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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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Sennheiser HD650


Sennheiser‘s former flagship, the HD650, is still an excellent headphone today and worth owning. When you find the HD650 a bit too ‘dark’ go buy the HD600 which doesn’t have the ‘boost’ in the lows. The plots below are of the earlier type, black internals and the flatter earpads. This is considered the ‘veiled’ version and indeed can sound slightly subdued in the highs. The newer version with thicker pads and white internals have slightly less midbass ‘hump’ and isn’t considered ‘veiled’. Very smooth FR with no alarming peaks or dips. The ‘warmth’ hump’ between 30Hz and 500Hz is almost flat in the HD600 version. Another way to make this headphone sound less ‘overly warm’ is by using the active filter described HERE. More about the pads and ‘veil’ further down.


Type: Over ear, open
Usage: Home, studio.
Driver type: dynamic
Pads: replaceable, velours
Collapsible: No
Headphone connector: Sennheiser 2 pin
Cable entry: dual sided, marked
Cable: 3m OFC cable terminated in 6.3mm TRS jack.
Driver size: 40mm (38mm membrane)
Inner Pad dimensions: depth = 20mm, Width = 40mm, height = 62mm, oval shaped
Max. power rating: 0.5W
Max. voltage: 12Vrms (35Vpp)
Max. current: 40mA
Max. S.P.L.: 125dB
Impedance: 300 Ω
Efficiency: 100 dB @ 1mW
Sensitivity: 104 dB @ 1V
Weight: 260 g. (without cable)
Clamping force: high
Accessories: 6.3mm to 3.5mm adapter cable, presentation box

White/Silver driver HD650 (2017)


Below the frequency response of the HD650 (Left, Right) 2017 version

FR HD650

Channel matching is excellent.  Below 50Hz this headphone drops off gradually.
No deep rumbles on an impressive level but still quite audible.
Bass sounds ‘tight’ and ‘dynamic’ and integrates seamlessly with the mids.
From 50Hz to 20kHz the response is very flat and accurate.
The small dip from 1.5kHz to 3kHz may be a little less ‘deep’ in reality compared to the measurement  because the measurement rig does not have a pinna which does alter the response exactly in that area. That small dip is not audible as a dip.
The treble response is exemplary.
From 5khz to >20khHz with very little sharp peaks and no dips.

As this headphone will pique the interest of HD660S owners below some comparative plots with a current (2107 stock) HD650 and a HD660S.

HD660vs HD650 nr 3

Lot’s of people are wondering how the current (silver screen) HD650 compares to the (silver screen) HD600.
Below the frequency response of the HD650 vs HD600.

HD600 vs HD650
As can be seen the HD600 has slightly less midbass hump. From 600Hz up the headphones are very similar. Above 10kHz the HD650 has about 1.5dB more output.

Below a direct comparison between HD600, HD650 and HD660S but the vertical (dB) scale is not 5dB per division but 2dB/div so ‘expanded’ in view to show the differences a bit better.HD600 vs 650 vs 660S
As it is hard to see the perceived tonal balance below the exact same plot but with some light ‘psychoacoustic smoothing’ applied.

HD600 vs 650 vs 660S tonal balWhen comparing directly we can see the HD600 is the ‘brightest’ of the bunch followed by the HD650 which really only has a few dB more upper-bass/lower mids in other words ‘warmth’. The HD660S has the ‘warmth’ of the HD650 but less clarity/presence and slightly less treble.
Look at the scale the differences are quite enlarged in this plot.
Below the differences are shown on a scale I normally use (5dB/division).HD600 vs 650 vs 660 5db div

Below the distortion plot of the HD650 : (only Left channel shown)
Note that this headphone was measured at ears-unlimited-logo where  background noises were present in the demo room. As this is an open headphone distortion levels and spectrum plot performance may thus be better in reality than as shown on the plots.

The distortion products are shown in dB.DIST L

Below the same data but shown in a percentage scale instead of a dB scale.DIST L percentNote: ignore the spikes below 100Hz these are ‘hum’ components in the portable measurement rig.
The distortion profile is typical for a dynamic driver. Higher 2nd harmonic distortion in the bass. 2% distortion around 50Hz is not bad but also not a poor value.
The measurement rig has a relatively high 2nd harmonic distortion so numbers below 0.2% will be there in reality above 200Hz.
Distortion from 200Hz to 10kHz is very low for the higher harmonics.

