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Modification for the prototype HE35X

When testing and probing the HE35X I found some odd behaviour in the lows. It turned out that the lower frequency extension got less at higher listening levels. I will spare everyone the multitude of tests and only show the relevant results.
Below the frequency response measured at different loudness levels at 500Hz.
90dB, 80dB and 70dB.
FR changes with amplitude
At 70dB SPL the bass extends flat down to 50Hz, at 90dB SPL it starts to drop-off below 150Hz.
Same measurement but the traces are over-layed so one can see the effect more clearly.Bass extension different SPL
There is definitely relatively less bass at higher listening levels.

To show this usually is not the case below the AKG K701 measured at 3 different levels.
90dB, 80dB, 70dB @ 1kHz
90dB, 80dB, 70dB K701 fr changes
As can be seen the tonal balance does not change with different SPL levels.
The fact that audibly the perceived tonal balance changes with listening level (true for all headphones and speakers) is for another reason namely the ‘equal loudness curves or Fletcher-Munson curves’
In any case such compression at higher excursions should not happen with drivers.
The various drivers/headphones I tested did not show this effect.
The reason this happens with the HE350 and HE35X is because the driver is over-damped under the dome/voicecoil. In the right upper corner one can see the driver magnet. In the middle of the driver magnet there is a hole that allows air pressure from the rear of the driver to escape/flow freely. Unfortunately this hole has been covered with ‘acoustic paper’ that restricts (or should I say blocks) air flow too much. Removing this paper sticker will increase the midbass way too much and the sound becomes ‘boomy’ and ‘bloated’ and ‘fat’ in the bass in a very negative way. The driver membrane (around the voiecoil) has damping paper around it but does not have a selfadhesive that blocks air and also has a small hole (on the left of the magnet in the picture below) that allows air at lower frequencies to flow. Just not in the center magnet hole.

The solution for this is relatively simple. Carefully remove the sticker (peel it off by using a sharpy) covering the magnet hole. Press the removed sticker on the hole around the driver. Basically the paper covering the magnet hole is moved to the hole in the paper ring around the magnet.
hole HE35X driver-1modified HE35X driver

 

 

 
The hole in the driver is covered by a selfadhesive felt disc. The type used under table or chair legs. Experiments showed the best response was obtained using a felt disc with a 3mm ∅ hole. This hole was punched using a leather-belt hole punch.magnet hole felt with hole with ring hole shut
Don’t expect this modification to perform miracles and turn it into another headphone. The tonal balance will be very similar and the sound at lower to normal listening levels will not change at all. At higher listening levels the modification will give some lower distortion in the bass and a few more dB of lower bass.

Below on the left driver with original sticker, on the right with the felt disc with hole.

stock HE35X driver

modif HE35X driver

 

 

 

 

 

 

 

Below the frequency response of the driver measured at different listening levels with the felt disc with a 3mm hole in it. 100dB, 90dB, 80dB, 70dB (at 500Hz)3mm felt disc, hole shut overlayed compression no legenda
NOTE: Ignore the little wiggles at 100Hz, 200Hz, 300Hz etc in the blue trace as this is mains hum.
Subbas roll-off is barely present at 100dB and just -1dB @ 50Hz (relative to 200Hz) at 90dB SPL where it is -5dB in the original driver.

Distortion numbers also improved. Below distortion levels of the stock HE-35X at 90dB (500Hz)dist stock 90dB percent

Below the distortion levels of the modified HE-35X at 90dB (500Hz)dist modified 90dB percent
Distortion levels dropped from 8% to 2.5% and above 100Hz the 3rd harm. distortion drops rapidly.
And this is at 90dB SPL for the lower frequencies.

Most people will say: well one never listens to 90dB as this way too loud.
What one has to consider here though is that 90dB SPL at 40Hz is ‘perceived’ as 70dBA due to the equal loudness contours.

Given the fact that the mids usually are -15dB opposite the bass notes the 90dB SPL would come down to 75dBA.
So the 90dB SPL, which is measured, is NOT equal to 90dBA (= hairdryer) but is rather a still ‘comfortably loud’ listening level. Yet the bass notes will reach 90dB SPL even though you won’t perceive the bass as loud.

For those wanting to know what the grille changes to the tonal balance this is shown below. With grille, without grille. The differences are very small.grille vs no grille

The good news is that the measured headphone is the prototype and Massdrop is trying to get this modification fitted in the production version of the HE35X (August 2019 drop).
Should this not be possible one can always apply the modification above.

Don’t expect major changes from this modification. The tonal balance and overall sound will be the same as the reviewed headphone. Only at louder levels and when compared directly to each other the bass response could be told apart.

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