Ever heard of the saying ‘devil lies in the details’ ? It could be an overused and tired cliché, but it makes lots of sense in the audio electronics’ world. For instance, most people won’t say much past plug-and-play with headphones, but real audiophiles and sound engineers know there is more. And it’s all in the specification sheet!
The specification sheet shows what you should know about a pair of headphones, but not all specs mean much to the average customer. And among the most underrated and misunderstood spec is the headphones’ impedance rated in ohms. But why is the headphone impedance rating important? Read on for more on headphones impedance below.
What Is Impedance?
Impedance is a measure of an electric circuit’s resistance to electrical current flow. In a simple electronic circuit, both the audio source and the headphones have individual impedances known as source and load impedances. Practically, it shows the amount of power needed to drive the headphones and whether you’ll need an external amp or not.
Headphones impedance ranges from 8-600 ohms, with 32 ohms being a popular standard. The source impedance usually is low, mostly ranging between 0-4 ohms but can go as high as 120 ohms in some specialized amplifiers. Mainly, manufacturers indicate the nominal impedance of a pair of cans and can vary in magnitude (ohms) and phase (degrees) as the sound frequency ranges vary.
Headphones impedance is a factor of the voice coil’s length and wire size, the coil material, and the number of turns, affecting the sound quality produced. Typically an impedance rating of between 20-40 ohms is decent for casual listening without the need for an amplifier.
Source vs. Load Impedance
As mentioned earlier, both the headphones and the audio sources have individual impedances. The source impedance is inherent in audio outputs, including headphone jacks on amplifiers, recording devices, audio interfaces and smartphones.
The load impedance is the electrical resistance of the headphones, which essentially is the input impedance. For a better illustration, when you connect your headphones to the jack output of an amp, the impedance at the headphone jack’s device becomes the source while the headphone drivers act as the load.
Why Impedance Matters
Impedance determines the amount of power needed to drive a pair of headphones. Meaning, high impedance rated headphones require more power to perform optimally while low-impedance cans require way less power. To further simplify the concept, low-impedance headphones will work fine with low power devices like iPads and smartphones, while high-impedance headphones need an amplifier. So, why does impedance matter when buying headphones?
For starters, high-impedance headphones can be pricey but with exceptionally high quality and clear audio output. However, you’ll need another budget for an amplifier to provide enough juice for them. So, if you want headphones for your iPad, then high-impedance headphones aren’t the best for you unless, of course, you have a budget for an amp.
Technically, if you used high-impedance headphones with your phone, the resulting sound would suffer from a reduced headroom. Also, low-impedance headphones are prone to damage if used with a powerful amp at high volumes.
Low vs. High-Impedance Headphones
When we talk of low-impedance headphones, the threshold of how low the impedance should be is somehow non-standardized. Some people consider low to mean 25 Ω or less, while others go as high as 50 Ω. So, low-impedance could fall at different limits, but all should be able to run on battery-powered devices without draining the battery too fast or working sub-optimally.
Also, high-impedance has no standard lower threshold, but any headphone with 100 Ω ratings could fit this category. Typically, high-impedance headphones will be inaudible if used with low-power audio sources. On the good side, though, these headphones sound better, with minimal distortions and excellent clarity, if powered adequately.
Mid-impedance headphones fall within a sort of “grey area”, between the highest limit of low-impedance headphones and the lower limit of high-impedance headphones. Typically, mid-impedance range between 32 Ω to 100 Ω and works well with or without an amplifier.
Headphones Models with Multiple Impedances
Customarily, headphone models come with a single impedance rating. But there are brands like the Beyerdynamic that offer different versions of a headphone model with differing impedance ratings. For instance, the Beyerdynamic DT880 and DT990 come with 32,250 and 600 ohms versions, while the DT770 comes with a 32, 80, or 250 ohms version.
Electrostatic Headphone Impedances
Standard headphones have impedance ranges between 8-600 Ω, but electrostatic headphones can go as high as 100,000 Ω. The extreme impedance serves to maintain the electric charges within the driver’s stator plates to achieve expected performance. Electrostatic headphones require audio signal amplified to meet the high-impedance and high voltage but low current signal requirement to sustain alternating charges on the stator plates with less dissipation.
We have talked about using amplifiers to drive high-impedance headphones, but how do you choose the best amp for your cans? There is a lot of mathematics and scientific jargon involved, but the baseline is, the source and load impedances must pair well for optimal output.
