It&#x27;s a Faraday cage (<a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Faraday_cage" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Faraday_cage</a>), It keeps external radio signals out so they can check for EMI (<a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Electromagnetic_interference" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Electromagnetic_interference</a>)