Metal doors cause the most signal attenuation in wireless design due to the properties of metal reflecting and absorbing radio waves, which can significantly reduce signal strength. Unlike materials such as cinder block or glass, metal can create a complete barrier for the signal, leading to a phenomenon known as "Faraday Cage" effect, where the metallic enclosure blocks electromagnetic fields. References: Designing Cisco Enterprise Wireless Networks (ENWLSD 300-425)