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published: Jul-13-2014, updated May-17-2021

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NO SMOOTHING is applied to the shown plots. Most measurement sites have some smoothing applied which ‘irons flat’ sharp peaks and ‘wiggles’. I do not use smoothing because some info about sound quality is lost when plots are smoothed.

Aside from a small correction of the microphone itself also some correction in the lowest frequencies is applied to the plots to compensate for the perceived loss of bass when using headphones. This is described HERE in more detail.
A ‘horizontal‘ frequency response curve on the shown frequency response plots on this website thus indicates a perceived ‘flat’ tonal signature.

ALL measurements are made with a good SEAL on a flatbed measurement rig.
shape of your head, bone structure, pad size, pad ‘softness,  (compliance), hair or no hair and or wearing glasses may (drastically) change the frequency response of some headphones, so… your personal experience may differ substantially from these plots.

Frequency response (tonal balance) is the most sound-determining aspect of headphones. A horizontal line shows audible neutral response in the plots on this website. Deviations in different severities at different frequency bands have an effect on the sound character.
The bigger the deviation the stronger the effect.

Below an aid to help determining the sound character of headphones with relation to the frequency response.


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AKG K400

AKG K400

The AKG K400 is a very comfortable headphone. The elastic bands that pull down the headband do wear after time and need to be replaced sooner or later. It has the familiar AKG look and is simple and elegant. The pads are cloth instead of pleather or velour and very comfortable for longer listening sessions. It is an open headphone design and sounds from the outside enter almost unattenuated. People around you can hear what you are playing.


Type: Over-ear, Open
Usage: Home, Studio
Isolation: poor, as it is an open design
Driver type: dynamic
Pads: replaceable, cloth
Inner pad dimensions: diameter: 60mm,  24mm
Collapsible: no
Headphone connector: fixed
Cable entry: left sided.
Cable: 3m with 3.5mm TRS plug and screw-on 6.3mm adapter.
Driver size: ø 40mm
Nom. power rating: 200mW
Max. voltage: 4.9V
Max. current: 41mA
Max. S.P.L.: 116dB
Impedance: 120Ω
Efficiency: 94dB/1mW (103dB/1V)
Weight: 220g. (without cable)
Colour: black
Clamping force: medium/low
Accessories: screw-on 6.3mm adapter


It sounds quite good. Bass is tight but lacks ‘body’. There is no bass extension at all and sounds ‘lean’ compared to other headphones. With some music this isn’t very obvious but with well made recordings the lack of bass extension is obvious. Tonal balance is good, instruments and voices sound very convincing. The treble peak is there and it makes the K400 sound detailed yet lacking finesse. Compared to better headphones the treble has some ‘spike’ to it and accentuates attacks with some instruments in a slightly artificial way. left, right

FR K400

This headphone does start to distort at higher listening levels much sooner than most other headphones.
While it did have an edge to it at normal listening levels (around 80dBA) it certainly did not sound as bad as the measurements would suggest.

Above 100Hz channel matching is excellent.

Below the distortion measurement (dB scale) of the K400.Dist K400

And below the same plot but using a dB scale.
Dist K400 R percent

The CSD below shows the 7kHz and 13kHz peaks to be the result of resonances, the 7kHz one quite severe even. The peak in FR and ringing explain the sharp ‘edge’ in the treble and the somewhat artificial sound on cymbals etc. Below 5kHz everything seems quite decent. left, right

CSD K400

Another way of looking in the time domain is the spectrum plot. This plot shows several issues in the treble region but the lows are decaying quite quickly.spectr K400 L

Below the step response of the K400.step K400
There is a whole lotta ringin’ going on… The sharpness is shown by the initial rise and peaks in the first 250μs.
The lack of bass extension and ‘thin’ sound is obvious as what would have to be a slowly sloping horizontal line is a quickly dropping line.


The positives are a neutral sound to the mids, no muddy-ness and good comfort.
Negatives are the lack of body (bass extension) and the slightly ‘edgy’ treble.
Well suited for monitoring instruments and voices. Lacking for musical enjoyment.

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