The RE15 is a super cardioid dynamic microphone originally released in late 1960’s. As with many Electro-Voice microphones the RE15 utilizes their patented “Variable-D” (Variable Distance) technology. There is an excellent video HERE that goes into detail about the design, but in short the perforated pipe along the rear of the microphone provides the appropriate damping to reduce proximity effect (low end boost as the microphone is placed closed to the source) and changes in frequency response as the source moves off-axis. The RE15 was original designed for stage use and the super cardioid pattern means the greatest rejection at 150 degrees off axis, which is ideal since most vocal microphones are placed at a 30 degree angle up at the performer. In the studio they are very forgiving with singers that move around while performing, and as a close spot mic when proximity effect is undesired.