639A is a Cardioid Directional Microphone
The 639-type Microphone is a general purpose magnetic mic­rophone with directional character­istics which are selectable by means of a switch. The three basic patterns available are non-directional, figure eight, and cardioid.

Sensitivity
The 639A open circuit output level is 84 dB below 1 volt/dyne/cm², 64 dB below 1 volt/10 dynes/cm².

Power Output Level
–56 dBm for a sound pressure level (SPL) of 10 dynes per square centi­meter, or –76 dBm for 1 dyne per square centi­meter when the micro­phone is terminated with a resistance equal to its internal impedance. Experience indicates that the sound pressure pro­duced at conversa­tional level three feet from a micro­phone ap­proaches 10 dynes/cm².

This beautiful mic sounds amazing, and has been housed in an upgraded bird cage, as unfortunately with a mic that is approximately 90 years old, the cages do suffer from wear and tear...

Reach out if you have any questions...