Anechoic chambers are specialized chambers designed for electromagnetic or sound waves. They have specialized walls to absorb and reflect waves of electromagnetic radiation. Acoustic anechoic chambers are soundproof and used for testing applications. They are usually constructed with cement or brick walls to keep outside sound from entering the chamber. Inside, the chamber is lined with fiberglass wedges to absorb the sound waves. Anechoic chambers that are used to study or test electromagnetic interference (EMI) are lined with an absorbent material, such as carbon-impregnated foam shaped into pyramids. The pyramidal shape acts to resist and dissipate the electromagnetic waves. EMI testing is done in anechoic chambers to analyze the properties of antennas and other electronics that are susceptible to radio or microwave interference.