What is Frequency Response?
Frequency response describes the range of audio frequencies a headphone, earphone, or speaker can reproduce, and how evenly it reproduces them across that range. It is typically expressed as a range (like “20Hz–20kHz”) and, more usefully, as a graph showing volume (in decibels) on the vertical axis against frequency (in hertz) on the horizontal axis.
If audio specifications had a “most important single metric” award, frequency response would win it. It tells you more about how a headphone will actually sound than any other spec on the box. A headphone with a flat frequency response reproduces every note at the same volume level. A headphone with elevated bass has a frequency response that rises at the low end. Understanding this concept – and learning to read frequency response graphs – is one of the most practical skills an audio enthusiast can develop.
In-Depth
The Basics: Frequency, Pitch, and Hearing
Sound is vibration, and frequency measures how fast something vibrates. It is expressed in hertz (Hz), meaning cycles per second. Low frequencies produce low-pitched sounds (bass), and high frequencies produce high-pitched sounds (treble).
Human hearing spans roughly 20Hz to 20,000Hz (20kHz), though the upper limit decreases with age – most adults over 30 cannot hear above 15–16kHz. Within this range, our hearing is not equally sensitive at all frequencies. We are most sensitive in the 2–5kHz range (the frequency band of human speech and crying babies – evolution was not subtle), and less sensitive at the extremes.
Audio frequencies are commonly divided into regions:
| Region | Frequency Range | What Lives Here |
|---|---|---|
| Sub-bass | 20–60Hz | Rumble, earthquake effects, pipe organs |
| Bass | 60–250Hz | Kick drums, bass guitars, male vocals (fundamentals) |
| Lower midrange | 250–500Hz | Body of instruments, warmth |
| Midrange | 500Hz–2kHz | Vocals, guitars, pianos |
| Upper midrange | 2–4kHz | Vocal presence, attack of instruments |
| Treble / presence | 4–8kHz | Sibilance, cymbal attack, detail |
| Upper treble / air | 8–20kHz | Sparkle, spaciousness, overtones |
The Specification on the Box
When a headphone lists “Frequency Response: 5Hz–40kHz,” it is stating the range of frequencies the driver can physically produce. This number is almost meaningless by itself for two reasons:
No tolerance is given. A driver might produce 5Hz, but at 40 dB below its midrange level – effectively inaudible. Without knowing how far the response drops at the extremes, the number is just marketing.
Flatness matters more than range. A headphone that covers 20Hz–20kHz with a smooth, even response will sound better than one that extends to 5Hz but has a 15 dB peak at 8kHz. The range tells you the boundaries; the shape tells you the sound.
This is why frequency response graphs – also called FR graphs or FR plots – are far more valuable than the spec sheet number.
Reading a Frequency Response Graph
A frequency response graph plots frequency (horizontal axis, logarithmic scale) against sound pressure level in decibels (vertical axis). Here is how to interpret one:
A perfectly flat line means the headphone reproduces every frequency at exactly the same level. In practice, no headphone is perfectly flat, nor would most listeners want it to be (more on this below).
A rising line in the bass region (left side) means the headphone has boosted bass. The more it rises, the bassier it will sound.
A dip in the midrange means vocals and instruments will sound recessed or distant.
A peak around 2–4kHz adds vocal presence and perceived clarity but can become harsh or fatiguing if overdone.
Elevated treble (8–15kHz) adds a sense of “air” and detail but can produce sibilance (harsh “s” and “t” sounds) if it rises too sharply.
A steep drop-off at the high end makes the headphone sound dark or muted.
The vertical scale matters enormously. A graph with a 5 dB range from top to bottom will make every headphone look wildly uneven. A graph with a 50 dB range will make everything look flat. Most reputable measurement sites use a 60–80 dB range with consistent scaling.
Target Curves: What “Good” Looks Like
A dead-flat frequency response from a headphone does not actually sound natural to our ears. This is because of the way sound interacts with the outer ear (pinna), ear canal, and head before reaching the eardrum. In a natural listening environment – speakers in a room – these interactions add specific frequency shaping that our brain expects.
To account for this, audio researchers have developed target curves – idealized frequency response shapes that represent what a headphone should look like on a measurement rig to sound “neutral” or “natural” to a human listener. The most widely referenced targets are:
Harman Target (Over-Ear and In-Ear). Developed by Sean Olive and the research team at Harman International, these targets are based on extensive listener preference studies. The Harman target features a modest bass elevation (about 3–5 dB above midrange) and a carefully shaped presence region. Most listeners in controlled tests prefer headphones that follow this curve.
