Introduction
Inside the earcups there are two foam screens that with time can get dirty and/or degrade.
As reported by Sony's partner Encompass, OEM part numbers are 4-738-572-01 (left) and 4-738-573-01 (right)
Third-party screens are usually sold together with ear cushions.
Tools
Parts
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Insert a plastic spudger between the cushion and the earcup frame
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There are six clips that hold the cushion in place, release them moving the spudger around the frame
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Conclusion
Once installed the new screens, reinstall earpad