Usually you could think that matching the load and source impedances solves the performance issue, but not exactly. Equal load and source maximizes power transfer but limits voltage transfer and lowers the frequency bandwidth. Simply put, with matched source and load impedances, you won’t be getting quality sound, especially the low and high frequencies.
To achieve optimal performance, the headphone impedance should be higher than the source impedance, a concept known as impedance matching or bridging. In other words, the source and load impedances should be complementary but not matching. And there are three reasons why impedance bridging between the audio source and headphones is essential.
1. Damping factor
The damping factor is a ratio between the load impedance and the source impedance. Essentially, the ratio shows how much control the amplifier has over headphones’ drivers when the audio signal is disconnected. In other words, the damping factor shows the control electrical circuit adds to kinetic friction to stop the oscillating drivers once the signal stops flowing.
Ideally, the damping factor should be between 2.5:1 to 8:1, or even greater to ensure the drivers remain under control even when responding to very low or high frequencies.
If the damping factor is low, the drivers’ ability to accurately oscillate as required in low frequencies is highly compromised. The bass becomes boomy with low clarity in a low-end transient response, which you won’t like. Also, as stated earlier, impedance fluctuates as the drivers respond to different frequencies, and there could a spike could occur sometimes. And a large damping factor helps eliminate resulting distortions or alterations to ensure the drivers react to frequencies accurately.
2. Bass roll-off
Bass roll-off refers to the decrease in response playback volume as the drivers handle lower frequencies. Typically, bass roll-off is clearly noticeable in low-damping factor setups, as the electrical current reduces with increasing output impedance. The bass becomes boomy and sounds less controlled, and it gets worse as the transient response roll-off sooner than expected. So, correct impedance bridging is vital in eliminating bass roll-off.
It goes without saying that impedance mismatch results in current overloading on the drivers’ circuits, which results in distortions and crosstalk in unbalanced stereo headphones. The damping factor should be as close to the recommended ratio of 8:1 or even higher to eliminate the overload.
The baseline is, you should test headphones with the intended audio source to ensure there is a proper impedance match. However, most headphones with an impedance rating of 32 Ω should fit most mobile audio devices and laptops.
Relationship between Impedance and Sensitivity
Headphone sensitivity ranges between 90-105 decibels and relates to how loud the headphones will be at a given power level. Typically, the sensitivity is measured at a frequency of 1 kHz and 1 mW of power. Technically, there is no sensitivity and impedance correlation in headphones, but impedance matching values change as the volume varies.
To clarify further, the impedance variations alter the power transfer between the audio source and the headphones, therefore affecting how loud the headphones can be, even when sensitivity remains constant.
The relationship between headphones’ impedance and sensitivity is pretty complicated. Though both play a part when doing impedance matching. Ideally, more sensitive headphones will be louder, but the relationship is not that direct when you factor in the impedance. Therefore, to completely understand the relationship between impedance and sensitivity, other factors such as load resistance come into play.
The math gets more complicated beyond basic Ohm’s law, as amplifiers normally deliver more voltage to higher impedance loads. All in all, the headphones’ audio volume and performance are factors of source impedance, sensitivity, and load impedance. And therefore, you’ll never be totally sure about the performance of a pair of headphones until you try them with the intended audio source.
Headphone Sensitivity vs. Impedance
Now that we know sensitivity and impedance have a relation, do manufacturers consider impedance and sensitivity consistency when designing headphones? They actually don’t, and it’s surprising that there is little consistency even among headphones from the same manufacturer. For instance, you’ll find low impedance headphones with low sensitivity and vice versa, while others with low sensitivity and high impedance. And, these varying combinations determine the ideal source requirements for each pair of cans. For example, the table below shows some popular headphones with their sensitivity and impedance ratings.
|Headphone model||Impedance (Ohm) at 1kHz||Sensitivity (dB)|
|Beyerdynamic T1 (gen 2)||600||102|
|Meze 99 Classics||32||103|
|Shure SRH 1840||65||96|
|Bowers & Wilkins PX7||20,000||111|
|Beyerdynamic DT 770 Pro||32 Ω 80 Ω 250 Ω||96 dB 96 dB 96 dB|
Which Headphone Impedance Best Suits What Situation?
Headphone impedance is a must-understand spec if you care about audio quality. However, when the maths and technical jargon comes into play, the issue can get complicated quickly. But there is a more straightforward way to wrap your head around the whole impedance issue and match headphones with ideal audio sources easily.
