What is impedance in headphones?

Even if you haven't attended any electronics classes, you have likely heard about electrical resistance:

If you force a flow of electrons across an object, it will apply resistance to this flow.

This is true for a DC signal (Direct current) where all electrons flow in the same direction.

With AC (Alternate Current) signals, electrons change direction periodically. Here, we talk about impedance and not resistance.

A device with high impedance will "impede" the movement of electrons more than one with low impedance.

In audio applications, we often deal with AC because it's the most convenient way of translating sound into an electrical signal. 
Sound is a vibration of particles, and AC is a vibration of electrons. 

Why does this matter for headphones?

Most headphones use electrodynamic speakers to generate sound. Like the ones used as woofers in studio monitors.

Inside these speakers, there is a fixed magnet. The coil and diaphragm are attached to it. 
The AC signal representing your favorite song is fed into the coil, and since it is immersed in a magnetic field, it starts moving back and forth. So does the connected diaphragm. This generates the sound waves you hear. 

The electrical signal passes through the coil with a particular impedance.

Depending on the coil, the signal will have a lower or higher impedance. This is the main factor that defines the headphone impedance and how it affects the audio signal.

Impedance is measured in Ohm, and headphones impedance can vary between 8 and 600 Ohm (or even above in rare cases).

Headphones between 8 and about 50 Ohm are generally considered low-impedance headphones.

Your typical consumer headphones are primarily low impedance. Low-impedance headphones are easier to drive.

This means that when using a device with low output voltage, you'll be able to reach a higher SPL (Sound Pressure Level), which means a higher volume.

But there are also a lot of pro-audio headphones that fall into this category.

Headphones with more than 50 Ohm impedance are considered high-impedance headphones.

High-impedance headphones are primarily for pro-audio use and so-called audiophiles.

So, why are there even high and low-impedance headphones?

Both high and low-impedance headphones exist because of historical context, different technologies, and different applications.

Portable devices such as smartphones or laptops deliver a lower output voltage than professional audio equipment. Therefore, low-impedance headphones are better when connected to those devices because they can reach a high volume despite the low output voltage.

You can try connecting a high-impedance headphone to your laptop. Even at maximum volume, it will not be loud enough.

But why do high-impedance headphones even exist?

The gear commonly used in studios does not have the limitations of modern portable devices. In fact, the output voltage of professional audio gear (audio interfaces, headphone amplifiers, etc.) is usually high. So, getting enough volume in your headphones is not a problem at all. Therefore, both high and low-impedance headphones are generally suitable in studio scenarios. 

In short, no. Even though impedance impacts sound, we cannot say that high impedance is better than low impedance.

These days, the opposite might be true. The advantage of low-impedance headphones is that you can use them with portable devices without problems. 
 
Suppose you want to connect your headphones to professional audio equipment. You should be fine.

Most modern audio interfaces and headphone amplifiers are designed to work well with high and low-impedance headphones.

Just be careful if you're using headphones with low impedance. As they are easier to drive, you can easily reach a volume that can be dangerous for your ears. 
 
If the output voltage is too high, you could even damage the headphones, so watch out!

Be aware that hearing loss is a real health threat as you can find out in this blog. 

What matters is the relationship between the headphone impedance (or load impedance) and the output impedance of the device to which the headphone is connected (or source impedance).

Regardless of the headphone impedance, you can use your headphones with any professional equipment.

As a rule of thumb, the load impedance should be at least 10 times higher than the source impedance. If this condition is not met, it can result in unwanted coloration or bandwidth reduction. 

This condition is commonly referred to as impedance matching. 
 
How can I know if I am in impedance-matching conditions?

Most professional equipment is designed to have low enough output impedance to meet these conditions regardless of the headphone that is connected to it. 
 
That's why you shouldn't worry too much if you have an audio interface or a dedicated headphone amplifier.

Sometimes, you will find sensitivity in headphone specifications instead of the impedance. 
 
The two topics are related. 
 
Sensitivity is measured in dB SPL/V @ 1 kHz (or sometimes in dB SPL/mW @ 1 kHz), which means that at a specific voltage, the headphones can generate a certain SPL at 1 kHz. 
 
For example, a headphone with a sensitivity of 105 dB SPL/V can generate 105 dB SPL at 1 kHz if fed with a signal of 1 V. 
 
The higher the sensitivity, the higher the SPL at a specific voltage. Therefore, the louder the headphones will be.

Low-impedance headphones have higher sensitivity, and high-impedance headphones have lower sensitivity.