Hence my question, are there any tests I can run to see if it has any impure elements. I know certain UV lights show stains but I don't know if they work on humans too. These days the way headphones are being sold, it isn't really clear if the product is a used one or new one.

No, I've had these concerns from long before the pandemic. Should I assume they came in contact with crap, piss and toilet water?

No you are absolutely right.

Tried that and it didn't improve. The voice coil came off with the membrane from the magnet, should it be fixed inside or come out with membrane?

Thanks for this suggestion. I tried using a mini vacuum cleaner with a straw attached, it was able to reduce the depression in the center, but there is some depression towards the side, and I'm unable to insert the straw or anything to reach that area because of the grill. Do these depression...

I was trying to repair a headphone and a screw from the earcup got attracted onto the driver, while trying to take it out, the screw got moved around on the center bump of the driver, this left a large depression and many smaller ones on the driver. Will this affect sound quality, how can I...

No, I mean dirty things.

Are there any tests I can run to see if there are any impure elements on a headphone or earphone?

There is no 7.1 headphone implementation in the market which sounds as good as the real 7.1 home theater. So stereo, I've tried 7.1 in gaming headsets and I think they make the quality worse than it should sound.

I don't know how you found that graph, on their main review rtings shows this: https://www.rtings.com/headphones/reviews/sennheiser/hd-800-s And it is almost flat, within +/- 3dB.

In Hi-Fi speakers from reputable speaker companies, they mention frequency response with amplitude variation, this gives an indication of how flat the speaker will sound. Headphone and earphone companies, never mention this information, I've only seen Chi-Fi gaming headphones and earphones...

So, in your opinion a company claiming their headphone is hi-res without certification is lying? I think it is, Japan Electronics and Information Technology Industries Association invented it in 2014 and they certify headphones as hi-res, like HD 800S, ATH-SR5, etc So it has no noticeable...

Should a headphone claiming to be Hi-Res have Hi-Res certification?

Beyerdynamic DT 990 Pro. What is the purpose of cotton paper over diaphragm? Can it be removed? How would I remove the plastic housing the diagram, they seem to be clamped to the black holder.