46 SecTion | 1 Test and Measurement FigURe 1.28 A popular polarity test set. devices), and a receiving device that collects the signal via an inter- nal microphone (loudspeaker testing) or XLR input jack. A green light indicates correct polarity and a red light indicates reverse polarity. The receive unit should be placed at the system output (in front of the loudspeaker) while the send unit is systematically moved from device to device toward the system input. A polarity reversal will manifest itself by a red light on the receive unit. 1.3.7.3 Impulse Response Tests The impulse response is perhaps the most fundamental of audio and acoustic measurements. The polarity of a loudspeaker or electronic device can be determined from observing its impulse response, Figs. 1.29 and 1.30. This is one of the few ways to test flown loudspeakers from a remote position. It is best to test the polarity of components of multiway loudspeakers individually, since all of the individual components may not be polarized the same. Filters in the signal path (i.e., active crossover network) make the results more difficult to interpret, so it may be necessary to carefully test a system component (e.g., woofer) full-range for definitive results. Be sure to return the crossover to its proper set- ting before continuing.