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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.
The 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 HD671



Type: Over ear, closed
Usage: Home, studio, portable.
Driver type: dynamic
Pads: replaceable, silicone alike composite material
Inner Pad dimensions: depth = 18mm, ø 60mm
Collapsible: No
Headphone connector: none, fixed cable.
Cable entry: single sided (left)
Cable: 2.5m terminated in 3.5mm TRS with screw-able 6.3mm adapter.
Driver size: ø 50mm
Nom. power rating: unknown, assumed 0.2W
Max. voltage: 2.5V (assumed 0.2W)
Max. current: 80mA (assumed 0.2W)
Max. S.P.L.: 118dB (assumed 0.2W)
Impedance: 32 Ω
Efficiency: 95 dB/1mW (110dB/1V)
Weight: 280g. (with cable)
Colour options: black and white
Clamping force: low
Accessories: Ø3.5mm to Ø6.3mm gold-plated connector, cloth carrying bag


The Superlux HD671 is a closed dynamic over-ear headphone intended to be driven directly from desktop and portable equipment. It will probably retail for around $ 35.- which is quite affordable to most people.
For this you do get a modern looking, light-weight and to some people comfortable headphone. It will be available in white and black.
The cable is 2.5m long which is somewhat on the short side for desktop duty and way too long for portable usage. It is non replaceable and only very slightly microphonic. This means mechanical noises from the cable rubbing on clothes etc. are heard only softly in the left earpiece.

The headband is comfortable and self adjusting over a reasonably wide range.
This is achieved by a elastic-band hidden inside the headband. The adjustment range is quite good and better than the HD681-EVO, HD662-EVO, HD687 & HD688. The construction seems decent. I have no idea how these elastic bands will age though.

Isolation from outside noises is decent but not exceptional.
Others around you can’t hear what you are playing unless you are playing very loud.

The ear-pads are what this headphone sets aside from (all ?) other headphones. They seem made of silicone. Superlux calls it ‘composite material earpads’ and are at least innovative. Easy to clean and won’t flake after a few years like a lot of pleather pads do.
They are quite soft as there is no foam inside which often decays over time. It gets its flexibility from its shape and thickness.
More about these pads can be read in this PDF file that can be downloaded.

Sound description

Out of the box the HD671 sounds very dynamic and forward. There is a slight ‘weirdness’ to the sound though. Bass is present and extends low but is not of the rumbling kind and also not very realistic. A bit speaker like in a small room. A bit artificial sounding. Not of poor quality though. It sounds as if the bass and lower mids are coming from a large barrel. The mids do sound very open and dynamic. Instruments are well separated but have a kind of ‘hollow-ness’ to it. The sound is forward but is somewhat lacking in clarity.
The treble is elevated but do not sound elevated nor harsh.
The HD671 can play pretty loud before sounding nasty (starting to distort). Of course this is also dependant of the maximum output voltage of the source.


Below the frequency response of the HD671 (Left, Right)

FR HD671

Bass extension is quite good and goes down to 10Hz. From 60Hz to 250Hz there is a substantial peak of +7dB. These are quite audible levels and make the HD671 sound not as good as its semi-open sibling the HD672.
The bass sounds hollow and gives a sort of ‘warming’ to the sound. Above 3kHz the HD672 shows some ‘problems’. A dip with some resonances up to 6kHz with a massive resonance at 7kHz. It is not really desirable to have a peak in that part of the frequency range as ‘sibilance’, ‘edgy sound’ and unnatural brightness is in that area (see the previous page).
The treble itself does not have many deep peaks and dips but is reasonably well behaved till 20kHz.
Highs thus have enough ‘air’ and ‘resolution’.

The HD672 is quite sensitive to loss of seal. Good seal, with glasses, with thick arm glasses and a seal breach.

seal stock

When the seal is breached (a gap between pads and head) a substantial loss of bass levels is observed. When wearing glasses with thin arms the bass loss is a few dB and not problematic yet, However, when thicker arms are used the loss of bass is already substantial and quite audible. A good seal is important. When a pad is not touching the skin in one or more places and the seal is fully breached and bass levels can drop over 15 dB.

