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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.

post separation


The HIFIMAN HE-400i is an entry level orthodynamic headphone. There is a lot to like about it.
Comfort is excellent, the clamping force is somewhat on the high side but because of the large pads the pressure is well distributed and ensures a good seal.
The weight of this planar headphone is low compared to ‘similar’ headphone types.This is largely due to the way the drivers are built.
The headphone cable is a bit short and stiff but can be replaced. I find the SMC connectors a bit less suited for this task but at least they aren’t unobtainium and of good quality.
The finish is quite nice. High gloss very dark blue-purple-ish. The pads are soft and pleasant to the touch. Pleather (vinyl based fake leather) on the outside, velours on the contact area with the skin and performated pleather on the inside. They angle the drivers ever so slightly. A very nice carying case comes with it.

Ofcourse it is all about the sound. This headphone doesn’t disappoint there. It is one of those headphones that don’t need modifications and can be used without EQ. That doesn’t need it can’t improve with EQ nor that this is the perfect headphone.
I see this headphone a bit like an HD600 with more subbass. It is a bit brighter but not clearer sounding and the treble is soft but sometimes has an ‘edge’ to it. Not ‘sharp’ sounding just ever so slightly ‘not exactly right’.

The bass and subbass is there. I find the bass nice and ‘tight’ and ‘dry’. Subbass could be a few dB more present though.
The mids are ‘fine’ but not exactly ‘flat’. The mids are forward and present yet not ‘ super clear’. Very hard to explain but will give it a try. Drums, guitars and singers etc are a bit more ‘present’ than on a perfectly flat headphone. Smaller ‘toms’ are a bit too emphasized for instance.
Yet it doesn’t have the ‘edge’ headphones like the HD600 has. In this regard it is more like the HD800 but the HD800 is warmer and has slightly less subbass.
Then there is the treble part. The treble is ‘soft’ in general. good ‘air’ and detail. It doesn’t sound rolled off nor accentuated nor exaggerated BUT there is a slight ‘rough edge’ to it.
This not always obvious but sometimes it just takes something away from the rest of the experience.

Most of it is explained by the Frequancy Response graph which is a good thing because this means EQ can address the minor niggles IF you should desire so. Left, Right.

FR HE400i

The channel balance is excellent, above 50Hz within 1 dB which is exceptional.
As can be seen the subbass is there down to 50Hz but a few dB lower than the mids.
Towards 1kHz the FR is tilted slightly upwards which gives the sound a ‘more present/clearer character as opposed to a ‘warmish’ mids as the HD650.
The 10dB dip at 2kHz lowers some sense of the ‘clarity’ and ‘edge/attack’ of certain instruments. It also prevents instruments to become ‘shouty’. The HD800 also shares this trait somewhat. A dip isn’t nearly as objectional as a peak so a dip here is no problem and doesn’t need to be compensated by EQ.
The 8kHz peak is very narrow (pointing to a resonance) and peaks 5dB above the rest.
Because it is very narrow and up high it doesn’t manifest itself as sibilance BUT it sometimes (when overtones are present in that narrow band) it gives the sound an unnatural ‘edge’ that, when removed’ will make the treble perfectly smooth and pleasant.

The distortion is comforatbly low thus it is no problem to play a bit louder with it and to apply some extra subbass/warmth without increasing distortion. Plot of the left channel belowDIST L HE400i

Slightly higher distortion around 300Hz (inaudible to me) and at the 8kHz resonance.

Below the CSD of this headphone, (Left and Right channel superimposed)

CSD HE400i

Fast decay below 3kHz. Then the ‘dip’ which has a resonance in the middle (4kHz). Fortunately for our brain the ear canal also ‘rings’ in that frequency band so it isn’t objectionable. The brain ignores the ringing there. The 8kHz peak is double that of the 4kHz. Probably not a coincidence ….. That 8kHz ringing is audible as that ‘unnatural’ sort of edge sometimes. The ringing can not be reduced but the level can which effectively also shortens the ringing slightly. Above 10kHz a very good response.

below the squarewave response plots. The 40Hz is excellent in waveform (nice horizontal red line) which shows the slight upwards tilt in FR as well.
The 440Hz shows the relative ‘lack’ of sharp attack but the horizontal part shows the mids are pretty good.
The 100µs impulse plot shows the ‘sharp’ attack is present (rising edge) and just below the target level. This make the treble ‘fast’ but not exaggerated, smooth. The ringing following the impulse shows the membrane loves to vibrate in a high frequency (8kHz).

SQR HE400i

The Spectrum plot below also shows the ‘lingering’ SPECT 400i L

When this plot is compared to non ortho headphones such as the beats Studio Wireless below you can see that the ‘energy’ lingers on longer (up to 20ms) compared to the one below (Beats in this case). Below 1kHz the HE-400i does a lot better than the beats though.
SPECT beats BW L

What’s not to like:
The slight ‘ringing’ at 8kHz is something that could be addressed with a very sharp notch EQ.
Subbass could be a tad more present, can also be adjusted with tone control.
Cable stiffness and connectors (screw-on)

What’s to like:
Fit and finish, comfort, weight (for an ortho).Stock sound signature which is ‘clear’ and open, not splashy nor sibilant.
All in all a headphone I can recommend be it that the price is somewhat on the high side (€ 500-600) and there are alternatives in that price range, though not that much.
Sound quality at lower volume is excellent.

Of course I designed a Kameleon module for it.
It is why the HE400i was sent in for anyway.

Frequency response on the Kameleon amplifier (with HE-400i module) Left, Right

FR HE400i Kam

No more 8kHz peak, increased clarity and ‘air’ and (sub)bass are now on more desirable levels.

Below the difference between the stock HE-400i and via the Kameleon.

HE400i stok vs Kameleon R

It doesn’t look like a LOT has changed but the +5dB compared to stock at 20Hz is audible.|
More ‘body’ to the sound. Also the removal of the 8kHz peak is audible. Extension to well beyond 25kHz is also improved.

Below the distortion plots of the left channel:

DIST L Kam HE400i.png

The distortion is quite low, certainly in the lowest frequency range. Exemplary !

Below the CSD (Left and Right channel superimposed) on the Kameleon.

CSD Kam HE400i

The 4kHz ringing is still there but the 8kHz (which is more offending) is substantially reduced.

Below the sqruare-wave and impulse response plots. On the left the stock HE-400i on the right the same headphone playing via the Kameleon amplifier.

SQR HE400i

I have rarely seen such nice 40Hz response plots. The 440Hz shows an increase in ‘attack’.
The impulse response shows VERY sharp risetimes, sharper than other headphones I measured. Ringing also is greatly reduced. The FR response also is very well extended.

The sound quality has increased. The ‘edge’ is gone, clarity has improved and the sub-bass is there, deep and tight.

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