An acoustic door is a special door designed to block noise or sound from passing one room to the next when closed. The material used for the door usually contain a foam insulation that have been proven to break soundwaves coming from the noise. It also comes with a special seal along the edges to prevent the sound from coming inside and enables to virtually airtight seal.
The sound hitting the door will be muffled due to the foam core that has been shaped to absorb the sound waves and prevent the sound from passing and weakening the signal greatly before it will able to pass through.