Principal investigator Ron Miles and his team created movies showing how a differential microphone works. In each case, the sound comes from the black loudspeaker shown at the right.  In the first video the sound is "white" random noise.  In the second video there is a recording of Doug Jones saying "the beer drinkers raised their mugs."  The most sensitive axis of the microphone is indicated by the arrow on the rotation stage.  As the microphone is rotated, one should be able to hear the sound get softer and louder.  The gain was kept constant so the changes in level are due to the directionality of the differential microphone.  Rough measurements of the directivity index of the microphone are found to be about 5 dB over a broad range of frequencies.  An ideal differential microphone has a DI of about 4.8 dB. 

Video: Differential Microphone Effectiveness Using White Noise

Video: Differential Microphone Effectiveness Using Human Voice