Frequency response and sound signature: how to read the sound of a pair of headphones

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 falls with age: around 15,000 Hz at forty, often lower after that.

What a response curve shows

To measure a pair of headphones, they are placed on an artificial head fitted with microphones in place of eardrums, then fed every frequency at the same level. The resulting curve shows, for each frequency, the level at which the headphones reproduce it. The horizontal axis carries the frequencies, from bass on the left to treble on the right; the vertical axis the level, in decibels.

A hump means the headphones push that region forward, a dip that they hold it back. A difference of 3 dB is clearly audible; 10 dB corresponds to a perceived volume twice as loud.

Why “neutral” does not mean “flat”

This is the most counter-intuitive point. When you listen to loudspeakers, the sound passes through your outer ear and ear canal before reaching the eardrum. This anatomy acts as an amplifier: it boosts frequencies around 3,000 Hz by ten decibels or so. Your brain has been used to it all your life and regards this lift as normal.

Headphones partly bypass this path. To sound natural, they therefore have to recreate the hump 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 by experiment. Hundreds of listeners, trained and untrained, from several countries, compared blind a series of headphones whose response had been altered by equalisation, then adjusted the bass and treble levels themselves to suit their taste.

The result was a target curve that roughly reproduces the sound of good loudspeakers in a good room: the ear's hump around 3 kHz, bass raised 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 has changed the industry: most recent headphones draw on it, and serious reviews refer to it to say how a given pair departs from it.

Recognising the main signatures

• Neutral. Close to the target. Nothing sticks out: voices are where they should be, and the bass is present without spilling over. Essential in the studio, valued in hi-fi. Can seem restrained on first listen.
• V-shaped. Boosted bass and treble, recessed mids. Spectacular with electronic music, rap and films; voices step back, and listening becomes tiring sooner.
• Warm. Generous bass and lower mids, softened treble. Restful, forgiving of poor recordings; sometimes short on detail.
• Bright or analytical. Treble to the fore. Gives an impression of precision and detail; quickly turns harsh at high volume or with hard-edged recordings.

The selector asks for your preference among three families (neutral, balanced, generous bass) and takes it into account in the ranking.

What the curve does not tell you

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

• Distortion: a driver pushed to high volume deforms the signal, especially in the bass.
• Soundstage: it depends on the geometry of the headphones, in particular whether they are open-back or closed-back.
• Your ears: the curve is measured on a standard head. The shape of your outer ear, wearing glasses or the seal of an ear tip all alter 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 equalisation

A signature is not set in stone. An equaliser, 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 is better to turn down what sticks out than to turn up what is lacking, to avoid clipping, and fine corrections above 8 kHz are best avoided, because that region varies too much from one ear to another.

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