Below the distortion measurements of the HD671 (Left channel).


Below the distortion plot but displayed in percentages.

DIST HD671 L percent

In the bass area the 3rd harmonic distortion is quite high and point towards compression. 10% is quite audible.
Above 1000Hz the distortion is lower than 2nd harmonics and around 0.5%.
Another soar point is the area around 4kHz. The distortion reaches 1% in a rather narrow band.
This is audible as the ear is most sensitive in that part of the frequency range. Also around 7kHz there is some distortion (1%) and ringing (see other measurements on the following pages).
For a headphone in the lower price class these values are not bad but not good either.

Below the CSD of the HD671 . (Left and Right are superimposed)


There is a lot of lingering at lower frequencies. At 500Hz the amplitude does not go down fast enough. At 2kHz and 4kHz some ringing is visible.
This driver resonates at 7kHz and 9kHz. Above 10kHz the response isn’t worryingly bad though and no alarming resonances are seen.
This peak at 7kHz not only consists of a peak in the frequency domain but also in the time domain.

Below the spectrum plot of the HD671 (Left channel). This plot is a bit like the CSD but looked at from above where the amplitude is colour coded and the time line is from bottom to top instead of rear to front. Also a wider part of the frequency range is visible.

spectr HD671 L

The area between 400Hz and 800Hz shows some considerable lingering. At 1.5kHz, 2.4kHz, 7kHz and 9.5kHz resonances are visible.

The step response (Left channel) below.

step L

This plot shows how the driver reacts to a jump in position from ‘central’ (no signal) to a position similar to producing about 90dB SPL.
The initial rise doesn’t reach the 0dB line but reaches -3dB and has some serious ringing.
It still rings visibly up to 2.5ms.
The sub-bass response is decent and only lacking a bit below 30Hz shown by the drop in the horizontal line.

Below the square-wave and impulse response plots of the HD671.

HD671 stock SQR

The 40Hz oscilloscope shot shows the sub-bass extension is good.
The 440Hz square-wave shot doesn’t look like a square-wave at all. It looks more like a bunch of 7kHz ringing somewhat following the applied signal.
The 100μs pulse overshoots and isn’t the correct width. The ringing following is is almost of equal amplitude and rings on for quite some time.
I am actually amazed that a headphone with this kind of square-wave and pulse response still sounds the way it does.

output resistance / damping-factor

As this is a low impedance dynamic headphone the frequency response might be amplifier output resistance dependent when certain higher output resistance amplifiers are used. In the high-end desktop amp world chances are higher that you can encounter a high output resistance amplifier.
To test this the headphone is measured via a low impedance amplifier (0.2Ω) and a high impedance amplifier (120Ω). On a higher output resistance amplifier the output level will be slightly lower. To compensate for this the amplifier is cranked up a bit to the same level (at 1kHz) as the low impedance amplifier. This way the plots are overlay-ed and it is easy to see how the tonal balance changes.120 ohm

It looks like a higher output resistance of the amplifier (or portable equipment) has very little influence on the tonal balance. The impedance plot thus is very ‘flat’ with only a minor wideband rise around 60Hz.

The HD671 also exists in a semi-open version (the HD672) which, to me, is the better sounding one. Below the HD671 versus the HD672HD672 vs HD671


The HD671 does not measure extremely well. The sound, on the other hand, decent but a bit muddy in the lows.
Even though the pads have something going for it to me they have more down sides than upsides.
Comfort of the headphone is good and so are the looks.
The lack of movement from the cups, the long fixed wire and pads are downsides that may put some people off.
The good news is that the pads can be changed into something more pleasant (velours) and the sound quality can be improved significantly with just a few simple modifications.

A more in-depth analysis with several experiments and modifications can be downloaded HERE


HD671 modif

Below a small preview of what can be achieved with this heaphone with a simple pad change and small modification which do not require many skills nor opening the headphones.

Below the frequency response of the modified version shown in the picture above.
HD671 modif FR



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