Below the distortion plot of the black driver HD650.HD650 dist

Below the CSD of the HD650 (Left and Right channel are superimposed)The CSD shows an excellent response with no obvious and deal-breaking resonances or lingering.

By lack of oscilloscope shots (not enough time to measure that) below a step response plot of the HD650 (Left channel)step HD650 LThe step response shows the lack of ‘bottom end’ as evidenced by the downwards sloping line. Headphones with a good sub-bass extension have a (close to) horizontal line after the initial rise.
The initial rise reaches the 0dB line very closely and shows there is nothing ‘slow’ about the HD650’s driver at all.
The subjective ‘speed’ of a headphone is often said to relate to the highest frequencies a headphone can reach. Nothing is further from the truth. Most of the ‘attack’ in instruments is in the 2kHz to 5kHz range in reality.

measurements of ‘black’ screen driver

Below the frequency response of the HD650 with old pads Left, Right


more extensive measurements of several possible modifications
(Warning large file >13MB, 64 pages)

The HD650 has an exceptional good frequency response but is ‘tuned’ to sound speaker like and lacks bass extension and to some sounds ‘veiled’ because of this. Fortunately the HD650 responds great to a proper EQ.

Below the plot of exactly the same headphone but measured through the Kameleon Portable amplifier with an ‘HD650 module’ installed.

The difference in sound is remarkable, no excessive warmth but pure realism, excellent and VERY deep and tight bass and an unrivalled midrange with a present and NEVER sibilant smooth and extended treble.hd650 kameleon

A combined CSD for both channels (stock HD650 NOT via the Kameleon amplifier), very clean and fast decay !csd hd650

Below the distortion plot of the HD650 (fitted with new pads)DIST new pads redone
Below the distortion plot from the HD600.distortion LThe 2nd harmonic distortion of the HD650 below 100Hz is considerably lower than that of the HD600, above 100Hz the HD650 shows higher distortion.
3rd harmonic distortion of the HD600 is lower than that of the HD650.

HD650 pads

The measurements of the headphone above are made with a black driver + old type pads.
Most likely the old pads once were a bit stiffer and the foam inside will be somewhat decayed/softened over time.

The newer pads indeed sound ‘brighter‘ and ‘clearer‘ and relatively the old pads sound ‘darker’ or ‘warmer’ with less clarity/treble presence.
I was curious what caused the differences as side by side they looked very similar in height.
The old pad = 30mm high, the new pad 32mm, the color of the new pad is darker and the velours of the newer pads feel less ‘soft’ to the skin.
The new pads also come with new foam inlays.
The new pads have the product code: 050635. They are suited for HD545 / HD565 / HD580 / HD600 / HD650 / HDI850.

HD650 pads

Time for some measurements to show the differences between the 2 pads (right channel only).
The measurements are done with the same test-voltages from a low output R amplifier and the same driver/channel.
The old pad has a good 2dB more presence below 1000Hz and is also a bit louder in SPL. The amount of treble (> 4kHz) is about the same level.
The new pad is more balanced and the ‘warmer’ signature is gone. More towards the HD580 / HD600 signature.

hd 650 r old pads vs new padsWhat isn’t very obvious when wearing these is the fact that the old pads compress a lot more than the newer (less compliant) ones.
When placed on the test rig this became quite evident though.
Quite possible the older pads may create a slightly better seal on some heads (depending on bone structure) as well.
Using the older pads the drivers are thus much closer to the ears which results in a slightly higher SPL below 1kHz.
To check if the pad thickness is indeed the only parameter that changed I pressed the new pads against the rig to the same driver-rig distance as the old pads did by themselves. Below the result of this small test.hd650 old pads vs compressed new padsThis test shows that the newer pads do not have any different absorption or other acoustical effects and the differences in sound signature and SPL between the two pads is now the same. They sound equally ‘warm’ under these conditions.

What became quite obvious is that while the old and new pads differ only slightly in height when laying flat on the table (30mm vs 32mm) as shown in the picture above, but the pads compress in a substantially different way when being pressed against the skull with the same clamping force.