At the fundamental definition, impedance tells you how much power a pair of headphones requires to perform optimally, and the relationship is direct. Therefore, the higher the impedance, the higher the power needed. And now, with that in mind, the following impedances fit the stated situations perfectly:
- 32 Ω and below: Works best with low-power level consumer devices such as smartphones, laptops, and iPads, among other mobile devices.
- 32 Ω to 100 Ω: Headphones at this range can work with low power level consumer products but would benefit perform-wise from a headphone amplifier. These ranges of headphones are used for recording in the studio.
- 100 Ω and above: Headphones at this range require an amplifier to give any meaningful performance.
Headphone Driver Types and Their Impedances
Headphone drivers are capable of reproducing a sound along a wide frequency range. Theoretically, a perfect driver would respond consistently across the sound frequency range, but typical drivers are far from being perfect. And the resonant frequency response varies significantly among different driver types.
Impedance response across audible frequencies is not perfectly flat in dynamic drivers. The response curve depends on the voice coil control on the drivers at the resonant frequency. In most cases, the actual impedance is significantly higher than the indicated impedance at 1 kHz for any dynamic driver. As the below frequency-impedance graph suggests, the actual impedance can increase many times at the resonant frequency.
Just as the name suggests, electrostatic drivers use electrostatic charge principles to work. A typical electrostatic driver will have a positively charged diaphragm sitting between two negatively charged stator plates that act like capacitors. Therefore, the audio signal has to have enough voltage to keep the plates charged, and thus typically, these drivers have very high impedances.
Many manufacturers rarely produce impedance graphs for electrostatic drivers due to the audio signal amplification requirement to keep the stray charges as negligible as possible. You’ll need a specialized amplifier to provide enough voltage and impedance to keep a pair of electrostatic headphones running. Good examples of electrostatic headphones include the 100,000 Ω impedance Koss ESP-950 and the 170,000 Ω impedance STAX SR-007A MK2 headphones.
Planar Magnetic Drivers
A planar magnetic driver sound can be likened to a combination of electrostatic and dynamic drivers and thus acts differently than either of the two. The S-shaped voice coil usually has a lower impedance than the traditional circular windings. And, the ease of driving any specific planer magnetic driver depends on its sensitivity. For instance, the HiFiMan HE 400s (22 ohms, -98dB) are easy to drive compared to HifiMan HE-6 (50 ohms – 83.5 dB). Also, planar magnetic drivers have no peaks and valleys and are thus very flat across the frequency range. For better illustration, check the below HiFiMAN HE1000 Frequency-Impedance graph.
Wireless Headphones and Impedance
All headphones have inherent impedance, but it becomes a lesser issue with wireless headphones. As we have discussed, impedance matching becomes a critical issue when the headphones have a wired connection to an audio source, which isn’t the case in wireless headphones.
Wireless headphones operate with an analog audio signal, and the inbuilt amplifier adjusts the received audio signal to run the drivers correctly. However, wireless headphones will have specific transmitters or standard wireless connection protocols like Bluetooth. For ways to overcome Bluetooth interference, see our article here.
Active Headphone Impedances
Lately, headphones with active components have become very popular. The main difference with regular wireless headphones is that the audio signal is applied to an active circuit rather than directly to the driver itself. Therefore, the impedance rating indicates the input impedance of the active circuitry and is usually higher than that of the driver.
Active circuits are often found in noise-cancelling and sometimes bass enhancing wireless headphones. An excellent example of a dynamic headphone is the Bowers & Wilkins PX7, with an impedance rating of 20,000 Ω.
Problems of Impedance Mismatch
What could happen if you mismatch the source and load impedances? Ideally, a mismatch between headphones and audio sources could result in two scenarios:
- Low output: It happens when high-impedance headphones are paired with a low-level audio source. The audio output becomes typically barely audible, as the audio signal is insufficient to properly drive the drivers, and consequently, poor signal to noise ratios.
- Blow out: It happens when low-impedance headphones are connected to high-power amplifiers. The power becomes too much for the drivers’ circuitry, which ends up tearing or getting damaged. If you love your cans, it’s good to avoid overloading at all costs, as the headphones will be damaged beyond repair.
Most people overlook the headphone impedance rating, but it’s an important spec if you care about audio quality. Also, now that you know much about it, you’re less likely to connect your headphones to the wrong audio source and end up blowing them out or underutilizing them. The baseline is, connect your headphones to the audio source that compliments their impedance, and you will always enjoy quality audio. We hope you are now in a more informed position when deciding headphones and whether they may need extra power. Happy listening!
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