Hi Pushpak,
Signal attenuation or signal loss occurs even as the signal passes through air. The loss of signal strength is more pronounced as the signal passes through different objects. A transmit power of 20 mW is equivalent to 13 dBm. Therefore, if the transmitted power at the entry point of a plasterboard wall is at 13 dBm, the signal strength will be reduced to 10 dBm when exiting that wall.Here some examples;
Plasterboard wall Signal Attenuation 3 dB
Glass wall with metal frame Signal Attenuation 6 dB
Cinder block wall Signal Attenuation 4 dB
Office window Signal Attenuation 3 dB
Metal door Signal Attenuation 6 dB
Metal door in brick wall Signal Attenuation 12 dB
Human body Signal Attenuation 3 dB
From this good site survey doc;
http://www.cisco.com/en/US/products/hw/phones/ps379/products_implementation_design_guide_chapter09186a00802a09e0.html
Hope this helps!
Rob