Headphone Impedance – Stop Believing The Lie

OK, yes, I will admit that to a small degree, that title is a little “click-baity”.
I don’t personally believe that the audio community has intentionally been lied to for so many years, but the whole headphone impedance thing is often a major misconception nonetheless.

Time and again you’ll see or hear someone say something to the effect of “these headphones have a high impedance, so make sure you’ve got an amplifier that’s powerful enough to drive them”, or perhaps even “bro, your amp is too weak to handle 300-ohms”.

What this misconception is, is simply people incorrectly assigning or otherwise assuming a direct relationship between a headphone’s rated impedance and how much power is required to drive them to a given volume level.
Whilst it is true that a higher impedance does require more power, what is being neglected is that the impedance figure is sometimes simply a correlation, not necessarily a direct causation.

What’s The Deal With Impedance?

What usually happened in the past was that many of the “Pro” headphones incidentally had higher impedance figures, which is what lead to this incorrect albeit innocent misunderstanding. But, what people missed was a far more telling specification – efficiency or sensitivity.

I say efficiency or sensitivity as these can be converted from one to another and there’s no hard set rule for which one a manufacturer has to list (often they don’t even list either), but it’s still somewhat important to understand the difference between them.
The efficiency is represented as the decibel level that can be reached with 1 milliwatt of power, and is indicative of the sound pressure level that can be achieved in relation to how much power is required to achieve it.
On the other hand, sensitivity is shown as the decibel level that can be reached with 1 Volt, which essentially means the comparative loudness irrespective of the power required.
However, I should also note that converting from the dB/V figure to the dB/mW figure (or vice versa) does require the impedance to be taken into account, but we’ll soon have a look at just how much (or how little) of an influence the impedance has on the end result.

So, the general rule of thumb that goes around is that a higher impedance requires more power. And again, this is true to some extent, but it’s only really relevant if the 2 headphones being compared have an equal sensitivity/efficiency rating.
To illustrate just how fallacious this notion of impedance being a telling factor is, let’s take a look at practical example.

 

Fitting It All Together

The famous Sennheiser HD650 has an impedance of 300-ohm, whereas the HiFiMAN HE6 has an impedance of just 50-ohms.
You may conclude then that the HD650 would be significantly, or at the very least noticeably harder to drive than the HE6. After all, it’s got a mere 1/13th of the impedance of the HD650. But you’d be wrong, very, very wrong.

The HE6 is notorious for being one of the hardest to drive headphones out there. In fact, HiFiMAN recommends using the HE6 with an amplifier that’s capable of delivering 2 watts per channel. Yeah, we’re no longer talking about milliwatts, we are in full wattage territory now.
In contrast, the HD650 does not require that much power to sound their best.
So, let’s take a look at the sensitivity/efficiency of these headphones to get a clearer understanding of what’s going on here.

The HD650 might have an impedance of 300-ohm, but it’s got a sensitivity of 103 dB/V, whereas the 50-ohm HE6 has a sensitivity of 83.5 dB/V.
So, as you can see, it’s the sensitivity that indicates to us more clearly how power-hungry a set of cans will be.

It’s also important to understand that, when it comes to sound pressure and the power required to make it, there are 2 other things you may want to consider.
For us humans, each increase of 10dB of sound pressure will be perceived as a doubling of the loudness. For example, if you increase the pressure of a particular sound from 100dB to 110dB, the 110dB one will sound double as loud to us.
However, when it comes to the power required to make those decibels, then it’s a different story as here exists a logarithmic relationship, rather than a linear one.
To simplify things – an increase of just 3dB requires double the power.
For example, if you want to increase the pressure of a particular sound from 100dB to 103dB, that increase of 3dB would require double the amount of power that the 100dB sound required to produce.

Let’s take a look at just one more example – Sennheiser’s recently-released HD660S.
It has an impedance of 150-ohm (half that of the HD650) and a sensitivity of 104dB/V (vs. the 103dB/V of the HD650). So really, not a huge difference between the sensitivity of those headphones.
And yet, you’ll see tons of people claiming that the HD660S will be much easier to drive.
Why? Well, because they’ve only considered the impedance. But, as we can see from the sensitivities of those 2 headphones, the HD660S in fact requires only mildly less power in reality.To back up this claim, we can refer to the measurements made by Innerfidelity.com (shown above).
If we compare the measurements of the HD660S to that of the HD650 (lower right of the graph reports) then we can see that the HD650 may have required 42% more voltage than the HD660S to reach 90dB SPL, but in terms of power it only required about 20% more.
And, just for the sake of another comparative illustration, the HE6 required 1.018V and 19.69mW to reach 90dB SPL, which is nearly 5 times as much voltage and 151 times as much power as required by the HD650.

As I mentioned, sometimes manufacturers will list the dB/mW figure, and other times they might use the dB/V figure…or sometimes they might omit either of those specs entirely. But, keep an eye out for either of those 2 specs as they will be far more useful in determining if your source has enough power to drive them.

So, hopefully, this little guide and case study would’ve made things at least a little easier for you to understand as to why headphone sensitivity (efficiency) is so much more important than merely looking at the impedance.

Leave a Reply

Your email address will not be published. Required fields are marked *