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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.

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Koss KSC-35


The headphone (earphone ?) below is the well known and universally loved (certainly by me) Koss KSC-35. It measures quite similar to the cheaper, and more pleasant to wear, KSC75 and also the PortaPro. They all share drivers that are very similar. The KSC35/75 are ear-clip models and the PortaPro is an on-ear headband model with the distinct difference that the earpieces do NOT rest on the ear (as with all other on-ears) but with small pads on the head, just above the ear. The FR is very smooth between 30Hz and 3kHz. No subwoofer rumbles but the bass reproduction is quite good. In most recordings there isn’t much there below 50Hz anyway. The highs extend to 13kHz and drop off rather fast. Because those highs are slightly lifted the small dip at 7kHz isn’t obvious nor is the lack of treble extension (“air’). These plots are made with the stock pads on it. blue=left, red = right channel. This is a very enjoyable earphone but the plastic earclip is NOT as comfortable as that of the cheaper KSC75 and, unlike that of the KSC75, can NOT be adjusted or bent as it is a thin hard plastic earclip. Left, Right channel

KSC35 stock (revisted)

Waterfall plot (CSD) below shows some coarse behaviour from 2kHz to 5kHz.

ksc35 l waterfall

On many websites some easy mods are mentioned, one of them is called the coin mod. Basically you cut out a round hole the size of a coin from the center of pad as shown below. (picture showing the coin mod is of a KSC75)

stock vs coinThis gives these headphones an apparent boost in the highs. Strangely enough you always read impressions of what it does to the sound but see no plots. In reality the boost in highs is NOT coming from the lack of foam in front of the driver but rather from the fact that the foam compresses more easily as there is less material and when the same clamping pressure is present the driver is somewhat closer to the ear and thus the highs are reproduced somewhat louder in comparison with the lows and mids. New foam pads are ‘springy’ but they will start to disintegrate  after a few year and get thinner. This means over time the highs will increase by itself as the driver comes closer to the ear. Easy to mistake this for ‘burn in‘. The foam pads are replaceable and can be found cheaply on the web (e-bay and headphone websites).

There is an added bonus to this modification though. Sometime hear(s) get caught in the foam and get stuck in there. In some cases those hairs come in contact with the driver and this may give a rattling bass reproduction as if the driver has become defective. Don’t think they are broken! You can remove the pads and clean them out. The quarter mod (the hole is the size of a quarter) prevents this problem.
Disadvantage, however, is that dust etc. can reach the driver.

Below the differences between a ‘coin moddedKSC35 and the same driver with stock pads. It brings a bit more ‘clarity’ overall as the upper mids are elevated a bit.


Even though the area around 7kHz shows a distinct dip this isn’t as ‘obvious’ as a peak in that area would be. There is a rather sharp drop above 13kHz. This suggests it should lack some air. It does so on direct comparison with better headphones yet isn’t very obvious when you listen to one for a longer period. The KSC35 is still available new but twice the price of the KSC75. Very enjoyable and ‘believable’ headphone. Highly recommended. It cannot play loud very well though and starts to distort audibly at higher levels

Because the impedance plot shows an increase of impedance these drivers have a different frequency response when driven from a high output resistance amplifier.Below the difference in response KSC35 from amplifier and from 120Ω output resistance amplifier. As can be seen there is about 1.5dB bass boost from a higher output resistance amplifier but at the cost of  -10dB in efficiency.

KSC35 120 Ohm vs 0 Ohm

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