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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
Koss PortaPro


The Koss PortaPro is the ‘headband’ version of the KSC35. It is a highly regarded headphone that is very portable, can be folded so it takes up less space when transporting and is very comfortable on the head. The earpieces do not press on the ears but are simply held right before your ears so there is little to no clamping force. The headband can be easily adjusted in height but does not feel very sturdy, yet is durable nonetheless. The pads above the earpieces are the ones that rest on the head. With a finicky and difficult to adjust ‘slider’ you can adjust the pressure on the ears somewhat but the system doesn’t work that well for me (it does not ‘hold’ its position). Another disadvantage to me are the sharp little steel hook and clamp just under the head rests. It will depend on how big ones ears are, but in my case they ‘touch’ in the upper part of the pinnae which is uncomfortable. I removed those clamp/hook construction in my version by sawing it off (steel so not easy to do). The sound quality is something most will NOT complain about, unless they bought on of the many FAKES out there.

The frequency plot below shows a sharp dip that isn’t seen on the other Koss models shown above. The peak at 5kHz also is not present in the KSC35. It should be noted that the PortaPro used here is the PortaPro Classic and is brand new. The KSC35 and KSC75 are much older and have MANY years of very frequent usage on them. The sharp dip and peak MAY thus be something that might still alter somewhat over time but could also be because of changes in the production process over the years. In any case the peak and dip are NOT obvious in the sound. It sounds very closely the same as the KSC35. realisticwith good lows and mids, enough clarity and no obvious roll-off in the highs. No piercing highs and no boomy sound !, very natural sounding. The very smooth and gently sloping part between 40Hz and 2kHz ensures very good lows and mids, only lacking in real bass extension (the deep movie type rumbles). left, right

PortaPro stock
When the frequency plot is ‘smoothed’ with 1/6 octave (as is usually done on most websites) the FR plot looks like this left, right:

portapro smoothed 1-6
The C.S.D. (or waterfall plot) below shows the peak at 5kHz to be a resonance. Ringing is not that long lived though and only marginally ‘worse’ than that of the KSC35.

portapro waterfall

A comparison between the KSC35, KSC75 and PortaPro can be found above in the KSC75 section.

As the pressure on the pads is very light, the ‘coin mod’ doesn’t make much difference for the PortaPro as the foam is not compressed more.
Stock pads versus coin modded pads on the PortaPro… no difference at all showing that the foam material itself is acoustically ‘transparent’ and the effect on the other models is due to the pads being compressed more due to the clamping force of the clips on the pinnae.
The coin mod still has the benefit that hairs cannot get stuck in it and touch the driver. Disadvantage is that dust etc. can reach the driver.

PortaPro r green QMod

The PortaPro is a very comfortable (if the steel spring doesn’t touch the top of your pinnae), lightweight and good sounding headphone that is easily driven by portable gear. It cannot play loud very well though and starts to distort audibly at higher levels. It is not very expensive nor very cheap either but well worth the price.

Be VERY aware of the fakes out there generally on e-bay or Chinese sites that sell cheap ‘original’ (ahem) products !

Most people will have heard of the Kramer mod. This involves removing the plastic grille in front of the driver and drilling more holes in them.


On the left a ‘Kramer modded grille, on the right original grille, measurements below.

portapro kramer mod

Interestingly enough the effect isn’t as big as many reviews and websites make it out to be. One can read reviews of bass getting increased to much more brighter sounding.  A lot of those reports seem to be based on expectation bias. The effect isn’t as big as some make it out to be. There seems to be slightly more extension in the highs but also less presence above 7kHz. Less pressure on the ear will decrease the amount of lows and thus make it sound a tad brighter.
Because the measurement rig is NOT built like a real head and the shape of the head as well as the Pinnae has a definite influence on the sound the PortaPro may not be perceived as measured by everyone.

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