Diffuse Field (DF). An older target based on how a flat speaker would measure at the eardrum in a room with reflections from all directions. DF-neutral headphones tend to sound thinner and brighter than Harman-target headphones.
IEF Neutral. A community-developed target (from the In-Ear Fidelity community) that represents a middle ground between Harman and diffuse field, adjusted for IEM-specific considerations.
When reviewers say a headphone “follows the Harman target closely,” they mean its frequency response matches this well-researched preference curve. Deviations from the target are not automatically bad – they represent the tuner’s artistic choices – but large deviations are a warning sign.
Why Frequency Response Is Not Everything
Frequency response is the most important single specification, but it does not capture the full picture of how a headphone sounds:
Distortion. A headphone can have a beautiful frequency response but introduce harmonic distortion that muddies the sound. Distortion measurements (THD%) complement FR graphs.
Impulse response and decay. How quickly a driver starts and stops producing sound affects perceived detail and clarity. A dynamic driver and a balanced armature might measure similarly in FR but sound different because of their transient behavior.
Soundstage and imaging. The perceived width and depth of the sound, and the ability to place instruments in space, are influenced by driver type, housing design, and ear pad shape – none of which appear on an FR graph.
Channel matching. How closely the left and right drivers match each other matters for imaging accuracy. Cheap headphones sometimes have significant left/right mismatch.
Unit variation. Every physical headphone is slightly different from the next one off the production line. FR measurements represent one sample, not every unit.
How Measurement Rigs Work
Frequency response measurements are taken using a coupler (for IEMs) or a head and torso simulator – HATS (for headphones). These devices contain a calibrated microphone positioned where the eardrum would be, inside an artificial ear canal and pinna that mimic human anatomy.
Different measurement rigs produce different results. A measurement taken on a GRAS RA0402 coupler will not look identical to one taken on a B&K 5128 or a MiniDSP EARS clone. This is why it is important to compare measurements taken on the same rig. Mixing measurements from different rigs leads to incorrect conclusions.
Reputable measurement databases include:
- Crinacle’s IEM and headphone database – one of the largest, using consistent rigs
- Rtings – standardized measurements with consistent methodology
- Audio Science Review (ASR) – detailed measurements with a focus on technical accuracy
- Super Review* – uses the industry-standard B&K 5128
Frequency Response and Hi-Res Audio
The hi-res audio specification requires a frequency response extending to at least 40kHz. This is well beyond human hearing, but proponents argue that ultrasonic content affects the perception of audible frequencies through intermodulation and that higher sample rates reduce in-band filter artifacts. Whether or not you subscribe to this theory, a headphone certified as hi-res capable must demonstrate measurable output above 20kHz, which requires a driver with exceptional speed and extension.
How to Choose
When using frequency response to evaluate headphones, keep these three principles in mind:
Look at the graph, not the spec sheet number. A “20Hz–40kHz” specification tells you almost nothing. An actual frequency response graph – from a reputable measurement source using a standard rig – tells you almost everything. Before buying any headphone over $50, look up its measured frequency response online. Sites like Crinacle, Rtings, and Audio Science Review provide free measurements for hundreds of models.
Compare against a target curve that matches your preference. If you like a balanced, “reference” sound, compare the headphone’s FR graph to the Harman target. If you prefer more bass, look for headphones that rise 5–10 dB in the sub-bass region. If you are sensitive to treble, avoid headphones with sharp peaks in the 6–10kHz region. Understanding your own preferences in terms of the frequency response graph is the fastest way to make good purchasing decisions.
Use FR as a starting point, not the final answer. Two headphones with nearly identical frequency response graphs can sound noticeably different due to distortion, driver speed, soundstage characteristics, and comfort. Use FR to narrow your options, then read detailed reviews or – ideally – audition the finalists in person to make the final call.
The Bottom Line
Frequency response is the single most informative specification in audio. It reveals whether a headphone will sound bright, warm, bass-heavy, neutral, or anything in between. Learning to read an FR graph and compare it against your preferred target curve is a skill that will save you from countless disappointing purchases. The spec sheet number (20Hz–20kHz) is nearly useless on its own, but the measured graph behind it is a window into how a headphone will actually perform on your ears. Combine it with distortion data, build quality assessment, and comfort evaluation, and you have a reliable framework for choosing audio gear that truly suits you.