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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

post separation
Superlux HD681


Below the FR plot of the cheap Superlux HD681 in stock form.

hd681 with stock pads
The HD681 can be modified to perform MUCH better. a heavily modified version is shown below. Heliharris aluminium ring set, leather headband made by Claus from my old motorcycle jacket, AKG K240 pads and a filter inside the cups.

The plots below are made with a modified HD681 which has AKG-K240 velour pads and a high-peak filter as well as a lows filter. What’s been done to it is described HERE. The Frequency Range is much flatter and well extended in the lows. It easily reaches 20Hz and even 10Hz isn’t that bad. The small dip around 4kHz is not very obvious but the slight peaky behaviour between 6kHz and 16kHz gives it a bit rougher highs than those that can can be found in some considerably more expensive headphones. Once modified the headphone is not that cheap any more as the pads alone are more expensive than the HD681 itself. The pads are worth it though. Also the filter adds to the costs depending on how it is made (external or internal). All in all the modified version is about 4x the price of a stock HD681. But as a stock version costs about E 20.- the modified HD681 will be around E 75. which still is very good VFM.

hd681 nieuwe meting

The CSD shows very slight ringing (when modified) and does not behave badly.

filtered hd681 k240 pads CSD

For comparison below the CSD of a stock HD681

stock hd681 CSDIt shows some ringing between 5kHz and 7kHz and a nastier one at 16kHz. The filter and pads have removed these artefacts.

Below the distortion plot of the (modified) HD681.

DIST HD681 modified

It shows quite some 2nd and 3rd harmonic distortion in the bass. The 3rd harmonics point to clipping alike behaviour.
In the treble area around 5kHz the distortion rises again.

Below the 40Hz squarewave plots of the (modified) HD681. It shows a strong bass response that extends well.

40Hz SQR HD681 mod

Below the 440Hz squarewave plot. It shows sharp rising and falling edges and reasonably controled ringing. Rest assured, without modifications it will look quite different.

440Hz SQR HD681 mod

And finally the 100μs needle pulse. Almost right on target (the green line) but shows quite some resonating effects afterwards but at a high frequency at 12.5kHz which is also visible in the FR plot.

100us HD681 mod

To me a modified HD681 cannot be beaten by many other headphones V.F.M. wise and together with the sound quality is it’s major selling point. Note that a stock HD681 does not sound anywhere as good as the modified version.


Above a picture of a modified HD681. It has K240 velourspads, the headband has been replaced by a real leather one (Made by Claus, Hoved-fi) and the red plastic rings have been replaced my machined aluminium rings (Mick, Heliharris). The cable has been replaced with a shorter and less microphonic cable.
There is damping material inside as well as an electronic filter reducing the treble and bass levels.

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