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published: Dec-5-2013, 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.

sound descriptions mine

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AKG K551



Type: Over-ear, Closed
Usage: Home, Studio
Isolation: decent
Driver type: dynamic
Pads: replaceable, pleather
Inner pad dimensions: diameter: 53mm,  25mm
Collapsible: no
Headphone connector: fixed
Cable entry: left sided.
Cable: 1.2m with 3.5mm TRS plug and screw-on 6.3mm adapter.
Driver size: ø 50mm
Nom. power rating: 200mW
Max. voltage: 2.5V
Max. current: 80mA
Max. S.P.L.: 121dB
Impedance: 32Ω
Efficiency: 99dB @ 1mW
Sensitivity: 114dB @ 1V
Weight: 305g. (without cable)
Colour: black
Clamping force: medium
Accessories: screw-on 6.3mm adapter, warranty card


The AKG K551 is intended for portable gear and has a remote control integrated in the cord (only for i-products) and a microphone. It has a good amount of bass, not accentuated and reaches as low as 5Hz (really). The upper mids/lower highs aren’t as good as I hoped it would be and the treble peak is noticeable, especially on recordings with distortion in it. The K550 is the ‘better’ headphone. The K551 is more ‘laid-back’ and a bit sharper in the treble. left, rightFR K551

Below the phase response of the K551.phase K551
Aside from the phase shifts above 5kHz this seems quite decent. Only sharp phase shifts in a narrow band may become somewhat audible.

Below the Group Delay plot for the K551.GD K551
Nothing strange, only some influence of the pads in the lower frequencies is visible.

Below the distortion plots of the K551.Dist K551 L
And below the same plot but in the more common percentage scale.Dist K551 L percent
Around 4kHz the distortion levels are reaching audible levels.

The CSD below shows some short lived ringing at some frequencies but nothing really alarming.CSD K551
Around 1.5kHz there is a resonance that takes its time to drop. At 5khz and 7kHz some resonances as well.

Below the Spectrum plot for the K551.spectr K551 L
It shows some lingering at 1.5kHz and short lived resonances. Nothing that is potentially sound degrading.

Below the step response for the K551step K551
This plot shows some ringing and the somewhat darker tonal balance is showing. Bass extension is enormous.
There is some difference between the left and right channel.


This headphone can be transformed into an exceptional performer with relatively little effort but some soldering skills are required (or ask someone to do it for you). The result of these modifications are shown below left, right.
HERE is where you can view/download the article that describes how to modify the K551. The felt discs for this modification can be ordered via the FORUM.

evo felt + low filter


The K551 is a rather big but not heavy closed headphone that is easily driven and does not require a lot of power.
It’s tonal balance is towards the warmer side of neutral but can be improved with some modifications.
Down side is that AKG does not have replacement pads for this headphone.
The K550 is a slightly better one but this version has the mic/remote.

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