Isolation cabinet (guitar)

A guitar speaker isolation cabinet is a sound-proof enclosure that surrounds the speaker and sound-capturing microphone and prevents sound leakage into the outside environment, enabling the guitar amplifier to be turned up without excessive listening volume. An amplifier and speaker at full volume can be extremely loud, posing a risk to hearing and an annoyance to neighbors, and will often drown out other instruments in a mix in live shows.

Source: Wikipedia — Isolation cabinet (guitar) (CC BY-SA 4.0)