That is called an anechoic chamber [1]. Basically, the walls absorb all the outward radiation from the device so the test instruments do not have to sort out reflected waves. It mimics the device being outside with infinite space in all directions.<p>[1] <a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Anechoic_chamber" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Anechoic_chamber</a>