examples).

Bone Conduction Driver Impedance

Bone conduction headphones work on the principle of piezoelectricity. Piezoelectricity refers to the electric charge that accumulates in certain solid materials in response to applied mechanical stress.

If we reverse that statement, we could say that by applying an electrical charge to a piezoelectric material, we could cause mechanical stress.

This is what happens with the piezoelectric crystals of bone conduction headphones.

The impedance of these crystals is rarely found on the specifications sheets of bone conduction headphones but we can assume that the impedance is very high due (in the tens of thousands of ohms) to the nature of the piezo material.

To learn more about bone conduction headphones, check out my article The Complete Guide To Bone Conduction Headphones (With Examples).

To learn more about each driver type, please consider reading my article What Is A Headphone Driver? (How All 5 Driver Types Work).

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Low-Impedance Headphones

There's no set standard threshold that deems a pair of headphones as “low-impedance.” Some people would argue that 32 Ω or less constitutes low-impedance while others state 50 Ω.

For the sake of this article, let's go with 32 Ω but keep an open mind to the fact that the following information may very well apply to higher impedance values.

The Grado Labs PS2000e is an example of a pair of low-impedance headphones with a nominal impedance rating of 32 Ω.

Low-impedance headphones are designed to work with most headphone jacks. This includes smartphones, laptops and other consumer electronics. They also work well with professional gear such as mixing consoles and recording devices.

However, connecting a low-impedance pair of headphones to a high-powered amplifier could cause trouble. Sending too much signal voltage to a low-impedance headphone driver could overload it and cause significant distortion or, even worse, a blow-out of the driver diaphragm.

Planar magnetic headphones tend to have low-impedances. Moving-coil dynamic mics with larger gauge coils often typically have lower impedances. Balanced armature designs, as we have discussed, also tend to have low-impedance ratings.

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High-Impedance Headphones

High-impedance headphones require higher voltages to drive their drivers but often benefit from added clarity and performance when matched properly to a compatible amp.

For the sake of this article, we'll set an arbitrary “high-impedance” threshold at 100 Ω. So headphones with an impedance rating above 100 Ω can be considered high-impedance though this is not set in stone, so to speak.

The AKG K240 is an example of a pair of high-impedance headphones with a nominal impedance rating of 300 Ω.

The AKG K240 is featured in the following My New Microphone articles:
• Top 5 Best Moving-Coil/Dynamic Headphones Under $100
• Top 5 Best Headphones For Podcasting Under $100
• Top 5 Best Open-Back Headphones Under $100
• Top 5 Best Circumaural (Over-Ear) Headphones Under $100

These same headphones tend to suffer when plugged into a typical headphone jack (like those found in consumer electronics) since the internal digital-to-analog audio converters/amplifiers cannot provide the signal voltage necessary to drive the drivers at their full potential.

Moving-coils with lower mass, thinner gauges and more turns will have higher impedances and tend to sound better. The coil fits tighter and produces a stronger electromagnetic field when a current is applied. This enhances audio quality by improving the driver's electromagnetic induction capabilities.

Electrostatic headphones have an incredibly high impedance in order for their capacitor-like drivers to function properly. We'll discuss this soon.

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Mid-Impedance Headphones

Headphones with impedance ratings between 32 Ω and 100 Ω find themselves in a sort of a “grey area” where they should sound decent with and without a dedicated amplifier.

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Electrostatic Headphone Impedances

It bears repeating that electrostatic headphones are way outside the normal 8 Ω – 600 Ω range for headphone impedance.

Their extremely high impedance ratings are typically above 100 kΩ (100,000 Ω). This seemingly extreme impedance is required to keep the electric charge from escaping the driver design.

Here is a simplified diagram to show the inner workings and amp/power requirements of the typical electrostatic headphone driver:

The audio signal is amplified to become an extremely high-impedance high-voltage low-current signal and is used to apply alternating charges on the stator plates. The high impedance helps to keep the charge from dissipating.

As one stator plate experiences a positive charge, the other experiences an equal but negative charge.

