Buy headphones? You should know the ‘Harman curve’

If you’ve recently purchased some high-end headphones, chances are you’ve come across something called the “Harman Target Curve”, or perhaps just the “Harman Curve”. Maybe you read a review that said a pair of headphones sounded amazing because they were tuned to said target. Or maybe you saw an angry comment suggesting that headphones tuned to the Harman curve suck.

But why exactly are more manufacturers, audiophiles and reviewers paying attention to the Harman Curve?

There’s a lot of science behind headphone acoustics, much of which is covered in our primer on speaker measurements, but in some ways the headphones are even more complex. This article is not intended to be an exhaustive guide to headphone measurements, but rather an introduction to help you make a more informed purchasing decision.

Let’s dive into it.

What is the Harman curve?

The Harman curve is a scientific frequency response target for headphones. It was created by Dr. Sean Olive and other researchers from Harman Audio in the 2010s (Harman is owned by Samsung and includes headphone makers like AKG and JBL) and is the result of several studies trying to figure out what makes good-sounding headphones.

Basically, the Harman curve seems to be the best current scientific answer to the question: “which frequency response is most preferred for most people?”

It should be noted that, as seen with frequency response measurements for loudspeakers, what people “prefer” is largely interchangeable with what people consider “neutral”. People just tend to like what feels most natural to them.

I should also note that there are several versions of the Harman curve; the target changes depending on whether it is over-ear headphones or in-ear headphones (IEM, AKA earphones), for example. And again, the target has been tweaked slightly over the years through ongoing research.

What does the Harman curve look like?

This is the latest version of the Harman curve for full size helmets (blue) and IEMs:

Not that the above only applies to “raw” headphone measurements without any particular calibration applied. This is also the response measured with the specific test equipment used by Harman – measurements created with other devices are not entirely comparable.

It doesn’t look flat at all! How could that sound neutral?

There is extensive evidence to suggest that loudspeakers which have a flat frequency response when measured in an anechoic chamber are preferred by listeners and perceived as neutral. So why is Harman’s target for headphones so far from a flat line?

The first part of the answer lies in our own body. When you’re listening to speakers — or, you know, instruments and vocals — a lot of the sound goes through your head and torso before it gets to your eardrums. Sound from the left speaker will pass through your noggin, causing its frequency response to change before it gets to your right ear.

We call this modification of the sound the head-related transfer function (HRTF). Our brains, smart as they are, use this change in sound to determine where the sound is coming from.

The problem with headphones is that their proximity to your ear does not allow these frequency response changes to occur. Sounds coming out of your headphones have a direct line of sight to your ear canals. It is therefore up to manufacturers to find a way to roughly emulate the HRTF in the frequency response of their headphones. Headphones with a really flat frequency response will sound terribly dull.

But we also don’t want the headphones to reproduce the sound of the speakers in an anechoic room. No one listens to speakers in an anechoic chamber, after all (well, except scientists). Thus, the second part of the Harman curve reproduces some of the characteristics of the response of a loudspeaker in the room.

When a good loudspeaker is placed in a room, its sound develops a slight tilt, (more on Why in our speaker measurements guide) with more bass and less treble. The speaker is usually still perceived as neutral, but your brain uses this tilt of the room to help place the sounds leaving the speaker as existing in your room.

Harman scientists reasoned that since music is typically mixed for listening to speakers in a room, the ideal headphones would try to replicate the sound of good speakers in a room, resulting in some of this big bass bump in the Harman curve.

The final curve was determined by the responses of study participants, but the fundamental principle remains the same: the best headphones sound the most like the best speakers.

But we all have different musical tastes! How can one frequency response fit all?

Here’s the thing: you’re probably a lot less special than you think.

Harman research found that, as with loudspeakers, the vast majority of people have similar tastes when it comes to headphones, and this preference can be largely predicted by frequency response data. In one study, researchers found that measurements could predict preference for on-ear and on-ear headphones with a remarkable 86% accuracy. For in-ear monitors, the model was even more accurate, at 91%.

One study (Olive et al., 2014) tested listeners in different countries, age groups and experience levels. Although participants gave different individual scores to different listeners – trained listeners tended to be more critical, for example – they classroom very similar helmet.

AKG K371