Figuring out: Headphones Impedance
/I have always wondered about impedance in the context of pro audio vs consumer audio. Don’t get me wrong, this is a deep topic for the audiophile crowd, but that’s not going to be my approach. If you want to get deep into it and chase the absolutely clearest listening experience, have a look at this article that goes deeper into the technical details.
In the meantime, i just wanted to get an overview of the situation so when I look at my headphones, I have some understanding of what is going on. As you can see, my daily headphones which I use for almost everything are a pair of Sennheiser HD25. And there is the impedance staring at us, 70 ohms. Let’s see what this number means.
About Impedance
Remember Ohm’s Law? Voltage = Current * Resistance. You may be tempted to think about impedance in a similar fashion and it kind of makes sense because impedance also reflects how hard it is for a current to run through a circuit… kinda. The problem is that Ohm’s law only applies to DC, that is direct current. DC is what most electric appliances and machines use and was the technology championed by Edison in the War of the currents of the 1880s.
On the other hand, Tesla defended AC, alternating current, as a better alternative to transport energy across distances. On an AC current, electrons change their direction at a certain frequency, usually at 50 or 60Hz. Long story short, Tesla ended up winning and today we use AC to transport electricity. It is then converted to DC so machines can use it. That’s why there is a transformer on anything that you plug in to the wall and also why the band ACDC bear that same name.
So why do we care, what’s the relation with audio? Well, since sound is air particles moving back and forth at certain frequencies, all analogue audio signals are AC. So whenever an audio signal goes into a pair of headphones, comes out of a microphone or moves through an analogue mixing desk, that’s AC. Only the audio signal is AC, a mixing desk will be powered with DC electricity, don’t confuse the two.
So since audio signals are AC, we can’t just use resistance to measure how hard it is to run them through a system, we must use impedance which takes into account resistance, capacitance and inductance. I won’t go into detail since the math gets much trickier than just Ohm’s law but you can learn more in the article I linked at the beginning.
Implications on Headphones
You can think of impedance as a measure of how inefficient your headphones are at generating an audio level. Generally, the lower the impedance, the easiest is for your headphones to create a loud signal. Does that mean that you just should get headphones with the lowest impedance possible? Not at all!
Higher audio levels are not the only thing we are after. Sound quality should also be a big factor. Lower impedance headphones usually have lower audio quality while higher impedances are best if we want to avoid distortion and improve frequency response and faithfulness to the original source.
So high impedance headphones will be the most crystalline BUT (and this is a big but) you will need much more power to drive them and get a proper signal from them. This means that in an ideal world, you would have high impedance headphones and also a high impedance headphone amplifier for the best audiophile experience possible. This is what people call “Impedance Matching”, making usr eimpedance levels at the source and desitnation aren’t too far apart.
This means that you may go and buy expensive high impedance headphones and then find out that they give you a tiny quiet signal on your phone, your computer or on any other consumer level audio product. Not good, particularly if you want or need a loud signal. For those consumer uses, you would be much better off with lower impedance headphones which is why almost all normie headphones are relatively low impedance.
Again, things are more complicated. Impedance varies across the frequency spectrum and there are other factors like distortion and sensitivity. All of this will contribute to how loud a pair of headphones will be and how accurate their response will be. But for now, let’s just get an intuitive sense of how impedance works in different contexts.
Some Numbers & Perspective
So how does my 70Ω Sennheiser headphones fit on the impedance scale? A good reference to use is the Beyerdynamic DT770 which is one of the few pairs of headphones where you can buy 3 different versions which only differ in impedance. These versions are:
32Ω: This is consumer level impedance. It will give good audio levels for phones and computers. As a reference, Apple Earpods are 42.2Ω, which by the way doesn’t mean they are better, as I said before there are other factors to consider, like distortion.
80Ω: This is a good mid level which will still be loud enough with consumer products but offers an overall better audio quality. As you can see, my headphones fit here.
250Ω: And this is now getting on the audiophile levels of impedance where you need to make sure you are using a good headphone amplifier if you want to have enough audio levels.
I also wanted to say that usually in pro audio, headphones are not that crucial since we don’t really use them to mix music or cinema. For this, speakers and a good sounding room are always preferable since we want to have a more natural listening experience and at the end of the day, sound is supposed to be propagated through the air. This is why you won’t see studios buying crazy high impedance headphones and this is mostly reserved to the audiophile tribe.
Conclusions
Now you know. Lower impedance will produce higher audio levels but with lower audio quality. It is important then to match impedance on the source and the headphones if we want to achieve the best quality/level relation possible.