Since the diaphragm is positively biased, it will be pushed and pulled by the stators in a way that mimics the original audio signal. The diaphragm movement produces sound waves, which is how electrostatic drivers work with incredibly high impedance ratings.

The specialized electrostatic headphone amplifiers will boost the voltage of the audio signal appropriately and may also be used to provide the power need to bias the diaphragm.

For more information on headphone power requirements, check out my article How Do Headphones Get Power & Why Do They Need Power?

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Wireless Headphones And Impedance

Impedance is not as big an issue with wireless headphones. Of course, the headphone drivers within wireless headphones operate with analog (AC) audio signals and certainly have inherent impedance but it's not an overly important factor.

This is because wireless headphones do not physically connect to different sources and amplifiers.

Wireless headphones may receive audio from various transmitters, though they are often designed with specific transmitters or with a standard wireless connection protocol like Bluetooth.

But even with the potential source/transmitter variation, the impedance of the typical wireless pair of headphones is not overly concerning.

The reason is that the headphone receivers have their own built-in amplifiers that will effectively adjust the received audio signal to properly drive the drivers.

For more information on wireless headphones, check out the following My New Microphone articles:
• How Bluetooth Headphones Work & How To Pair Them To Devices
• How Do Wireless Headphones Work? + Bluetooth & True Wireless

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Active Headphone Impedances

Headphones may be designed with active components other than wireless receivers.

These active circuits are often used for noise-cancelling and sometimes for bass enhancement.

The impedance values of these headphones may seem considerably higher than normal.

Take the aforementioned Bowers & Wilkins PX7 wireless noise-cancelling headphones for example. This pair of headphones has an impedance rating of 20,000 Ω which is very high for moving-coil dynamic drivers.

The Bowers & Wilkins PX7 is featured in the following My New Microphone articles:
• Top 5 Best Wireless Headphones Under $500
• Top 5 Best Closed-Back Headphones Under $500
• Top 5 Best Circumaural (Over-Ear) Headphones Under $500
• Top 5 Best Noise-Cancelling Headphones Under $500

This rating is due to the signal having to be applied to the active circuit rather than straight to the driver itself. The impedance rating is, therefore, referencing the input impedance of the active circuitry, which is often remarkably higher than the driver.

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Headphone Amplifiers

In this article, we've been discussing headphone amplifiers as a potential source for driver headphones. Let's now define headphone amps in a bit more detail.

A headphone amplifier is a relatively low-level amp that acts to boost the voltage of an audio signal to properly drive connected headphones. Headphone amps also act to better match the impedance at their outputs to provide optimal signal transfer and headphone performance.

When we discuss headphone amps, we typically mean standalone units. However, technically speaking, the majority of headphone amps are found just within the headphone jacks of devices that have headphone jacks (smartphones, computers, mp3 players, etc.) and are often integrated into digital-to-analog audio converters.

Most low-impedance headphones will not require a dedicated headphone amp. The digital-to-analog converter and/or amp within a device's headphone jack will often be sufficient to drive these headphones.

Think of connecting a low-impedance pair of earbuds into your laptop. There's no issue.

However, high-impedance headphones generally always need a dedicated headphone amplifier to perform at their best. Headphone amplifiers boost the voltage and power of the audio signal, allowing the signal to drive the high-impedance headphones with improved accuracy and volume.

Note that the output impedance of the headphone amplifier should ideally follow the rule of eights mentioned earlier. A source impedance (headphone output impedance) should be an eighth of the load impedance (headphone impedance).

Headphone amps can effectively drop the impedance of a signal at their inputs relative to their outputs to better drive the headphones in addition to providing the necessary gain to the signal.

Let's take a look at the Rupert Neve RNHP headphone amplifier as an example.

The Rupert Neve RNHP has an astonishingly low output impedance of 0.08 Ω. This low source impedance will work amazingly well with any pair of headphones.

The inputs include professional line level (left and right mono), RCA consumer line level (left and right mono), and auxiliary 3.5mm (stereo). These inputs allow the RNHP to take a variety of input signal impedances. The amplifier will “convert” them to the low-impedance output level of 0.08 Ω.

