:Anechoic chamber:

Pronunciation GB: ˌænɪˈkəʊɪk, GA: ˌænɪˈkoʊɪk

A sound-proof enclosure whose walls, floor and ceiling are treated to minimise sound reflection. This commonly involves covering the internal surfaces with projecting cones or wedges of sound absorbing material such as fibre-glass wool. Anechoic chambers are used to produce high quality, distortion-free recordings for the purposes of speech research.

Link to a description of the anechoic chamber at UCL.