Frequency response and sound signature: how to read a headphone's sound

Frequencies, from bass to treble

The pitch of a sound is measured in hertz (Hz): the number of vibrations per second. The human ear hears, at best, from 20 Hz to 20,000 Hz. The upper limit drops with age: around 15,000 Hz at forty, often lower after that.

What a response curve shows

To measure a pair of headphones, you place it on an artificial head fitted with microphones where the eardrums would be, then feed it every frequency at the same level. The resulting curve shows, for each frequency, how loudly the headphones reproduce it. The horizontal axis carries the frequencies, from bass on the left to treble on the right; the vertical axis carries the level, in decibels.

A bump means the headphones emphasize that region; a dip means they tone it down. A 3 dB difference is clearly audible; 10 dB corresponds to a perceived volume twice as loud.

Why "neutral" doesn't mean "flat"

This is the most counterintuitive point. When you listen to speakers, the sound passes through your outer ear and your ear canal before it reaches the eardrum. That anatomy acts as an amplifier: it boosts the frequencies around 3,000 Hz by roughly ten decibels. Your brain has been used to it all your life and treats that boost as normal.

Headphones partly bypass that path. To sound natural, they therefore have to recreate the bump themselves. A perfectly flat curve, measured at the eardrum, would sound dull and distant. The real question becomes: which curve, measured at the eardrum, is perceived as neutral?

The Harman curve

Between 2012 and 2018, Sean Olive's team at Harman International looked for the answer experimentally. Hundreds of listeners, trained and untrained, from several countries, compared headphones in blind tests, with the response altered by equalization, then set the bass and treble levels themselves to suit their taste.

The result was a target curve that roughly reproduces the sound of good speakers in a good room: the ear's bump around 3 kHz, bass lifted by a few decibels below 100 Hz, and gently sloping treble. According to the 2018 study, about two-thirds of listeners prefer this curve; the rest are split between those who want more bass, who tend to be younger, and those who want less, who tend to be older.

This target is not an absolute truth, but it changed the industry: most recent headphones take their cue from it, and serious reviews refer to it to describe how a given model departs from it.

Recognizing the main signatures

• Neutral. Close to the target. Nothing sticks out: voices are where they belong, and the bass is present without spilling over. Essential in the studio, valued in hi-fi. Can seem tame on first listen.
• V-shaped. Boosted bass and treble, recessed mids. Thrilling with electronic music, hip-hop and movies; voices step back, and listening fatigue sets in sooner.
• Warm. Generous bass and low mids, softened treble. Relaxing, forgiving of poor recordings; sometimes short on detail.
• Bright or analytical. Forward treble. Gives an impression of precision and detail; quickly turns aggressive at high volume or with harsh recordings.

The selector asks which of three families you prefer (neutral, balanced, punchy bass) and factors it into the ranking.

What the curve doesn't tell you

Frequency response explains most of what we perceive from a pair of headphones, but not everything.

• Distortion: a driver pushed to high volume distorts the signal, especially in the bass.
• Soundstage: it depends on the geometry of the headphones, in particular on whether they are open-back or closed-back.
• Your ear: the curve is measured on a standard head. The shape of your outer ear, wearing glasses or the seal of an ear tip all change the result, especially in the bass and above 8 kHz.
• Listening volume: at low levels, the ear loses sensitivity in the bass and treble (equal-loudness contours, ISO 226 standard). A slightly V-shaped signature compensates for this effect, which partly explains its popularity.

Correcting headphones with EQ

A signature isn't set in stone. An equalizer, found in most wireless headphone apps, music players and operating systems, lets you bring a pair of headphones closer to the target or to your own taste. Two rules: it's better to cut what sticks out than to boost what's missing, to avoid clipping, and fine corrections above 8 kHz are best avoided, because that region varies too much from one ear to the next.

Sources and references

1. Olive, Welti, Khonsaripour — A Statistical Model that Predicts Listeners' Preference Ratings of Around-Ear and On-Ear Headphones, Audio Engineering Society Convention 144, 2018
2. Olive, Welti — The Relationship between Perception and Measurement of Headphone Sound Quality, Audio Engineering Society Convention 133, 2012
3. ISO 226:2023 — Acoustics: normal equal-loudness-level contours