After 2 years i changed my 1 more pads,because just sometimes the pads were very uncomfy.Not a long ago i bought a back up ear pads for my ndh 20 just in case.Its from the offical sennheiser support page in Hungary.The pads same material,but just slightly smaller than the original.I thought it will fit to my Tridents,and they are, really perfect.Ndh 20 always were my one of the best comfortable headphone so no problem here too with the comfort after several hours!!And i think the sound remaind the same or even better.Tridents 
Ohh and those holes inside the 1 more original pads! i pretty sure the holes just for let breath the foam inside the protein leather! Nothing to do with the sound!!
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In theory the holes in the pad are for the offset Piezo driver. Surprised you say the NDH20 has one of the most comfortable pads, always hear they are so stiff and they look quite small.
That picture aint valid!That 5 mm driver almost middle of the dynamic driver!Also you can feel it with your fingers,its in the middle not the side close to those holes!And the two driver covered with plus foam under the left-right logos.Theory and experience not the same!Thats why good to try things on your own!
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Ndh 20 not just so comfy for me but these are one of the best sounding,including some openback too(sundara,hd6xx series,etc about that prices)
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