soundproofing

US /ˈsaʊndˌpruː.fɪŋ/
UK /ˈsaʊndˌpruː.fɪŋ/
1.

geluidsisolatie

special material put in a room or building in order to prevent sound from passing out of or into it, or the act of putting in this material:

:
He added soundproofing in response to neighbors' complaints about noise.