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Signal Loss

DB loss and real world examples.  
Signal Strength Description Required for
-30 dBm Max achievable signal strength. The client can only be a few feet from the AP to achieve this. Not typical or desirable in the real world. N/A
-67 dBm Minimum signal strength for applications that require very reliable, timely packet delivery. VoIP/VoWiFi, streaming video
-70 dBm Minimum signal strength for reliable packet delivery. Email, web
-80 dBm Minimum signal strength for basic connectivity. Packet delivery may be unreliable. N/A
-90 dBm Approaching or drowning in the noise floor. Any functionality is highly unlikely. N/A

Chart listing some materials and approximate DB loss when transmitting through them. 
Material DB loss
Plasterboard wall 3dB
Glass wall with metal frame 6dB
Cinder block wall 4dB
Office window 3dB
Metal door 6dB
Metal door in brick wall 12.4dB