Below 2 pictures of both pads with 1kg weight on top of them. Roughly equating to about 10 Newton force which is a bit north of firm pressure but it shows the differences better.
With 10N on the entire surface of the pads the total height of the pads was reduced from 30mm to 20mm. Considering the mounting plate was also included (which is 5mm) the actual pad thickness reduced from 25mm to 15mm a 40% reduction.

old HD650 10NBelow a picture of the new pads with the exact same weight pressing on it. With 10N on the entire surface of the pads the total height of the pads was only reduced from 32mm to 27mm. Considering the mounting plate was also included (which is 5mm) the actual pad thickness reduced from 27mm to 22mm a 20% reduction.

new HD650The question of course is whether or not the old pads had been (much) stiffer when new which is quite likely as the foam inside the pads deteriorates over the years.
If that’s indeed the case then the sonic signature must have become ‘warmer’ over the years without me really noticing it….

Amplifier output resistance effect

Due to the impedance characteristics of the HD650 the headphone changes its sonic signature slightly when fed from either a low output R amplifier (0.1Ω) or  a higher output R amplifier (120Ω). Because of the relatively high impedance of the headphone (around 300Ω) the effect isn’t nearly as dramatic as it can be on lower impedance headphones.
Below a plot of the HD650 (with new pads)  driven from a 0.1Ω, 10Ω, 32Ω and 120Ω output amplifier.HD650 R120 meting
As can be seen the mid bass is increased by about +1dB and the upper treble by about +0.5dB when using a 120Ω output R amplifier, anything below 32Ω is not audible.
The SPL was level matched at 1kHz to compensate for the level differences caused by voltage division. The CSD showed no differences other than the small level differences.

The Veil… [insert ominous music]

I was sent a HD650 which the owner found veiled/dark. He noticed one of the drivers of his headphone came with the dome of the driver pressed inwards and wondered if that may have an impact on the sound.
Below the driver in question. It is not easy to see but the dome is ‘folded’ inwards.

OLYMPUS DIGITAL CAMERAThe fix for this is quite easy. Remove the rear grille. Put your mouth over the middle part of the rearside of the driver. Gently apply some pressure by blowing a little and the dome pops back in original position. The warped dome does alter the response slightly.
HD650 driver dented and repaired.L dented vs fixed
There is less upper treble (around 15kHz) and the upper mids between 1kHz and 3kHz is boosted a few dB.

FR H650After this rather easy repair the headphone measured similary again between L and R channel, at least within expected tolerances as shown above.

This HD650 seemed a bit ‘darker’ than what I have seen earlier so compared these with HD650’s measured earlier. An early (black screen) HD650 fitted with new pads. A HD650 from 2017 (silver screen) and the fixed but ‘veiled’ HD650 (2014) below.
(mind the dB scale which is 2dB/div in this plot)HD650 Bl vs HD650 BL+new pads vs HD650 veiledThe veiled HD650 (a 2014 silverscreen by the way) clearly has 3 to 4 dB less output compared to the quite new HD650 (2017).
Even the older black screen HD650 (fitted with new pads) has about 2dB more presence & treble than the veiled HD650. Also the lows roll-off somewhat sooner and steeper.
This HD650 thus has audibly less output and sounds ‘darker’ than the HD650’s I heard and measured before.

It looks like there is quite some spread in driver performance and Sennheiser matches drivers to sets that are then used in one headphone. This means that one can get HD650’s that sound darker or clearer depending on which matching set of drivers was used.
This also means that when replacing a blown driver and ordering a new one chances are you may be getting a driver that isn’t matched with the still O.K. driver.


The silver/white screen HD650 performs closely the same as the older black screen driver when the same (new) pads are fitted.
The HD650 is a well known and capable headphone that does a lot of things right.
One of them is tonal balance. Also distortion is quite low and it shows why it had been the top model of Sennheiser for quite some time. It was considered an expensive headphone once. These days it is considered a cheap headphone…
Those that find the HD650 ‘veiled’ should look for headphones with more treble presence or audition another HD650 because not all HD650’s are created equal as shown above.

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