An anechoic room is a man-made free sound field, which cannot be considered a complete free sound field and can only absorb about 99% of the sound.
Firstly, let's understand what a free sound field is. A free sound field refers to the sound field formed by sound waves propagating freely in an open space. In the free sound field, the path of sound wave propagation is free, without encountering any obstacles or restrictions of reflective surface, and the propagation direction and speed of sound wave are also uniform and irregular. The sound pressure level in this sound field exhibits a square attenuation pattern with increasing distance, that is, the sound pressure level is inversely proportional to the square of distance. Free sound fields are often used in acoustic experiments and measurements, and are also the foundation of many acoustic models and theories.
So how does an anechoic chamber achieve a free sound field?

An anechoic chamber is a closed space with an internal surface filled with sound-absorbing materials, which can absorb most of the sound energy and reduce the indoor noise level. In order to achieve a free sound field, the anechoic chamber needs to undergo multiple designs to achieve the expected effect, including overall structural design, vibration and sound insulation design, sound absorption design, etc. (specific solutions can be consulted for static environmental protection). Sound absorbing wedges are essential materials for anechoic chambers. In order to achieve a free sound field, multiple evenly distributed sound absorbing wedges need to be arranged inside the anechoic chamber to form a free sound field.