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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
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 below.  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 hd650Below the distortion plot of the HD650 (with new pads)

DIST new pads redone
Below the distortion plot from the HD600 .

The 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.

distortion L


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 pads

What 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 pads

This 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 10N

Below 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 HD650
The 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) when driven from a 0.1Ω amplifier and a 120Ω output amplifier (Ember prototype was used).

hd650 new pads 0 ohm vs 120 ohm

As can be seen the mid bass is increased by about +1dB and the upper treble by about +0.5dB.
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.

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