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Audio Files: Headphones and the Harman Curve

How recent scientific research is making new headphones sound better and more consistent

The Harman Curve
A visual representation of the Harman curve

Jazz and headphones are both around 100 years old. But while jazz is codified to the point that four players who’ve never met can play a standards gig together with no rehearsal, the audio industry is only now establishing a useful set of guidelines for how headphones should sound. Fortunately, the headphones that are based on these guidelines may have been worth the wait.

Headphone sales unexpectedly boomed with the introduction of the Apple iPod, and later the iPhone, but headphone manufacturers were stuck with an outdated set of engineering guidelines created in the 1980s. As Sean Olive, senior fellow at Harman International (parent company of AKG, JBL, Mark Levinson, and other audio brands) puts it, “There were standards for headphone response, but no one was following them so there must have been something wrong.” As a result, headphones of different brands often sounded radically different—and sometimes even headphones of the same brand sounded radically different.

In the early 2010s, Olive and his colleagues began doing blind tests of headphones to see what type of sound most listeners like. Their initial project involved only six sets of headphones and 10 listeners, but the resulting 2012 paper, titled “The Relationship Between Perception and Measurement of Headphone Sound Quality,” rocked the industry. Eighteen subsequent papers saw Olive and his colleagues testing hundreds of headphones with hundreds of listeners worldwide.

The result of this research is the so-called “Harman curve.” It’s a specific target for the frequency response (or relative balance of the various bands of audio, such as bass, midrange, and treble) that most listeners prefer. By measuring the response of a set of headphones using a simulated ear (basically a rubber earlobe with a microphone inside), a headphone designer can tell how it compares to the Harman curve, instead of having to do extensive testing with multiple listeners.

So what does the Harman curve sound like? Ideally, it gives headphones much the same sound you’d hear when listening to a good set of stereo speakers in an acoustically treated room. Simply put, that means there’s going to be a little extra bass (to simulate the bass boost that a typical room lends to speakers), balanced out by some extra treble. While this might seem like it would sound unnatural, when you wear a set of Harman-curve headphones, they really do sound like speakers in a room.

AKG’s K371-BT
AKG’s K371-BT

A slow start

The Harman curve didn’t revolutionize the headphone industry overnight, though. Almost eight years after the original paper was published, there are still only a few headphones that conform to it. But we’ll soon see more. Olive told me that all new AKG headphones are designed to be as close as possible to the Harman curve, and all new JBL headphones use the curve as well, but with a little added bass to please young male listeners (who, according to Harman’s research, tend to like more bass than females and older males).

For me, the headphone that best demonstrates the validity of the Harman curve is the AKG K371, a professional model that debuted earlier this year. Olive told me it comes within one decibel (a barely perceptible difference) of the Harman curve. The K371 has a natural, wonderfully detailed sound; no instrument sticks out of the mix unless it’s supposed to, and the bass, midrange, and treble seem almost perfectly balanced. It’s also a comfortable, sturdy headphone, easily worth its $149 price. I know several audio pros who are already using the K371 in the studio. A Bluetooth version, the K371-BT, costs $179. There’s a slightly different Harman curve for earphones, as used in AKG’s flagship N5005 ($999).

AKG N5005
AKG’s N5005 earphones

Harman has published its research, so other manufacturers are free to use the Harman curve. As it happens, the Sony MDR-7506, probably the world’s most popular professional headphone, comes close to that curve. Paul Barton, who designs PSB and NAD headphones, conducted his own research and also arrived at a curve similar to Harman’s, which he calls RoomFeel and uses in all of his headphones.

However, the Harman curve reflects what most listeners prefer, not what every listener prefers. Some audiophiles prefer more treble, because they want to hear, for example, every last whisper of Eric Dolphy’s flute loud and clear, even if it might make the music sound thin. I’ve also found headphones that deviate from the Harman curve yet still sound good. Typically, these models have a more prominent midrange, but their designers were careful to preserve the natural balance of bass to treble that the Harman curve mandates.

PSB M4U8
PSB M4U8

I think it’s important for every headphone enthusiast—and every jazz enthusiast—to give a long listen to the K371 or one of the other headphones designed along the Harman curve guidelines. There’s no guarantee that it’ll be your favorite headphone, but at least you’ll have a baseline that demonstrates what most people prefer, and that will allow you to make more informed headphone purchases in the future.

Brent Butterworth

Brent Butterworth has been a professional audio journalist since 1989, and has evaluated and measured thousands of audio products. He is currently a writer at Wirecutter and editor of the SoundStage Solo headphone site; served as an editor at such magazines as Sound & Vision and Home Theater; and worked as marketing director for Dolby Laboratories. He also plays double bass with several jazz groups in Los Angeles.