I've put together the following table consolidating the typical impedance values of various audio inputs and outputs:

Input/Output Type	Typical Impedance Range	Typical Voltage Range (Nominal)
Mic Level Output	50 Ω to 600 Ω	-60 dBV (1 mVRMS) to -40 dBV (10 mVRMS)
Mic Level Input	1.5 to 15 kΩ	-60 dBV (1 mVRMS) to -40 dBV (10 mVRMS)
Instrument (Hi-Z) Level Output	10 kΩ to 100 kΩ	-20 dBu (77.5 mVRMS)
Instrument (Hi-Z) Level Input	47 kΩ to over 10 MΩ	-20 dBu (77.5 mVRMS)
Line Level (Professional) Output	75 to 600 Ω	+4 dBu (1.228 VRMS)
Line Level (Professional) Input	10 kΩ to 50 kΩ	+4 dBu (1.228 VRMS)
Line Level (Consumer) Output	75 to 600 Ω	-10 dBV (316 mVRMS)
Line Level (Consumer) Input	10 kΩ to 50 kΩ	-10 dBV (316 mVRMS)
Speaker Level Output	<100 mΩ	20 dBV to 40 dBV (10 VRMS to 100 VRMS)
Speaker Level Input	4 Ω to 16 Ω (4,8 or 16 Ω)	20 dBV to 40 dBV (10 VRMS to 100 VRMS)
Aux Output	75Ω to 150 Ω	-10 dBV (0.300 VRMS)
Aux Input	>10 kΩ	-10 dBV (0.300 VRMS)
Headphone Jack Output	0.1 Ω to <24 Ω	N/A
Headphone Amplifier Output	0.5 Ω to >120 Ω	N/A
Headphone Input	8 Ω to 600 Ω	N/A

To learn more about headphone amplifiers, check out my article What Is A Headphone Amplifier & Are Headphone Amps Worth It?

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Low Output And Blow-Out

Pairing headphones and headphone outputs with compatible impedances (load and source, respectively) is important for optimal headphone performance.

Failing to do so could result in one of the following occurrences:

• Low output
• Blow-out

When connecting a high-impedance pair of headphones to a low-level output, the audio signal may have difficulty driving the drivers properly.

This stops the headphones' performance short of their potential; leads to low output levels and poor signal-to-noise ratios.

For example, connecting a pair of AKG K240s (600 Ω) to a smartphone will lead to lacklustre results. Even at max volume, the signal will not be able to drive the K240s to their full potential. The sound will have low-volume; it may suffer in its signal-to-noise ratio, and artifacts may even be introduced (all unwanted).

The iPhone 6S (the last Apple model with a dedicated headphone jack) has an output impedance of 5.9 Ω.

On the other end of the spectrum, we have blow-outs. Blow-outs are what they sound like: the headphone driver blowing out (stretching, tearing or otherwise damaging) its diaphragm. This happens when low-impedance headphones are connected to high-powered amplifiers and should be avoided at all costs.

For example, blow-out would likely happen if we connected Apple EarPods (45 Ω) to a professional headphone amplifier like the Rupert Neve RNHP and proceeded to max out the amp's volume.

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Related Questions

What is headphone sensitivity? Headphone sensitivity relates the loudness of the headphones to a given power level. Sensitivity ratings are given in decibels of Sound Pressure Level per milliwatt (dB SPL/mW) or simply as a dB value. Headphones with higher sensitivity ratings are considered to be louder in general.

Related article: The Complete Guide To Headphones Sensitivity Ratings.

What are the specifications that determine the quality of a pair of headphones? The electrical specifications that determine the audio quality of a pair of headphones include frequency response, impedance and sensitivity. Physical factors such as weight and subjective comfort and fitting also play a role in determining overall headphone quality.

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Sources

• https://www.innerfidelity.com/images/SennheiserHD280Pro.pdf
• https://www.innerfidelity.com/images/BeyerdynamicDT88032ohm.pdf
• https://www.innerfidelity.com/images/BeyerdynamicDT880250ohm.pdf
• https://www.innerfidelity.com/images/BeyerdynamicDT880600ohm.pdf
• https://www.innerfidelity.com/images/AudezeLCDXSample3.pdf
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