A room which creates free field conditions but above a reflective plane.
Above a certain frequency all sound waves that hit the walls or ceiling of the chamber will be absorbed.
This type of chamber is normally used in vehicle testing with a rolling round so that the vehicle may be driven. The semi-anechoic chamber then forms a controlled environment with the surrounding acoustic conditions behaving as a sterile version of the real world. In this way accurate measurements of the sound inside and outside the vehicle can be made under operating conditions.
ISO 3745:1983, Acoustics - Determination of sound power levels of noise sources - Precision methods for anechoic and semi-anechoic rooms.